diff options
Diffstat (limited to 'erts/emulator/internal_doc')
22 files changed, 2145 insertions, 79 deletions
diff --git a/erts/emulator/internal_doc/CarrierMigration.md b/erts/emulator/internal_doc/CarrierMigration.md index 2a9594db25..3a796d11b7 100644 --- a/erts/emulator/internal_doc/CarrierMigration.md +++ b/erts/emulator/internal_doc/CarrierMigration.md @@ -3,17 +3,17 @@ Carrier Migration The ERTS memory allocators manage memory blocks in two types of raw memory chunks. We call these chunks of raw memory -*carriers*. Singleblock carriers which only contain one large block, -and multiblock carriers which contain multiple blocks. A carrier is +*carriers*. Single-block carriers which only contain one large block, +and multi-block carriers which contain multiple blocks. A carrier is typically created using `mmap()` on unix systems. However, how a carrier is created is of minor importance. An allocator instance -typically manages a mixture of single- and multiblock carriers. +typically manages a mixture of single- and multi-block carriers. Problem ------- When a carrier is empty, i.e. contains only one large free block, it -is deallocated. Since multiblock carriers can contain both allocated +is deallocated. Since multi-block carriers can contain both allocated blocks and free blocks at the same time, an allocator instance might be stuck with a large amount of poorly utilized carriers if the memory load decreases. After a peak in memory usage it is expected that not @@ -23,9 +23,9 @@ can usually be reused if the memory load increases again. However, since each scheduler thread manages its own set of allocator instances, and memory load is not necessarily correlated to CPU load, we might get into a situation where there are lots of poorly utilized -multiblock carriers on some allocator instances while we need to -allocate new multiblock carriers on other allocator instances. In -scenarios like this, the demand for multiblock carriers in the system +multi-block carriers on some allocator instances while we need to +allocate new multi-block carriers on other allocator instances. In +scenarios like this, the demand for multi-block carriers in the system might increase at the same time as the actual memory demand in the system has decreased which is both unwanted and quite unexpected for the end user. @@ -34,7 +34,7 @@ Solution -------- In order to prevent scenarios like this we've implemented support for -migration of multiblock carriers between allocator instances of the +migration of multi-block carriers between allocator instances of the same type. ### Management of Free Blocks ### @@ -44,7 +44,7 @@ and add it to another we need to be able to move references to the free blocks of the carrier between the allocator instances. The allocator instance specific data structure referring to the free blocks it manages often refers to the same carrier from multiple -places. For example, when the address order bestfit strategy is used +places. For example, when the address order best-fit strategy is used this data structure is a binary search tree spanning all carriers that the allocator instance manages. Free blocks in one specific carrier can be referred to from potentially every other carrier that is @@ -135,7 +135,7 @@ carriers between scheduler specific allocator instances of the same allocator type. Each allocator instance keeps track of the current utilization of its -multiblock carriers. When the total utilization falls below the "abandon +multi-block carriers. When the total utilization falls below the "abandon carrier utilization limit" it starts to inspect the utilization of the current carrier when deallocations are made. If also the utilization of the carrier falls below the "abandon carrier utilization limit" it @@ -144,31 +144,45 @@ and inserts the carrier into the pool. Since the carrier has been unlinked from the data structure of available free blocks, no more allocations will be made in the -carrier. The allocator instance putting the carrier into the pool, -however, still has the responsibility of performing deallocations in -it while it remains in the pool. The allocator instance with this -deallocation responsibility is here called the **employer**. - -Each carrier has a flag field containing information about the -employing allocator instance, a flag indicating if the carrier is in -the pool or not, and a flag indicating if it is busy or not. When the -carrier is in the pool, the employing allocator instance needs to mark it -as busy while operating on it. If another thread inspects it in order -to try to fetch it from the pool, it will skip it if it is busy. When -fetching the carrier from the pool, employment will change and further +carrier. + +The allocator instance that created a carrier is called its **owner**. +Ownership never changes. + +The allocator instance that has the responsibility to perform deallocations in a +carrier is called its **employer**. The employer may also perform allocations if +the carrier is not in the pool. Employment may change when a carrier is fetched from +or inserted into the pool. + +Deallocations in a carrier, while it remains in the pool, is always performed +the owner. That is, all pooled carriers are employed by their owners. + +Each carrier has an atomic word containing a pointer to the employing allocator +instance and three bit flags; IN_POOL, BUSY and HOMECOMING. + +When fetching a carrier from the pool, employment may change and further deallocations in the carrier will be redirected to the new employer using the delayed dealloc functionality. -If a carrier in the pool becomes empty, it will be withdrawn from the -pool. All carriers that become empty are also always passed to its -**owning** allocator instance for deallocation using the delayed -dealloc functionality. Since carriers this way always will be -deallocated by the owner that allocated the carrier, the +When a foreign allocator instance abandons a carrier back into the pool, it will +also pass it back to its **owner** using the delayed dealloc queue. When doing +this it will set the HOMECOMING bit flag to mark it as "enqueued". The owner +will later clear the HOMECOMING bit when the carrier is dequeued. This mechanism +prevents a carrier from being enqueued again before it has been dequeued. + +When a carrier becomes empty, it will be deallocated. Carrier deallocation is +always done by the owner that allocated the carrier. By doing this, the underlying functionality of allocating and deallocating carriers can remain simple and doesn't have to bother about multiple threads. In a NUMA system we will also not mix carriers originating from multiple NUMA nodes. +If a carrier in the pool becomes empty, it will be withdrawn from the +pool and be deallocated by the owner which already employs it. + +If a carrier employed by a foreign allocator becomes empty, it will be passed +back to the owner for deallocation using the delayed dealloc functionality. + In short: * The allocator instance that created a carrier **owns** it. @@ -177,34 +191,31 @@ In short: * The allocator instance that uses a carrier **employs** it. * An **employer** can abandon a carrier into the pool. * Pooled carriers are not allocated from. -* Deallocation in a pooled carrier is still performed by its **employer**. -* **Employment** can only change when a carrier is fetched from the pool. +* Pooled carriers are always **employed** by their **owner**. +* **Employment** can only change from **owner** to a foreign allocator + when a carrier is fetched from the pool. + ### Searching the pool ### +When an allocator instance needs more carrier space, it inspects the pool. If no +carrier could be fetched from the pool, it will allocate a new +carrier. Regardless of where the allocator instance gets the carrier from, it +just links in the carrier into its data structure of free blocks. + To harbor real time characteristics, searching the pool is limited. We only inspect a limited number of carriers. If none of those carriers had a free block large enough to satisfy the allocation -request, the search will fail. A carrier in the pool can also be busy +request, the search will fail. A carrier in the pool can also be BUSY if another thread is currently doing block deallocation work on the -carrier. A busy carrier will also be skipped by the search as it can +carrier. A BUSY carrier will also be skipped by the search as it can not satisfy the request. The pool is lock-free and we do not want to block, waiting for the other thread to finish. -#### Before OTP 17.4 #### +### The bad cluster problem ### -When an allocator instance needs more carrier space, it always begins -by inspecting its own carriers that are waiting for thread progress -before they can be deallocated. If no such carrier could be found, it -then inspects the pool. If no carrier could be fetched from the pool, -it will allocate a new carrier. Regardless of where the allocator -instance gets the carrier from it the just links in the carrier into -its data structure of free blocks. - -#### After OTP 17.4 #### - -The old search algorithm had a problem as the search always started at -the same position in the pool, the sentinel. This could lead to +Before OTP-17.4 the search algorithm had a problem as the search always started +at the same position in the pool, the sentinel. This could lead to contention from concurrent searching processes. But even worse, it could lead to a "bad" state when searches fail with a high rate leading to new carriers instead being allocated. These new carriers @@ -236,26 +247,27 @@ The result is that we prefer carriers created by the thread itself, which is good for NUMA performance. And we get more entry points when searching the pool, which will ease contention and clustering. +### Our own pooled tree ### + To do the first search among own carriers, every allocator instance -has two new lists: `pooled_list` and `traitor_list`. These lists are only -accessed by the allocator itself and they only contain the allocator's -own carriers. When an owned carrier is abandoned and put in the -pool, it is also linked into `pooled_list`. When we search our -`pooled_list` and find a carrier that is no longer in the pool, we -move that carrier from `pooled_list` to `traitor_list` as it is now -employed by another allocator. If searching `pooled_list` fails, we -also do a limited search of `traitor_list`. When finding an abandoned -carrier in `traitor_list` it is either employed or moved back to -`pooled_list` if it could not satisfy the allocation request. - -When searching `pooled_list` and `traitor_list` we always start at the -point where the last search ended. This to avoid clustering -problems and increase the probability to find a "good" carrier. As -`pooled_list` and `traitor_list` are only accessed by the owning -allocator instance, they need no thread synchronization at all. +has a `pooled_tree` of carriers. This tree is only accessed by the allocator +itself and can only contain its own carriers. When a carrier is +abandoned and put in the pool, it is also inserted into `pooled_tree`. This is +either done direct, if the carrier was already employed by its owner, or by +first passing it back to the owner via the delayed dealloc queue. + +When we search our `pooled_tree` and find a carrier that is no longer in the +pool, we remove that carrier from `pooled_tree` and mark it as TRAITOR, as it is +now employed by a foreign allocator. We will not find any carriers in +`pooled_tree` that are marked as BUSY by other threads. + +If no carrier in `pooled_tree` had a large enough free block, we search it again +to find any carrier that may act as an entry point into the shared list of all +pooled carriers. This in order to, if possible, avoid starting at the sentinel +and thereby ease the "bad clustering" problem. Furthermore, the search for own carriers that are scheduled -for deallocation is now done as the last search option. The idea is +for deallocation is done as the last search option. The idea is that it is better to reuse a poorly utilized carrier than to resurrect an empty carrier that was just about to be released back to the OS. @@ -271,14 +283,14 @@ load did not. When using the `aoffcaobf` or `aoff` strategies compared to `gf` or `bf`, we loose some performance since we get more modifications in the data structure of free blocks. This performance penalty is however -reduced using the `aoffcbf` strategy. A tradeoff between memory +reduced using the `aoffcbf` strategy. A trade off between memory consumption and performance is however inevitable, and it is up to the user to decide what is most important. Further work ------------ -It would be quite easy to extend this to allow migration of multiblock +It would be quite easy to extend this to allow migration of multi-block carriers between all allocator types. More or less the only obstacle is maintenance of the statistics information. diff --git a/erts/emulator/internal_doc/DelayedDealloc.md b/erts/emulator/internal_doc/DelayedDealloc.md index b7d87b839f..4b7c774141 100644 --- a/erts/emulator/internal_doc/DelayedDealloc.md +++ b/erts/emulator/internal_doc/DelayedDealloc.md @@ -19,7 +19,7 @@ the Erlang VM where memory allocation/deallocation is frequent and references to memory also are passed around between threads this solution will also scale poorly due to lock contention. -Functionality Used to Adress This problem +Functionality Used to Address This problem ----------------------------------------- In order to reduce contention due to locking of allocator instances we @@ -44,12 +44,12 @@ deallocation. The "message box" is implemented using a lock free single linked list through the memory blocks to deallocate. The order of the elements in this list is not important. Insertion of new free blocks will be made -somewhere near the end of this list. Requirering that the new blocks +somewhere near the end of this list. Requiring that the new blocks need to be inserted at the end would cause unnecessary contention when large amount of memory blocks are inserted simultaneous by multiple threads. -The data structure refering to this single linked list cover two cache +The data structure referring to this single linked list cover two cache lines. One cache line containing information about the head of the list, and one cache line containing information about the tail of the list. This in order to reduce cache line ping ponging of this data @@ -65,21 +65,21 @@ list. In the uncontended case it will point to the end of the list, but when simultaneous insert operations are performed it will point to something near the end of the list. -When insterting an element one will try to write a pointer to the new +When inserting an element one will try to write a pointer to the new element in the next pointer of the element pointed to by the last pointer. This is done using an atomic compare and swap that expects -the next pointer to be `NULL`. If this succeds the thread performing +the next pointer to be `NULL`. If this succeeds the thread performing this operation moves the last pointer to point to the newly inserted element. If the atomic compare and swap described above failed, the last pointer didn't point to the last element. In this case we need to -insert the new element somewhere inbetween the element that the last +insert the new element somewhere between the element that the last pointer pointed to and the actual last element. If we do it this way the last pointer will eventually end up at the last element when threads stop adding new elements. When trying to insert somewhere near the end and failing to do so, the inserting thread sometimes moves to -the next element and somtimes tries with the same element again. This +the next element and sometimes tries with the same element again. This in order to spread the inserted elements during heavy contention. That is, we try to spread the modifications of memory to different locations instead of letting all threads continue to try to modify the @@ -87,7 +87,7 @@ same location in memory. ### Head ### -The head contains pointers to begining of the list (`head.first`), and +The head contains pointers to beginning of the list (`head.first`), and to the first block which other threads may refer to (`head.unref_end`). Blocks between these pointers are only refered to by the head part of the data structure which is only used by the @@ -142,7 +142,7 @@ contains this "marker" element. ### Contention ### -When elements are continously inserted by threads not owning the +When elements are continuously inserted by threads not owning the allocator instance, the thread owning the allocator instance will be able to work more or less undisturbed by other threads at the head end of the list. At the tail end large amounts of simultaneous inserts may diff --git a/erts/emulator/internal_doc/GarbageCollection.md b/erts/emulator/internal_doc/GarbageCollection.md new file mode 100644 index 0000000000..1d9e3f4160 --- /dev/null +++ b/erts/emulator/internal_doc/GarbageCollection.md @@ -0,0 +1,187 @@ +# Erlang Garbage Collector + +Erlang manages dynamic memory with a [tracing garbage collector](https://en.wikipedia.org/wiki/Tracing_garbage_collection). More precisely a per process generational semi-space copying collector using [Cheney's](#cheney) copy collection algorithm together with a global large object space. + +## Overview + +Each Erlang process has its own stack and heap which are allocated in the same memory block and grow towards each other. When the stack and the heap [meet](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/beam_emu.c#L387), the garbage collector is triggered and memory is reclaimed. If not enough memory was reclaimed, the heap will grow. + +### Creating Data + +Terms are created on the heap by evaluating expressions. There are two major types of terms: [immediate terms](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_term.h#L88-L97) which require no heap space (small integers, atoms, pids, port ids etc) and cons or [boxed terms](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_term.h#L106-L120) (tuple, big num, binaries etc) that do require heap space. Immediate terms do not need any heap space because they are embedded into the containing structure. + +Let's look at an example that returns a tuple with the newly created data. + +```erlang +data(Foo) -> + Cons = [42|Foo], + Literal = {text, "hello world!"}, + {tag, Cons, Literal}. +``` + +In this example we first create a new cons cell with an integer and a tuple with some text. Then a tuple of size three wrapping the other values with an atom tag is created and returned. + +On the heap tuples require a word size for each of its elements as well as for the header. Cons cells always require two words. Adding these things together, we get seven words for the tuples and 26 words for the cons cells. The string `"hello world!"` is a list of cons cells and thus requires 24 words. The atom `tag` and the integer `42` do not require any additional heap memory since it is an *immediate*. Adding all the terms together, the heap space required in this example should be 33 words. + +Compiling this code to beam assembly (`erlc -S`) shows exactly what is happening. + +```erlang + ... + {test_heap,6,1}. + {put_list,{integer,42},{x,0},{x,1}}. + {put_tuple,3,{x,0}}. + {put,{atom,tag}}. + {put,{x,1}}. + {put,{literal,{text,"hello world!"}}}. + return. +``` + +Looking at the assembler code we can see three things; The heap requirement in this function turns out to be only six words, as seen by the `{test_heap,6,1}` instruction. All the allocations are combined to a single instruction. The bulk of the data `{text, "hello world!"}` is a *literal*. Literals, sometimes referred to as constants, are not allocated in the function since they are a part of the module and allocated at load time. + +If there is not enough space available on the heap to satisfy the `test_heap` instructions request for memory, then a garbage collection is initiated. It may happen immediately in the `test_heap` instruction, or it can be delayed until a later time depending on what state the process is in. If the garbage collection is delayed, any memory needed will be allocated in heap fragments. Heap fragments are extra memory blocks that are a part of the young heap, but are not allocated in the contigious area where terms normally reside. See [The young heap](#the-young-heap) for more details. + +### The collector + +Erlang has a copying semi-space garbage collector. This means that when doing a garbage collection, the terms are copied from one distinct area, called the *from space*, to a new clean area, called the *to space*. The collector starts by [scanning the root-set](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L1980) (stack, registers, etc). + + + +It follows all the pointers from the root-set to the heap and copies each term word by word to the *to space*. + +After the header word has been copied a [*move marker*](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.h#L45-L46) is destructively placed in it pointing to the term in the *to space*. Any other term that points to the already moved term will [see this move marker](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L1125) and copy the referring pointer instead. For example, if the have the following Erlang code: + +```erlang +foo(Arg) -> + T = {test, Arg}, + {wrapper, T, T, T}. +``` + +Only one copy of T exists on the heap and during the garbage collection only the first time T is encountered will it be copied. + + + +After [all terms](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L1089) referenced by the root-set have been copied, the collector scans the *to space* and copies all terms that these terms reference. When scanning, the collector steps through each term on the *to space* and any term still referencing the *from space* is copied over to the *to space*. Some terms contain non-term data (the payload of a on heap binary for instance). When encountered by the collector, these values are simply skipped. + + + +Every term object we can reach is copied to the *to space* and stored on top off the *scan stop* line, and then the scan stop is moved to the end of the last object. + + + +When *scan stop* marker [catches up](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L1103) to the *scan start* marker, the garbage collection is done. At this point we can [deallocate](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L1206) the entire *from space* and therefore reclaim the entire young heap. + +## Generational Garbage Collection + +In addition to the collection algorithm described above, the Erlang garbage collector also provides generational garbage collection. An additional heap, called the old heap, is used where the long lived data is stored. The original heap is called the young heap, or sometimes the allocation heap. + +With this in mind we can look at the Erlang's garbage collection again. During the copy stage anything that should be copied to the young *to space* is instead copied to the old *to space* *if* it is [below the *high-watermark*](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L1127). + + + +The [*high-watermark*](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_process.h#L1021) is placed where the previous garbage collection (described in [Overview](#overview)) ended and we have introduced a new area called the old heap. When doing the normal garbage collection pass, any term that is located below the high-watermark is copied to the old *to space* instead of the young. + + + +In the next garbage collection, any pointers to the old heap will be ignored and not scanned. This way the garbage collector does not have to scan the long-lived terms. + +Generational garbage collection aims to increase performance at the expense of memory. This is achieved because only the young, smaller, heap is considered in most garbage collections. + +The generational [hypothesis](#ungar) predicts that most terms tend to die young, and for an immutable language such as Erlang, young terms die even faster than in other languages. So for most usage patterns the data in the new heap will die very soon after it is allocated. This is good because it limits the amount of data copied to the old heap and also because the garbage collection algorithm used is proportional to the amount of live data on the heap. + +One critical issue to note here is that any term on the young heap can reference terms on the old heap but *no* term on the old heap may refer to a term on the young heap. This is due to the nature of the copy algorithm. Anything referenced by an old heap term is not included in the reference tree, root-set and its followers, and hence is not copied. If it was, the data would be lost, fire and brimstone would rise to cover the earth. Fortunately, this comes naturally for Erlang because the terms are immutable and thus there can be no pointers modified on the old heap to point to the young heap. + +To reclaim data from the old heap, both young and old heaps are included during the collection and copied to a common *to space*. Both the *from space* of the young and old heap are then deallocated and the procedure will start over from the beginning. This type of garbage collection is called a full sweep and is triggered when the size of the area under the high-watermark is larger than the size of the free area of the old heap. It can also be triggered by doing a manual call to [erlang:garbage_collect()](http://erlang.org/doc/man/erlang.html#garbage_collect-0), or by running into the young garbage collection limit set by [spawn_opt(fun(),[{fullsweep_after, N}])](http://erlang.org/doc/man/erlang.html#spawn_opt-4) where N is the number of young garbage collections to do before forcing a garbage collection of both young and old heap. + +## The young heap + +The young heap, or the allocation heap, consists of the stack and heap as described in the Overview. However, it also includes any heap fragments that are attached to the heap. All of the heap fragments are considered to be above the high-watermark and part of the young generation. Heap fragments contain terms that either did not fit on the heap, or were created by another process and then attached to the heap. For instance if the bif binary_to_term created a term which does not fit on the current heap without doing a garbage collection, it will create a heap-fragment for the term and then schedule a garbage collection for later. Also if a message is sent to the process, the payload may be placed in a heap-fragment and that fragment is added to young heap when the message is matched in a receive clause. + +This procedure differs from how it worked prior to Erlang/OTP 19.0. Before 19.0, only a contiguous memory block where the young heap and stack resided was considered to be part of the young heap. Heap fragments and messages were immediately copied into the young heap before they could be inspected by the Erlang program. The behaviour introduced in 19.0 is superior in many ways - most significantly it reduces the number of necessary copy operations and the root set for garbage collection. + +## Sizing the heap + +As mentioned in the Overview the size of the heap [grows](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L247) to accommodate more data. Heaps grow in two stages, first a [variation of the Fibonacci sequence](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L199-L208) is used starting at 233 words. Then at about 1 mega words the heap only [grows in 20% increments](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L215-L227). + +There are two occasions when the young heap grows: + +* if the total size of the heap + message and heap fragments exceeds the current heap size. +* if after a fullsweep, the total amount of live objects is greater than 75%. + +There are two occasions when the young heap is shrunk: + +* if after a young collection, the total amount of live objects is less than 25% of the heap and the young heap is "big" +* if after a fullsweep, the total amount of live objects is less than 25% of the heap. + +The old heap is always one step ahead in the heap growth stages than the young heap. + +## Literals + +When garbage collecting a heap (young or old) all literals are left in place and not copied. To figure out if a term should be copied or not when doing a garbage collection the following pseudo code is used: + +```c +if (erts_is_literal(ptr) || (on_old_heap(ptr) && !fullsweep)) { + /* literal or non fullsweep - do not copy */ +} else { + copy(ptr); +} +``` + +The [`erts_is_literal`](https://github.com/erlang/otp/blob/OTP-19.0/erts/emulator/beam/global.h#L1452-L1465) check works differently on different architectures and operating systems. + +On 64 bit systems that allow mapping of unreserved virtual memory areas (most operating systems except Windows), an area of size 1 GB (by default) is mapped and then all literals are placed within that area. Then all that has to be done to determine if something is a literal or not is [two quick pointer checks](https://github.com/erlang/otp/blob/OTP-19.0/erts/emulator/beam/erl_alloc.h#L322-L324). This system relies on the fact that a memory page that has not been touched yet does not take any actual space. So even if 1 GB of virtual memory is mapped, only the memory which is actually needed for literals is allocated in ram. The size of the literal area is configurable through the +MIscs erts_alloc option. + +On 32 bit systems, there is not enough virtual memory space to allocate 1 GB for just literals, so instead small 256 KB sized literal regions are created on demand and a card mark bit-array of the entire 32 bit memory space is then used to determine if a term is a literal or not. Since the total memory space is only 32 bits, the card mark bit-array is only 256 words large. On a 64 bit system the same bit-array would have to be 1 tera words large, so this technique is only viable on 32 bit systems. Doing [lookups in the array](https://github.com/erlang/otp/blob/OTP-19.0/erts/emulator/beam/erl_alloc.h#L316-L319) is a little more expensive then just doing the pointer checks that can be done in 64 bit systems, but not extremely so. + +On 64 bit windows, on which erts_alloc cannot do unreserved virtual memory mappings, a [special tag](https://github.com/erlang/otp/blob/OTP-19.0/erts/emulator/beam/erl_term.h#L59) within the Erlang term object is used to determine if something [is a literal or not](https://github.com/erlang/otp/blob/OTP-19.0/erts/emulator/beam/erl_term.h#L248-L252). This is very cheap, however, the tag is only available on 64 bit machines, and it is possible to do a great deal of other nice optimizations with this tag in the future (like for instance a more compact list implementation) so it is not used on operating systems where it is not needed. + +This behaviour is different from how it worked prior to Erlang/OTP 19.0. Before 19.0 the literal check was done by checking if the pointer pointed to the young or old heap block. If it did not, then it was considered a literal. This lead to considerable overhead and strange memory usage scenarios, so it was removed in 19.0. + +## Binary heap + +The binary heap works as a large object space for binary terms that are greater than 64 bytes (from now on called off-heap binaries). The binary heap is [reference counted](https://en.wikipedia.org/wiki/Reference_counting) and a pointer to the off-heap binary is stored on the process heap. To keep track of when to decrement the reference counter of the off-heap binary, a linked list (the MSO - mark and sweep object list) containing funs and externals as well as off-heap binaries is woven through the heap. After a garbage collection is done, the [MSO list is swept](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L2299) and any off-heap binary that does not have a [move marker](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L2325) written into the header words has its reference [decremented and is potentially freed](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L2344-L2367). + +All items in the MSO list are ordered by the time they were added to the process heap, so when doing a minor garbage collection, the MSO sweeper only has to sweep until it [encounters an off-heap binary that is on the old heap](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_gc.c#L2369). + +### Virtual Binary heap + +Each process has a virtual binary heap associated with it that has the size of all the current off-heap binaries that the process has references to. The virtual binary heap also has a limit and grows and shrinks depending on how off-heap binaries are used by the process. The same growth and shrink mechanisms are used for the binary heap and for the term heap, so first a Fibonacci like series and then 20% growth. + +The virtual binary heap exists in order to [trigger](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/beam_emu.c#L364) garbage collections earlier when potentially there is a very large amount of off-heap binary data that could be reclaimed. This approach does not catch all problems with binary memory not being released soon enough, but it does catch a lot of them. + +## Messages + +Messages can become a part of the process heap at different times. This depends on how the process is configured. +We can configure the behaviour of each process using `process_flag(message_queue_data, off_heap | on_heap)` or we can set a default for all processes at start using the option `+hmqd`. + +What do these different configurations do and when should we use them? +Let's start by going through what happens when one Erlang process sends a message to another. +The sending process needs to do a couple of things: + +1. calculate [how large](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_message.c#L1031) the message to be sent is +2. [allocate enough space](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_message.c#L1033) to fit the entire message +3. [copy](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_message.c#L1040) the message payload +4. allocate a message container with some meta data +5. [insert](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_message.c#L502) the message container in the receiver process' [message queue](https://github.com/erlang/otp/blob/OTP-18.0/erts/emulator/beam/erl_process.h#L1042) + +The process flag `message_queue_data`, of the receiver process, controls the message allocating strategy of the sender process in step 2 and also how the message data is treated by the garbage collector. + +The procedure above is different from how it worked prior to 19.0. Before 19.0 there was no configuration option, the behaviour was always very similar to how the `on_heap` option is in 19.0. + +### Message allocating strategies + +If set to `on_heap`, the sending process will first attempt to allocate the space for the message directly on the young heap block of the receiving process. +This is not always possible as it requires taking the *main lock* of the receiving process. The main lock is also held when the process is executing. The possibility for a lock conflict is thus likely in an intensely collaborating system. +If the sending process cannot acquire the main lock, a heap fragment is instead created for the message and the message payload is copied onto that. +With the `off_heap` option the sender process always creates heap fragments for messages sent to that process. + +There are a bunch of different tradeoffs that come into play when trying to figure out which of the strategies you want to use. + +Using `off_heap` may seem like a nice way to get a more scalable system as you get very little contention on the main locks, however, allocating a heap fragment is more expensive than allocating on the heap of the receiving process. So if it is very unlikely that contention will occur, it is more efficient to try to allocate the message directly on the receiving process' heap. + +Using `on_heap` will force all messages to be part of on the young heap which will increase the amount of data that the garbage collector has to move. So if a garbage collection is triggered while processing a large amount of messages, they will be copied to the young heap. This in turn will lead to that the messages will quickly be promoted to the old heap and thus increase its size. This may be good or bad depending on exactly what the process does. A large old heap means that the young heap will also be larger, which in turn means that less garbage collections will be triggered while processing the message queue. This will temporarly increase the throughput of the process at the cost of more memory usage. However, if after all the messages have been consumed the process enters a state where a lot less messages are being received. Then it may be a long time before the next fullsweep garbage collection happens and the messages that are on the old heap will be there until that happens. So while `on_heap` is potentially faster than the other modes, it uses more memory for a longer time. This mode is the legacy mode which is almost how the message queue was handled before Erlang/OTP 19.0. + +Which one of these strategies is best depends a lot on what the process is doing and how it interacts with other processes. So, as always, profile the application and see how it behaves with the different options. + + <a name="cheney">[1]</a>: C. J. Cheney. A nonrecursive list compacting algorithm. Commun. ACM, 13(11):677–678, Nov. 1970. + + <a name="ungar">[2]</a>: D. Ungar. Generation scavenging: A non-disruptive high performance storage reclamation algorithm. SIGSOFT Softw. Eng. Notes, 9(3):157–167, Apr. 1984. diff --git a/erts/emulator/internal_doc/PortSignals.md b/erts/emulator/internal_doc/PortSignals.md index b1afb7c5cb..8782ae4e17 100644 --- a/erts/emulator/internal_doc/PortSignals.md +++ b/erts/emulator/internal_doc/PortSignals.md @@ -204,7 +204,7 @@ high limit is 8 KB and the low limit is 4 KB. Previously all operations sending signals to ports began by acquiring the port lock, then performed preparations for sending the signal, and -then finaly sent the signal. The preparations typically included +then finally sent the signal. The preparations typically included inspecting the state of the port, and preparing the data to pass along with the signal. The preparation of data is frequently quite time consuming, and did not really depend on the port. That is we would diff --git a/erts/emulator/internal_doc/SuperCarrier.md b/erts/emulator/internal_doc/SuperCarrier.md index 0ad6af41de..acf722ea37 100644 --- a/erts/emulator/internal_doc/SuperCarrier.md +++ b/erts/emulator/internal_doc/SuperCarrier.md @@ -151,7 +151,7 @@ To find the smallest free segment that will satisfy a carrier allocation size (`stree`). We search in this tree at allocation. If no free segment of sufficient size was found, the area (`sa` or `sua`) is instead expanded. If two or more free segments with equal size exist, the one at lowest -address is choosen for `sa` and highest address for `sua`. +address is chosen for `sa` and highest address for `sua`. At carrier deallocation, we want to coalesce with any adjacent free segments, to form one large free segment. To do that, all free diff --git a/erts/emulator/internal_doc/ThreadProgress.md b/erts/emulator/internal_doc/ThreadProgress.md index 6118bcf0f6..03a802f904 100644 --- a/erts/emulator/internal_doc/ThreadProgress.md +++ b/erts/emulator/internal_doc/ThreadProgress.md @@ -60,7 +60,7 @@ threads are managed threads. ### Thread Progress Events ### Any thread in the system may use the thread progress functionality in -order to determine when the following events have occured at least +order to determine when the following events have occurred at least once in all managed threads: 1. The thread has returned from other code to a known state in the @@ -160,7 +160,7 @@ calling the following functions: * `int erts_thr_progress_leader_update(ErtsSchedulerData *esdp)` - Leader update thread progress. -Unmanaged threads can delay thread progress beeing made: +Unmanaged threads can delay thread progress being made: * `ErtsThrPrgrDelayHandle erts_thr_progress_unmanaged_delay(void)` - Delay thread progress. @@ -251,7 +251,7 @@ doing so. If not zero, the leader isn't allowed to increment the global counter, and needs to wait before it can do this. When it is zero, it swaps the `waiting` and `current` counters before increasing the global counter. From now on the new `waiting` counter will -decrease, so that it eventualy will reach zero, making it possible to +decrease, so that it eventually will reach zero, making it possible to increment the global counter the next time. If we only used one reference counter it would potentially be held above zero for ever by different unmanaged threads. @@ -261,7 +261,7 @@ prevent the next increment of the global counter, but instead the increment after that. This is sufficient since the global counter needs to be incremented two times before thread progress has been made. It is also desirable not to prevent the first increment, since -the likelyhood increases that the delay is withdrawn before any +the likelihood increases that the delay is withdrawn before any increment of the global counter is delayed. That is, the operation will cause as little disruption as possible. diff --git a/erts/emulator/internal_doc/Tracing.md b/erts/emulator/internal_doc/Tracing.md index 728f315263..7f97f64765 100644 --- a/erts/emulator/internal_doc/Tracing.md +++ b/erts/emulator/internal_doc/Tracing.md @@ -51,7 +51,7 @@ the new instrumented code. Normally loaded code can only be reached through external functions calls. Trace settings must be activated instantaneously without the need of external function calls. -The choosen solution is instead for tracing to use the technique of +The chosen solution is instead for tracing to use the technique of replication applied on the data structures for breakpoints. Two generations of breakpoints are kept and indentified by index of 0 and 1. The global atomic variables `erts_active_bp_index` will determine diff --git a/erts/emulator/internal_doc/beam_makeops.md b/erts/emulator/internal_doc/beam_makeops.md new file mode 100644 index 0000000000..1da8d2ab05 --- /dev/null +++ b/erts/emulator/internal_doc/beam_makeops.md @@ -0,0 +1,1846 @@ +The beam\_makeops script +======================= + +This document describes the **beam\_makeops** script. + +Introduction +------------ + +The **beam\_makeops** Perl script is used at build-time by both the +compiler and runtime system. Given a number of input files (all with +the extension `.tab`), it will generate source files used by the +Erlang compiler and by the runtime system to load and execute BEAM +instructions. + +Essentially those `.tab` files define: + +* External generic BEAM instructions. They are the instructions that +are known to both the compiler and the runtime system. Generic +instructions are stable between releases. New generic instructions +with high numbers than previous instructions can be added in major +releases. The OTP 20 release has 159 external generic instructions. + +* Internal generic instructions. They are known only to the runtime +system and can be changed at any time without compatibility issues. +They are created by transformation rules (described next). + +* Rules for transforming one or more generic instructions to other +generic instructions. The transformation rules allow combining, +splitting, and removal of instructions, as well as shuffling operands. +Because of the transformation rules, the runtime can have many +internal generic instructions that are only known to runtime system. + +* Specific BEAM instructions. The specific instructions are the +instructions that are actually executed by the runtime system. They +can be changed at any time without causing compatibility issues. +The loader translates generic instructions to specific instructions. +In general, for each generic instruction, there exists a family of +specific instructions. The OTP 20 release has 389 specific +instructions. + +* The implementation of specific instructions. + +Generic instructions have typed operands. Here are a few examples of +operands for `move/2`: + + {move,{atom,id},{x,5}}. + {move,{x,3},{x,0}}. + {move,{x,2},{y,1}}. + +When those instructions are loaded, the loader rewrites them +to specific instructions: + + move_cx id 5 + move_xx 3 0 + move_xy 2 1 + +Corresponding to each generic instruction, there is a family of +specific instructions. The types that an instance of a specific +instruction can handle are encoded in the instruction names. For +example, `move_xy` takes an X register number as the first operand and +a Y register number as the second operand. `move_cx` takes a tagged +Erlang term as the first operand and an X register number as the +second operand. + +An example: the move instruction +-------------------------------- + +Using the `move` instruction as an example, we will give a quick +tour to show the main features of **beam\_makeops**. + +In the `compiler` application, in the file `genop.tab`, there is the +following line: + + 64: move/2 + +This is a definition of an external generic BEAM instruction. Most +importantly it specifices that the opcode is 64. It also defines that +it has two operands. The BEAM assembler will use the opcode when +creating `.beam` files. The compiler does not really need the arity, +but it will use it as an internal sanity check when assembling the +BEAM code. + +Let's have a look at `ops.tab` in `erts/emulator/beam`, where the +specific `move` instructions are defined. Here are a few of them: + + move x x + move x y + move c x + +Each specific instructions is defined by following the name of the +instruction with the types for each operand. An operand type is a +single letter. For example, `x` means an X register, `y` +means a Y register, and `c` is a "constant" (a tagged term such as +an integer, an atom, or a literal). + +Now let's look at the implementation of the `move` instruction. There +are multiple files containing implementations of instructions in the +`erts/emulator/beam` directory. The `move` instruction is defined in +`instrs.tab`. It looks like this: + + move(Src, Dst) { + $Dst = $Src; + } + +The implementation for an instruction largely follows the C syntax, +except that the variables in the function head don't have any types. +The `$` before an identifier denotes a macro expansion. Thus, +`$Src` will expand to the code to pick up the source operand for +the instruction and `$Dst` to the code for the destination register. + +We will look at the code for each specific instruction in turn. To +make the code easier to understand, let's first look at the memory +layout for the instruction `{move,{atom,id},{x,5}}`: + + +--------------------+--------------------+ + I -> | 40 | &&lb_move_cx | + +--------------------+--------------------+ + | Tagged atom 'id' | + +--------------------+--------------------+ + +This example and all other examples in the document assumes a 64-bit +archictecture, and furthermore that pointers to C code fit in 32 bits. + +`I` in the BEAM virtual machine is the instruction pointer. When BEAM +executes an instruction, `I` points to the first word of the +instruction. + +`&&lb_move_cx` is the address to C code that implements `move_cx`. It +is stored in the lower 32 bits of the word. In the upper 32 bits is +the byte offset to the X register; the register number 5 has been +multiplied by the word size size 8. + +In the next word the tagged atom `id` is stored. + +With that background, we can look at the generated code for `move_cx` +in `beam_hot.h`: + + OpCase(move_cx): + { + BeamInstr next_pf = BeamCodeAddr(I[2]); + xb(BeamExtraData(I[0])) = I[1]; + I += 2; + ASSERT(VALID_INSTR(next_pf)); + GotoPF(next_pf); + } + +We will go through each line in turn. + +* `OpCase(move_cx):` defines a label for the instruction. The +`OpCase()` macro is defined in `beam_emu.c`. It will expand this line +to `lb_move_cx:`. + +* `BeamInstr next_pf = BeamCodeAddr(I[2]);` fetches the pointer to +code for the next instruction to be executed. The `BeamCodeAddr()` +macro extracts the pointer from the lower 32 bits of the instruction +word. + +* `xb(BeamExtraData(I[0])) = I[1];` is the expansion of `$Dst = $Src`. +`BeamExtraData()` is a macro that will extract the upper 32 bits from +the instruction word. In this example, it will return 40 which is the +byte offset for X register 5. The `xb()` macro will cast a byte +pointer to an `Eterm` pointer and dereference it. The `I[1]` on +the right side of the `=` fetches an Erlang term (the atom `id` in +this case). + +* `I += 2` advances the instruction pointer to the next +instruction. + +* In a debug-compiled emulator, `ASSERT(VALID_INSTR(next_pf));` makes +sure that `next_pf` is a valid instruction (that is, that it points +within the `process_main()` function in `beam_emu.c`). + +* `GotoPF(next_pf);` transfers control to the next instruction. + +Now let's look at the implementation of `move_xx`: + + OpCase(move_xx): + { + Eterm tmp_packed1 = BeamExtraData(I[0]); + BeamInstr next_pf = BeamCodeAddr(I[1]); + xb((tmp_packed1>>BEAM_TIGHT_SHIFT)) = xb(tmp_packed1&BEAM_TIGHT_MASK); + I += 1; + ASSERT(VALID_INSTR(next_pf)); + GotoPF(next_pf); + } + +We will go through the lines that are new or have changed compared to +`move_cx`. + +* `Eterm tmp_packed1 = BeamExtraData(I[0]);` picks up both X register +numbers packed into the upper 32 bits of the instruction word. + +* `BeamInstr next_pf = BeamCodeAddr(I[1]);` pre-fetches the address of +the next instruction. Note that because both X registers operands fits +into the instruction word, the next instruction is in the very next +word. + +* `xb((tmp_packed1>>BEAM_TIGHT_SHIFT)) = xb(tmp_packed1&BEAM_TIGHT_MASK);` +copies the source to the destination. (For a 64-bit architecture, +`BEAM_TIGHT_SHIFT` is 16 and `BEAM_TIGHT_MASK` is `0xFFFF`.) + +* `I += 1;` advances the instruction pointer to the next instruction. + +`move_xy` is almost identical to `move_xx`. The only difference is +the use of the `yb()` macro instead of `xb()` to reference the +destination register: + + OpCase(move_xy): + { + Eterm tmp_packed1 = BeamExtraData(I[0]); + BeamInstr next_pf = BeamCodeAddr(I[1]); + yb((tmp_packed1>>BEAM_TIGHT_SHIFT)) = xb(tmp_packed1&BEAM_TIGHT_MASK); + I += 1; + ASSERT(VALID_INSTR(next_pf)); + GotoPF(next_pf); + } + +### Transformation rules ### + +Next let's look at how we can do some optimizations using transformation +rules. For simple instructions such as `move/2`, the instruction dispatch +overhead can be substantial. A simple optimization is to combine common +instructions sequences to a single instruction. One such common sequence +is multiple `move` instructions moving X registers to Y registers. + +Using the following rule we can combine two `move` instructions +to a `move2` instruction: + + move X1=x Y1=y | move X2=x Y2=y => move2 X1 Y1 X2 Y2 + +The left side of the arrow (`=>`) is a pattern. If the pattern +matches, the matching instructions will be replaced by the +instructions on the right side. Variables in a pattern must start +with an uppercase letter just as in Erlang. A pattern variable may be +followed `=` and one or more type letters to constrain the match to +one of those types. The variables that are bound on the left side can +be used on the right side. + +We will also need to define a specific instruction and an implementation: + + # In ops.tab + move2 x y x y + + // In instrs.tab + move2(S1, D1, S2, D2) { + Eterm V1, V2; + V1 = $S1; + V2 = $S2; + $D1 = V1; + $D2 = V2; + } + +When the loader has found a match and replaced the matched instructions, +it will match the new instructions against the transformation rules. +Because of that, we can define the rule for a `move3/6` instruction +as follows: + + move2 X1=x Y1=y X2=x Y2=y | move X3=x Y3=y => \ + move3 X1 Y1 X2 Y2 X3 Y3 + +(A `\` before a newline can be used to break a long line for readability.) + +It would also be possible to define it like this: + + move X1=x Y1=y | move X2=x Y2=y | move X3=x Y3=y => \ + move3 X1 Y1 X2 Y2 X3 Y3 + +but in that case it must be defined before the rule for `move2/4` +because the first matching rule will be applied. + +One must be careful not to create infinite loops. For example, if we +for some reason would want to reverse the operand order for the `move` +instruction, we must not do like this: + + move Src Dst => move Dst Src + +The loader would swap the operands forever. To avoid the loop, we must +rename the instruction. For example: + + move Src Dst => assign Dst Src + +This concludes the quick tour of the features of **beam\_makeops**. + +Short overview of instruction loading +------------------------------------- + +To give some background to the rest of this document, here follows a +quick overview of how instructions are loaded. + +* The loader reads and decodes one instruction at a time from the BEAM +code and creates a generic instruction. Many transformation rules +must look at multiple instructions, so the loader will +keep multiple generic instructions in a linked list. + +* The loader tries to apply transformation rules against the +generic instructions in the linked list. If a rule matches, the +matched instructions will be removed and replaced with new +generic instructions constructed from the right side of the +transformation. + +* If a transformation rule matched, the loader applies the +transformation rules again. + +* If no transformation rule match, the loader will begin rewriting +the first of generic instructions to a specific instruction. + +* First the loader will search for a specific operation where the +types for all operands match the type for the generic instruction. +The first matching instruction will be selected. **beam\_makeops** +has ordered the specific instructions so that instructions with more +specific operands comes before instructions with less specific +operands. For example, `move_nx` is more specific than `move_cx`. If +the first operand is `[]` (NIL), `move_nx` will be selected. + +* Given the opcode for the selected specific instruction, the loader +looks up the pointer to the C code for the instruction and stores +in the code area for the module being loaded. + +* The loader translates each operand to a machine word and stores it +in the code area. The operand type for the selected specific +instruction guides the translation. For example, if the type is `e`, +the value of the operand is an index into an arry of external +functions and will be translated to a pointer to the export entry for +the function to call. If the type is `x`, the number of the X +register will be multiplied by the word size to produce a byte offset. + +* The loader runs the packing engine to pack multiple operands into a +single word. The packing engine is controlled by a small program, +which is a string where each character is an instruction. For +example, the code to pack the operands for `move_xy` is `"22#"` (on a +64-bit machine). That program will pack the byte offsets for both +registers into the same word as the pointer to C code. + +Running beam_makeops +-------------------- + +**beam\_makeops** is found in `$ERL_TOP/erts/emulator/utils`. Options +start with a hyphen (`-`). The options are followed by the name of +the input files. By convention, all input files have the extension +`.tab`, but is not enforced by **beam\_makeops**. + +### The -outdir option ### + +The option `-outdir Directory` specifies the output directory for +the generated files. Default is the current working directory. + +### Running beam_makeops for the compiler ### + +Give the option `-compiler` to produce output files for the compiler. +The following files will be written to the output directory: + +* `beam_opcodes.erl` - Used primarily by `beam_asm` and `beam_diasm`. + +* `beam_opcode.hrl` - Used by `beam_asm`. It contains tag definitions +used for encoding instruction operands. + +The input file should only contain the definition of BEAM_FORMAT_NUMBER +and external generic instructions. (Everything else would be ignored.) + +### Running beam_makeops for the emulator ### + +Give the option `-emulator` to produce output files for the emulator. +The following output files will be generated in the output directory. + +* `beam_hot.h`, `beam_warm.h`, `beam_cold.`h - Implementation of +instructions. Included inside the `process_main()` function in +`beam_emu.c`. + +* `beam_opcodes.c` - Defines static data used by the loader +(`beam_load.c`). Data about generic instructions, specific +instructions (including how to pack their operands), and +transformation rules are all part of this file. + +* `beam_opcodes.h` - Miscellanous preprocessor definitions, mainly +used by `beam_load.c` but also by `beam_{hot,warm,cold}.h`. + +* `beam_pred_funcs.h` - Included by `beam_load.c`. Contains defines +needed to call guard constraints in transformation rules. + +* `beam_tr_funcs.h` - Included by `beam_load.c`. Contains defines +needed to call a C function to the right of a transformation rule. + +The following options can be given: + +* `wordsize 32|64` - Defines the word size. Default is 32. + +* `code-model Model` - The code model as given to `-mcmodel` option +for GCC. Default is `unknown`. If the code model is `small` (and +the word size is 64 bits), **beam\_makeops** will pack operands +into the upper 32 bits of the instruction word. + +* `DSymbol=0|1` - Defines the value for a symbol. The symbol can be +used in `%if` and `%unless` directives. + +Syntax of .tab files +-------------------- + +### Comments ### + +Any line starting with `#` is a comment and is ignored. + +A line with `//` is also a comment. It is recommended to only +use this style of comments in files that define implementations of +instructions. + +A long line can be broken into shorter lines by a placing a`\` before +the newline. + +### Variable definitions ### + +A variable definition binds a variable to a Perl variable. It is only +meaningful to add a new definition if **beam\_makeops** is updated +at the same time to use the variable. A variable definition looks this: + +*name*=*value*[;] + +where *name* is the name of a Perl variable in **beam\_makeops**, +and *value* is the value to be given to the variable. The line +can optionally end with a `;` (to avoid messing up the +C indentation mode in Emacs). + +Here follows a description of the variables that are defined. + +#### BEAM\_FORMAT\_NUMBER #### + +`genop.tab` has the following definition: + + BEAM_FORMAT_NUMBER=0 + +It defines the version of the instruction set (which will be +included in the code header in the BEAM code). Theoretically, +the version could be bumped, and all instructions changed. +In practice, we would have two support two instruction sets +in the runtime system for at least two releases, so it will +probably never happen in practice. + +#### GC\_REGEXP #### + +In `macros.tab`, there is a definition of `GC_REGEXP`. +It will be described in [a later section](#the-gc_regexp-definition). + +### Directives ### + +There are directives to classify specific instructions depending +on how frequently used they are: + +* `%hot` - Implementation will be placed in `beam_hot.h`. Frequently +executed instructions. + +* `%warm` - Implementation will be placed in `beam_warm.h`. Binary +syntax instructions. + +* `%cold` - Implementation will be placed in `beam_cold.h`. Trace +instructions and infrequently used instructions. + +Default is `%hot`. The directives will be applied to declarations +of the specific instruction that follow. Here is an example: + + %cold + is_number f? xy + %hot + +#### Conditional compilation directives #### + +The `%if` directive includes a range of lines if a condition is +true. For example: + + %if ARCH_64 + i_bs_get_integer_32 x f? x + %endif + +The specific instruction `i_bs_get_integer_32` will only be defined +on a 64-bit machine. + +The condition can be inverted by using `%unless` instead of `%if`: + + %unless NO_FPE_SIGNALS + fcheckerror p => i_fcheckerror + i_fcheckerror + fclearerror + %endif + +It is also possible to add an `%else` clause: + + %if ARCH_64 + BS_SAFE_MUL(A, B, Fail, Dst) { + Uint64 res = ($A) * ($B); + if (res / $B != $A) { + $Fail; + } + $Dst = res; + } + %else + BS_SAFE_MUL(A, B, Fail, Dst) { + Uint64 res = (Uint64)($A) * (Uint64)($B); + if ((res >> (8*sizeof(Uint))) != 0) { + $Fail; + } + $Dst = res; + } + %endif + +#### Symbols that are defined in directives #### + +The following symbols are always defined. + +* `ARCH_64` - is 1 for a 64-bit machine, and 0 otherwise. +* `ARCH_32` - is 1 for 32-bit machine, and 1 otherwise. + +The `Makefile` for building the emulator currently defines the +following symbols by using the `-D` option on the command line for +**beam\_makeops**. + +* `NO_FPE_SIGNALS` - 1 if FPE signals are not enable in runtime system, +0 otherwise. +* `USE_VM_PROBES` - 1 if the runtime system is compiled to use VM probes (support for dtrace or systemtap), 0 otherwise. + +### Defining external generic instructions ### + +External generic BEAM instructions are known to both the compiler and +the runtime system. They remain stable between releases. A new major +release may add more external generic instructions, but must not change +the semantics for a previously defined instruction. + +The syntax for an external generic instruction is as follows: + +*opcode*: [-]*name*/*arity* + +*opcode* is an integer greater than or equal to 1. + +*name* is an identifier starting with a lowercase letter. *arity* is +an integer denoting the number of operands. + +*name* can optionally be preceded by `-` to indicate that it has been +obsoleted. The compiler is not allowed to generate BEAM files that +use obsolete instructions and the loader will refuse to load BEAM +files that use obsolete instructions. + +It only makes sense to define external generic instructions in the +file `genop.tab` in `lib/compiler/src`, because the compiler must +know about them in order to use them. + +New instructions must be added at the end of the file, with higher +numbers than the previous instructions. + +### Defining internal generic instructions ### + +Internal generic instructions are known only to the runtime +system and can be changed at any time without compatibility issues. + +There are two ways to define internal generic instructions: + +* Implicitly when a specific instruction is defined. This is by far +the most common way. Whenever a specific instruction is created, +**beam\_makeops** automatically creates an internal generic instruction +if it does not previously exist. + +* Explicitly. This is necessary only when a generic instruction does +not have any corresponding specific instruction. + +The syntax for an internal generic instruction is as follows: + +*name*/*arity* + +*name* is an identifier starting with a lowercase letter. *arity* is +an integer denoting the number of operands. + +### About generic instructions in general ### + +Each generic instruction has an opcode. The opcode is an integer, +greater than or equal to 1. For an external generic instruction, it +must be explicitly given `genop.tab`, while internal generic +instructions are automatically numbered by **beam\_makeops**. + +The identity of a generic instruction is its name combined with its +arity. That means that it is allowed to define two distinct generic +instructions having the same name but with different arities. For +example: + + move_window/5 + move_window/6 + +Each operand of a generic instruction is tagged with its type. A generic +instruction can have one of the following types: + +* `x` - X register. + +* `y` - Y register. + +* `l` - Floating point register number. + +* `i` - Tagged literal integer. + +* `a` - Tagged literal atom. + +* `n` - NIL (`[]`, the empty list). + +* `q` - Literal that don't fit in a word, that is an object stored on +the heap such as a list or tuple. Any heap object type is supported, +even types that don't have real literals such as external references. + +* `f` - Non-zero failure label. + +* `p` - Zero failure label. + +* `u` - Untagged integer that fits in a machine word. It is used for many +different purposes, such as the number of live registers in `test_heap/2`, +as a reference to the export for `call_ext/2`, and as the flags operand for +binary syntax instructions. When the generic instruction is translated to a +specific instruction, the type for the operand in the specific operation will +tell the loader how to treat the operand. + +* `o` - Overflow. If the value for an `u` operand does not fit in a machine +word, the type of the operand will be changed to `o` (with no associated +value). Currently only used internally in the loader in the guard constraint +function `binary_too_big()`. + +* `v` - Arity value. Only used internally in the loader. + + +### Defining specific instructions ### + +The specific instructions are known only to the runtime system and +are the instructions that are actually executed. They can be changed +at any time without causing compatibility issues. + +A specific instruction can have at most 6 operands. + +A specific instruction is defined by first giving its name followed by +the types for each operand. For example: + + move x y + +Internally, for example in the generated code and in the output from +the BEAM disassembler, the instruction `move x y` will be called `move_xy`. + +The name for a specific instruction is an identifier starting with a +lowercase letter. A type is an lowercase or uppercase letter. + +All specific instructions with a given name must have the same number +of operands. That is, the following is **not** allowed: + + move x x + move x y x y + +Here follows the type letters that more or less directly corresponds +to the types for generic instructions. + +* `x` - X register. Will be loaded as a byte offset to the X register +relative to the base of X register array. (Can be packed with other +operands.) + +* `y` - Y register. Will be loaded as a byte offset to the Y register +relative to the stack frame. (Can be packed with other operands.) + +* `r` - X register 0. An implicit operand that will not be stored in +the loaded code. + +* `l` - Floating point register number. (Can be packed with other +operands.) + +* `i` - Tagged literal integer (a SMALL that will fit in one word). + +* `a` - Tagged atom. + +* `n` - NIL or the empty list. (Will not be stored in the loaded code.) + +* `q` - Tagged CONS or BOXED pointer. That is, a term such as a list +or tuple. Any heap object type is supported, even types that don't +have real literals such as external references. + +* `f` - Failure label (non-zero). The target for a branch +or call instruction. + +* `p` - The 0 failure label, meaning that an exception should be raised +if the instruction fails. (Will not be stored in the loaded code.) + +* `c` - Any literal term; that is, immediate literals such as SMALL, +and CONS or BOXED pointers to literals. (Can be used where the +operand in the generic instruction has one of the types `i`, `a`, `n`, +or `q`.) + +The types that follow do a type test of the operand at runtime; thus, +they are generally more expensive in terms of runtime than the types +described earlier. However, those operand types are needed to avoid a +combinatorial explosion in the number of specific instructions and +overall code size of `process_main()`. + +* `s` - Tagged source: X register, Y register, or a literal term. The +tag will be tested at runtime to retrieve the value from an X +register, a Y register, or simply use the value as a tagged Erlang +term. (Implementation note: An X register is tagged as a pid, and a Y +register as a port. Therefore the literal term must not contain a +port or pid.) + +* `S` - Tagged source register (X or Y). The tag will be tested at +runtime to retrieve the value from an X register or a Y register. Slighly +cheaper than `s`. + +* `d` - Tagged destination register (X or Y). The tag will be tested +at runtime to set up a pointer to the destination register. If the +instrution performs a garbarge collection, it must use the +`$REFRESH_GEN_DEST()` macro to refresh the pointer before storing to +it (there are more details about that in a later section). + +* `j` - A failure label (combination of `f` and `p`). If the branch target 0, +an exception will be raised if instruction fails, otherwise control will be +transfered to the target address. + +The types that follows are all applied to an operand that has the `u` +type. + +* `t` - An untagged integer that will fit in 12 bits (0-4096). It can be +packed with other operands in a word. Most often used as the number +of live registers in instructions such as `test_heap`. + +* `I` - An untagged integer that will fit in 32 bits. It can be +packed with other operands in a word on a 64-bit system. + +* `W` - Untagged integer or pointer. Not possible to pack with other +operands. + +* `e` - Pointer to an export entry. Use by call instructions that call +other modules, such as `call_ext`. + +* `L` - A label. Only used by the `label/1` instruction. + +* `b` - Pointer to BIF. Used by instructions that BIFs, such as +`call_bif`. + +* `A` - A tagged arityvalue. Used in instructions that test the arity +of a tuple. + +* `P` - A byte offset into a tuple. + +* `Q` - A byte offset into the stack. Used for updating the frame +pointer register. Can be packed with other operands. + +When the loader translates a generic instruction a specific +instruction, it will choose the most specific instruction that will +fit the types. Consider the following two instructions: + + move c x + move n x + +The `c` operand can encode any literal value, including NIL. The +`n` operand only works for NIL. If we have the generic instruction +`{move,nil,{x,1}}`, the loader will translate it to `move_nx 1` +because `move n x` is more specific. `move_nx` could be slightly +faster or smaller (depending on the architecture), because the `[]` +is not stored explicitly as an operand. + +#### Syntactic sugar for specific instructions #### + +It is possible to specify more than one type letter for each operand. +Here is an example: + + move cxy xy + +This is syntactic sugar for: + + move c x + move c y + move x x + move x y + move y x + move y y + +Note the difference between `move c xy` and `move c d`. Note that `move c xy` +is equivalent to the following two definitions: + + move c x + move c y + +On the other hand, `move c d` is a single instruction. At runtime, +the `d` operand will be tested to see whether it refers to an X +register or a Y register, and a pointer to the register will be set +up. + +#### The '?' type modifier #### + +The character `?` can be added to the end of an operand to indicate +that the operand will not be used every time the instruction is executed. +For example: + + allocate_heap t I t? + is_eq_exact f? x xy + +In `allocate_heap`, the last operand is the number of live registers. +It will only be used if there is not enough heap space and a garbage +collection must be performed. + +In `is_eq_exact`, the failure address (the first operand) will only be +used if the two register operands are not equal. + +Knowing that an operand is not always used can improve how packing +is done for some instructions. + +For the `allocate_heap` instruction, without the `?` the packing would +be done like this: + + +--------------------+--------------------+ + I -> | Stack needed | &&lb_allocate_heap + + +--------------------+--------------------+ + | Heap needed | Live registers + + +--------------------+--------------------+ + +"Stack needed" and "Heap needed" are always used, but they are in +different words. Thus, at runtime the `allocate_heap` instruction +must read both words from memory even though it will not always use +"Live registers". + +With the `?`, the operands will be packed like this: + + +--------------------+--------------------+ + I -> | Live registers | &&lb_allocate_heap + + +--------------------+--------------------+ + | Heap needed | Stack needed + + +--------------------+--------------------+ + +Now "Stack needed" and "Heap needed" are in the same word. + +### Defining transformation rules ### + +Transformation rules are used to rewrite generic instructions to other +generic instructions. The transformations rules are applied +repeatedly until no rule match. At that point, the first instruction +in the resulting instruction sequence will be converted to a specific +instruction and added to the code for the module being loaded. Then +the transformation rules for the remaining instructions are run in the +same way. + +A rule is recognized by its right-pointer arrow: `=>`. To the left of +the arrow is one or more instruction patterns, separated by `|`. To +the right of the arrow is zero or more instructions, separated by `|`. +If the instructions from the BEAM code matches the instruction +patterns on the left side, they will be replaced with instructions on +the right side (or removed if there are no instructions on the right). + +#### Defining instruction patterns #### + +We will start looking at the patterns on the left side of the arrow. + +A pattern for an instruction consists of its name, followed by a pattern +for each of its operands. The operand patterns are separated by spaces. + +The simplest possible pattern is a variable. Just like in Erlang, +a variable must begin with an uppercase letter. If the same variable is +used in multiple operands, the pattern will only match if the operands +are equal. For example: + + move Same Same => + +This pattern will match if the operands for `move` are the same. If +the pattern match, the instruction will be removed. (That used to be an +actual rule a long time ago when the compiler would occasionally produce +instructions such as `{move,{x,2},{x,2}}`.) + +Variables that have been bound on the left side can be used on the +right side. For example, this rule will rewrite all `move` instructions +to `assign` instructions with the operands swapped: + + move Src Dst => assign Dst Src + +If we only want to match operands of a certain type, we can +use a type constraint. A type constraint consists of one or more +lowercase letters, each specifying a type. For example: + + is_integer Fail an => jump Fail + +The second operand pattern, `an`, will match if the second operand is +either an atom or NIL (the empty list). In case of a match, the +`is_integer/2` instruction will be replaced with a `jump/1` +instruction. + +An operand pattern can bind a variable and constrain the type at the +same time by following the variable with a `=` and the constraint. +For example: + + is_eq_exact Fail=f R=xy C=q => i_is_eq_exact_literal Fail R C + +Here the `is_eq_exact` instruction is replaced with a specialized instruction +that only compares literals, but only if the first operand is a register and +the second operand is a literal. + +#### Further constraining patterns #### + +In addition to specifying a type letter, the actual value for the type can +be specified. For example: + + move C=c x==1 => move_x1 C + +Here the second operand of `move` is constrained to be X register 1. + +When specifying an atom constraint, the atom is written as it would be +in the C source code. That is, it needs an `am_` prefix, and it must +be listed in `atom.names`. For example: + + is_boolean Fail=f a==am_true => + is_boolean Fail=f a==am_false => + +There are several constraints available for testing whether a call is to a BIF +or a function. + +The constraint `u$is_bif` will test whether the given operand refers to a BIF. +For example: + + call_ext u Bif=u$is_bif => call_bif Bif + call_ext u Func => i_call_ext Func + +The `call_ext` instruction can be used to call functions written in +Erlang as well as BIFs (or more properly called SNIFs). The +`u$is_bif` constraint will match if the operand refers to a BIF (that +is, if it is listed in the file `bif.tab`). Note that `u$is_bif` +should only be applied to operands that are known to contain an index +to the import table chunk in the BEAM file (such operands have the +type `b` or `e` in the corresponding specific instruction). If +applied to other `u` operands, it will at best return a nonsense +result. + +The `u$is_not_bif` constraint matches if the operand does not refer to +a BIF (not listed in `bif.tab`). For example: + + move S X0=x==0 | line Loc | call_ext_last Ar Func=u$is_not_bif D => \ + move S X0 | call_ext_last Ar Func D + +The `u$bif:Module:Name/Arity` constraint tests whether the given +operand refers to a specific BIF. Note that `Module:Name/Arity` +**must** be an existing BIF defined in `bif.tab`, or there will +be a compilation error. It is useful when a call to a specific BIF +should be replaced with an instruction as in this example: + + gc_bif2 Fail Live u$bif:erlang:splus/2 S1 S2 Dst => \ + gen_plus Fail Live S1 S2 Dst + +Here the call to the GC BIF `'+'/2` will be replaced with the instruction +`gen_plus/5`. Note that the same name as used in the C source code must be +used for the BIF, which in this case is `splus`. It is defined like this +in `bit.tab`: + + ubif erlang:'+'/2 splus_2 + +The `u$func:Module:Name/Arity` will test whether the given operand is a +a specific function. Here is an example: + + bif1 Fail u$func:erlang:is_constant/1 Src Dst => too_old_compiler + +`is_constant/1` used to be a BIF a long time ago. The transformation +replaces the call with the `too_old_compiler` instruction which will produce +a nicer error message than the default error would be for a missing guard BIF. + +#### Type constraints allowed in patterns #### + +Here are all type letters that are allowed on the left side of a transformation +rule. + +* `u` - An untagged integer that fits in a machine word. + +* `x` - X register. + +* `y` - Y register. + +* `l` - Floating point register number. + +* `i` - Tagged literal integer. + +* `a` - Tagged literal atom. + +* `n` - NIL (`[]`, the empty list). + +* `q` - Literals that don't fit in a word, such as list or tuples. + +* `f` - Non-zero failure label. + +* `p` - The zero failure label. + +* `j` - Any label. Equivalent to `fp`. + +* `c` - Any literal term. Equivalent to `ainq`. + +* `s` - X register, Y register, or any literal term. Equivalent to `xyc`. + +* `d` - X or Y register. Equivalent to `xy`. (In a pattern `d` will +match both source and destination registers. As an operand in a specific +instruction, it must only be used for a destination register.) + +* `o` - Overflow. An untagged integer that does not fit in a machine word. + +#### Guard constraints #### + +If the constraints described so far is not enough, additional +constraints can be written in C in `beam_load.c` and be called as a +guard function on the left side of the transformation. If the guard +function returns a non-zero value, the matching of the rule will +continue, otherwise the match will fail. For example: + + ensure_map Lit=q | literal_is_map(Lit) => + +The guard test `literal_is_map/1` tests whether the given literal is a map. +If the literal is a map, the instruction is unnecessary and can be removed. + +It is outside the scope for this document to describe in detail how such +guard functions are written, but for the curious here is the implementation +of `literal_is_map()`: + + static int + literal_is_map(LoaderState* stp, GenOpArg Lit) + { + Eterm term; + + ASSERT(Lit.type == TAG_q); + term = stp->literals[Lit.val].term; + return is_map(term); + } + +#### Handling instruction with variable number of operands #### + +Some instructions, such as `select_val/3`, essentially has a variable +number of operands. Such instructions have a `{list,[...]}` operand +as their last operand in the BEAM assembly code. For example: + + {select_val,{x,0}, + {f,1}, + {list,[{atom,b},{f,4},{atom,a},{f,5}]}}. + +The loader will convert a `{list,[...]}` operand to an `u` operand whose +value is the number of elements in the list, followed by each element in +the list. The instruction above would be translated to the following +generic instruction: + + {select_val,{x,0},{f,1},{u,4},{atom,b},{f,4},{atom,a},{f,5}} + +To match a variable number of arguments we need to use the special +operand type `*` like this: + + select_val Src=aiq Fail=f Size=u List=* => \ + i_const_select_val Src Fail Size List + +This transformation renames a `select_val/3` instruction +with a constant source operand to `i_const_select_val/3`. + +#### Constructing new instructions on the right side #### + +The most common operand on the right side is a variable that was bound while +matching the left side. For example: + + trim N Remaining => i_trim N + +An operand can also be a type letter to construct an operand of that type. +Each type has a default value. For example, the type `x` has the default +value 1023, which is the highest X register. That makes `x` on the right +side a convenient shortcut for a temporary X register. For example: + + is_number Fail Literal=q => move Literal x | is_number Fail x + +If the second operand for `is_number/2` is a literal, it will be moved to +X register 1023. Then `is_number/2` will test whether the value stored in +X register 1023 is a number. + +This kind of transformation is useful when it is rare that an operand can +be anything else but a register. In the case of `is_number/2`, the second +operand is always a register unless the compiler optimizations have been +disabled. + +If the default value is not suitable, the type letter can be followed +by `=` and a value. Most types take an integer value. The value for +an atom is written the same way as in the C source code. For example, +the atom `false` is written as `am_false`. The atom must be listed in +`atom.names`. + +Here is an example showing how values can be specified: + + bs_put_utf32 Fail=j Flags=u Src=s => \ + i_bs_validate_unicode Fail Src | \ + bs_put_integer Fail i=32 u=1 Flags Src + +#### Type letters on the right side #### + +Here follows all types that are allowed to be used in operands for +instructions being constructed on the right side of a transformation +rule. + +* `u` - Construct an untagged integer. The default value is 0. + +* `x` - X register. The default value is 1023. That makes `x` convenient to +use as a temporary X register. + +* `y` - Y register. The default value is 0. + +* `l` - Foating point register number. The default value is 0. + +* `i` - Tagged literal integer. The default value is 0. + +* `a` - Tagged atom. The default value is the empty atom (`am_Empty`). + +* `n` - NIL (`[]`, the empty list). + +#### Function call on the right side #### + +Transformations that are not possible to describe with the rule +language as described here can be written as a C function in +`beam_load.c` and called from the right side of a transformation. The +left side of the transformation will perform the match and bind +operands to variables. The variables can then be passed to a +generator function on the right side. For example: + + bif2 Fail=j u$bif:erlang:element/2 Index=s Tuple=xy Dst=d => \ + gen_element(Jump, Index, Tuple, Dst) + +This transformation rule matches a call to the BIF `element/2`. +The operands will be captured and the function `gen_element()` will +be called. + +`gen_element()` will produce one of two instructions depending +on `Index`. If `Index` is an integer in the range from 1 up to +the maximum tuple size, the instruction `i_fast_element/2` will +be produced, otherwise the instruction `i_element/4` will be +produced. The corresponding specific instructions are: + + i_fast_element xy j? I d + i_element xy j? s d + +The `i_fast_element/2` instruction is faster because the tuple is +already an untagged integer. It also knows that the index is at least +1, so it does not have to test for that. The `i_element/4` +instruction will have to fetch the index from a register, test that it +is an integer, and untag the integer. + +It is outside the scope of this document to describe in detail how +generator functions are written, but for the curious, here is the +implementation of `gen_element()`: + + static GenOp* + gen_element(LoaderState* stp, GenOpArg Fail, + GenOpArg Index, GenOpArg Tuple, GenOpArg Dst) + { + GenOp* op; + + NEW_GENOP(stp, op); + op->arity = 4; + op->next = NULL; + + if (Index.type == TAG_i && Index.val > 0 && + Index.val <= ERTS_MAX_TUPLE_SIZE && + (Tuple.type == TAG_x || Tuple.type == TAG_y)) { + op->op = genop_i_fast_element_4; + op->a[0] = Tuple; + op->a[1] = Fail; + op->a[2].type = TAG_u; + op->a[2].val = Index.val; + op->a[3] = Dst; + } else { + op->op = genop_i_element_4; + op->a[0] = Tuple; + op->a[1] = Fail; + op->a[2] = Index; + op->a[3] = Dst; + } + + return op; + } +} + +### Defining the implementation ### + +The actual implementation of instructions are also defined in `.tab` +files processed by **beam\_makeops**. For practical reasons, +instruction definitions are stored in several files, at the time of +writing in the following files: + + bif_instrs.tab + arith_instrs.tab + bs_instrs.tab + float_instrs.tab + instrs.tab + map_instrs.tab + msg_instrs.tab + select_instrs.tab + trace_instrs.tab + +There is also a file that only contains macro definitions: + + macros.tab + +The syntax of each file is similar to C code. In fact, most of +the contents *is* C code, interspersed with macro invocations. + +To allow Emacs to auto-indent the code, each file starts with the +following line: + + // -*- c -*- + +To avoid messing up the indentation, all comments are written +as C++ style comments (`//`) instead of `#`. Note that a comment +must start at the beginning of a line. + +The meat of an instruction definition file are macro definitions. +We have seen this macro definition before: + + move(Src, Dst) { + $Dst = $Src; + } + +A macro definitions must start at the beginning of the line (no spaces +allowed), the opening curly bracket must be on the same line, and the +finishing curly bracket must be at the beginning of a line. It is +recommended that the macro body is properly indented. + +As a convention, the macro arguments in the head all start with an +uppercase letter. In the body, the macro arguments can be expanded +by preceding them with `$`. + +A macro definition whose name and arity matches a family of +specific instructions is assumed to be the implementation of that +instruction. + +A macro can also be invoked from within another macro. For example, +`move_deallocate_return/2` avoids repeating code by invoking +`$deallocate_return()` as a macro: + + move_deallocate_return(Src, Deallocate) { + x(0) = $Src; + $deallocate_return($Deallocate); + } + +Here is the definition of `deallocate_return/1`: + + deallocate_return(Deallocate) { + //| -no_next + int words_to_pop = $Deallocate; + SET_I((BeamInstr *) cp_val(*E)); + E = ADD_BYTE_OFFSET(E, words_to_pop); + CHECK_TERM(x(0)); + DispatchReturn; + } + +The expanded code for `move_deallocate_return` will look this: + + OpCase(move_deallocate_return_cQ): + { + x(0) = I[1]; + do { + int words_to_pop = Qb(BeamExtraData(I[0])); + SET_I((BeamInstr *) cp_val(*E)); + E = ADD_BYTE_OFFSET(E, words_to_pop); + CHECK_TERM(x(0)); + DispatchReturn; + } while (0); + } + +When expanding macros, **beam\_makeops** wraps the expansion in a +`do`/`while` wrapper unless **beam\_makeops** can clearly see that no +wrapper is needed. In this case, the wrapper is needed. + +Note that arguments for macros cannot be complex expressions, because +the arguments are split on `,`. For example, the following would +not work because **beam\_makeops** would split the expression into +two arguments: + + $deallocate_return(get_deallocation(y, $Deallocate)); + +#### Code generation directives #### + +Within macro definitions, `//` comments are in general not treated +specially. They will be copied to the file with the generated code +along with the rest of code in the body. + +However, there is an exception. Within a macro definition, a line that +starts with whitespace followed by `//|` is treated specially. The +rest of the line is assumed to contain directives to control code +generation. + +Currently, two code generation directives are recognized: + +* `-no_prefetch` +* `-no_next` + +##### The -no_prefetch directive ##### + +To see what `-no_prefetch` does, let's first look at the default code +generation. Here is the code generated for `move_cx`: + + OpCase(move_cx): + { + BeamInstr next_pf = BeamCodeAddr(I[2]); + xb(BeamExtraData(I[0])) = I[1]; + I += 2; + ASSERT(VALID_INSTR(next_pf)); + GotoPF(next_pf); + } + +Note that the very first thing done is to fetch the address to the +next instruction. The reason is that it usually improves performance. + +Just as a demonstration, we can add a `-no_prefetch` directive to +the `move/2` instruction: + + move(Src, Dst) { + //| -no_prefetch + $Dst = $Src; + } + +We can see that the prefetch is no longer done: + + OpCase(move_cx): + { + xb(BeamExtraData(I[0])) = I[1]; + I += 2; + ASSERT(VALID_INSTR(*I)); + Goto(*I); + } + +When would we want to turn off the prefetch in practice? + +In instructions that will not always execute the next instruction. +For example: + + is_atom(Fail, Src) { + if (is_not_atom($Src)) { + $FAIL($Fail); + } + } + + // From macros.tab + FAIL(Fail) { + //| -no_prefetch + $SET_I_REL($Fail); + Goto(*I); + } + +`is_atom/2` may either execute the next instruction (if the second +operand is an atom) or branch to the failure label. + +The generated code looks like this: + + OpCase(is_atom_fx): + { + if (is_not_atom(xb(I[1]))) { + ASSERT(VALID_INSTR(*(I + (fb(BeamExtraData(I[0]))) + 0))); + I += fb(BeamExtraData(I[0])) + 0;; + Goto(*I);; + } + I += 2; + ASSERT(VALID_INSTR(*I)); + Goto(*I); + } + +##### The -no_next directive ##### + +Next we will look at when the `-no_next` directive can be used. Here +is the `jump/1` instruction: + + jump(Fail) { + $JUMP($Fail); + } + + // From macros.tab + JUMP(Fail) { + //| -no_next + $SET_I_REL($Fail); + Goto(*I); + } + +The generated code looks like this: + + OpCase(jump_f): + { + ASSERT(VALID_INSTR(*(I + (fb(BeamExtraData(I[0]))) + 0))); + I += fb(BeamExtraData(I[0])) + 0;; + Goto(*I);; + } + +If we remove the `-no_next` directive, the code would look like this: + + OpCase(jump_f): + { + BeamInstr next_pf = BeamCodeAddr(I[1]); + ASSERT(VALID_INSTR(*(I + (fb(BeamExtraData(I[0]))) + 0))); + I += fb(BeamExtraData(I[0])) + 0;; + Goto(*I);; + I += 1; + ASSERT(VALID_INSTR(next_pf)); + GotoPF(next_pf); + } + +In the end, the C compiler will probably optimize this code to the +same native code as the first version, but the first version is certainly +much easier to read for human readers. + +#### Macros in the macros.tab file #### + +The file `macros.tab` contains many useful macros. When implementing +new instructions it is good practice to look through `macros.tab` to +see if any of existing macros can be used rather than re-inventing +the wheel. + +We will describe a few of the most useful macros here. + +##### The GC_REGEXP definition ##### + +The following line defines a regular expression that will recognize +a call to a function that does a garbage collection: + + GC_REGEXP=erts_garbage_collect|erts_gc|GcBifFunction; + +The purpose is that **beam\_makeops** can verify that an instruction +that does a garbage collection and has an `d` operand uses the +`$REFRESH_GEN_DEST()` macro. + +If you need to define a new function that does garbage collection, +you should give it the prefix `erts_gc_`. If that is not possible +you should update the regular expression so that it will match your +new function. + +##### FAIL(Fail) ##### + +Branch to `$Fail`. Will suppress prefetch (`-no_prefetch`). Typical use: + + is_nonempty_list(Fail, Src) { + if (is_not_list($Src)) { + $FAIL($Fail); + } + } + +##### JUMP(Fail) ##### + +Branch to `$Fail`. Suppresses generation of dispatch of the next +instruction (`-no_next`). Typical use: + + jump(Fail) { + $JUMP($Fail); + } + +##### GC_TEST(NeedStack, NeedHeap, Live) ##### + +`$GC_TEST(NeedStack, NeedHeap, Live)` tests that given amount of +stack space and heap space is available. If not it will do a +garbage collection. Typical use: + + test_heap(Nh, Live) { + $GC_TEST(0, $Nh, $Live); + } + +##### AH(NeedStack, NeedHeap, Live) ##### + +`AH(NeedStack, NeedHeap, Live)` allocates a stack frame and +optionally additional heap space. + +#### Pre-defined macros and variables #### + +**beam\_makeops** defines several built-in macros and pre-bound variables. + +##### The NEXT_INSTRUCTION pre-bound variable ##### + +The NEXT_INSTRUCTION is a pre-bound variable that is available in +all instructions. It expands to the address of the next instruction. + +Here is an example: + + i_call(CallDest) { + SET_CP(c_p, $NEXT_INSTRUCTION); + $DISPATCH_REL($CallDest); + } + +When calling a function, the return address is first stored in `c_p->cp` +(using the `SET_CP()` macro defined in `beam_emu.c`), and then control is +transferred to the callee. Here is the generated code: + + OpCase(i_call_f): + { + SET_CP(c_p, I+1); + ASSERT(VALID_INSTR(*(I + (fb(BeamExtraData(I[0]))) + 0))); + I += fb(BeamExtraData(I[0])) + 0;; + DTRACE_LOCAL_CALL(c_p, erts_code_to_codemfa(I)); + Dispatch();; + } + +We can see that that `$NEXT_INSTRUCTION` has been expanded to `I+1`. +That makes sense since the size of the `i_call_f/1` instruction is +one word. + +##### The IP_ADJUSTMENT pre-bound variable ##### + +`$IP_ADJUSTMENT` is usually 0. In a few combined instructions +(described below) it can be non-zero. It is used like this +in `macros.tab`: + + SET_I_REL(Offset) { + ASSERT(VALID_INSTR(*(I + ($Offset) + $IP_ADJUSTMENT))); + I += $Offset + $IP_ADJUSTMENT; + } + +Avoid using `IP_ADJUSTMENT` directly. Use `SET_I_REL()` or +one of the macros that invoke such as `FAIL()` or `JUMP()` +defined in `macros.tab`. + +#### Pre-defined macro functions #### + +##### The IF() macro ##### + +`$IF(Expr, IfTrue, IfFalse)` evaluates `Expr`, which must be a valid +Perl expression (which for simple numeric expressions have the same +syntax as C). If `Expr` evaluates to 0, the entire `IF()` expression will be +replaced with `IfFalse`, otherwise it will be replaced with `IfTrue`. + +See the description of `OPERAND_POSITION()` for an example. + +##### The OPERAND\_POSITION() macro ##### + +`$OPERAND_POSITION(Expr)` returns the position for `Expr`, if +`Expr` is an operand that is not packed. The first operand is +at position 1. + +Returns 0 otherwise. + +This macro could be used like this in order to share code: + + FAIL(Fail) { + //| -no_prefetch + $IF($OPERAND_POSITION($Fail) == 1 && $IP_ADJUSTMENT == 0, + goto common_jump, + $DO_JUMP($Fail)); + } + + DO_JUMP(Fail) { + $SET_I_REL($Fail); + Goto(*I)); + } + + // In beam_emu.c: + common_jump: + I += I[1]; + Goto(*I)); + + +#### The $REFRESH\_GEN\_DEST() macro #### + +When a specific instruction has a `d` operand, early during execution +of the instruction, a pointer will be initialized to point to the X or +Y register in question. + +If there is a garbage collection before the result is stored, +the stack will move and if the `d` operand refered to a Y +register, the pointer will no longer be valid. (Y registers are +stored on the stack.) + +In those circumstances, `$REFRESH_GEN_DEST()` must be invoked +to set up the pointer again. **beam\_makeops** will notice +if there is a call to a function that does a garbage collection and +`$REFRESH_GEN_DEST()` is not called. + +Here is a complete example. The `new_map` instruction is defined +like this: + + new_map d t I + +It is implemented like this: + + new_map(Dst, Live, N) { + Eterm res; + + HEAVY_SWAPOUT; + res = erts_gc_new_map(c_p, reg, $Live, $N, $NEXT_INSTRUCTION); + HEAVY_SWAPIN; + $REFRESH_GEN_DEST(); + $Dst = res; + $NEXT($NEXT_INSTRUCTION+$N); + } + +If we have forgotten the `$REFRESH_GEN_DEST()` there would be a message +similar to this: + + pointer to destination register is invalid after GC -- use $REFRESH_GEN_DEST() + ... from the body of new_map at beam/map_instrs.tab(30) + +#### Combined instructions #### + +**Problem**: For frequently executed instructions we want to use +"fast" operands types such as `x` and `y`, as opposed to `s` or `S`. +To avoid an explosion in code size, we want to share most of the +implementation between the instructions. Here are the specific +instructions for `i_increment/5`: + + i_increment r W t d + i_increment x W t d + i_increment y W t d + +The `i_increment` instruction is implemented like this: + + i_increment(Source, IncrementVal, Live, Dst) { + Eterm increment_reg_source = $Source; + Eterm increment_val = $IncrementVal; + Uint live; + Eterm result; + + if (ERTS_LIKELY(is_small(increment_reg_val))) { + Sint i = signed_val(increment_reg_val) + increment_val; + if (ERTS_LIKELY(IS_SSMALL(i))) { + $Dst = make_small(i); + $NEXT0(); + } + } + live = $Live; + HEAVY_SWAPOUT; + reg[live] = increment_reg_val; + reg[live+1] = make_small(increment_val); + result = erts_gc_mixed_plus(c_p, reg, live); + HEAVY_SWAPIN; + ERTS_HOLE_CHECK(c_p); + if (ERTS_LIKELY(is_value(result))) { + $REFRESH_GEN_DEST(); + $Dst = result; + $NEXT0(); + } + ASSERT(c_p->freason != BADMATCH || is_value(c_p->fvalue)); + goto find_func_info; + } + +There will be three almost identical copies of the code. Given the +size of the code, that could be too high cost to pay. + +To avoid the three copies of the code, we could use only one specific +instruction: + + i_increment S W t d + +(The same implementation as above will work.) + +That reduces the code size, but is slower because `S` means that +there will be extra code to test whether the operand refers to an X +register or a Y register. + +**Solution**: We can use "combined instructions". Combined +instructions are combined from instruction fragments. The +bulk of the code can be shared. + +Here we will show how `i_increment` can be implemented as a combined +instruction. We will show each individual fragment first, and then +show how to connect them together. First we will need a variable that +we can store the value fetched from the register in: + + increment.head() { + Eterm increment_reg_val; + } + +The name `increment` is the name of the group that the fragment +belongs to. Note that it does not need to have the same +name as the instruction. The group name is followed by `.` and +the name of the fragment. The name `head` is pre-defined. +The code in it will be placed at the beginning of a block, so +that all fragments in the group can access it. + +Next we define the fragment that will pick up the value from the +register from the first operand: + + increment.fetch(Src) { + increment_reg_val = $Src; + } + +We call this fragment `fetch`. This fragment will be duplicated three +times, one for each value of the first operand (`r`, `x`, and `y`). + +Next we define the main part of the code that do the actual incrementing. + + increment.execute(IncrementVal, Live, Dst) { + Eterm increment_val = $IncrementVal; + Uint live; + Eterm result; + + if (ERTS_LIKELY(is_small(increment_reg_val))) { + Sint i = signed_val(increment_reg_val) + increment_val; + if (ERTS_LIKELY(IS_SSMALL(i))) { + $Dst = make_small(i); + $NEXT0(); + } + } + live = $Live; + HEAVY_SWAPOUT; + reg[live] = increment_reg_val; + reg[live+1] = make_small(increment_val); + result = erts_gc_mixed_plus(c_p, reg, live); + HEAVY_SWAPIN; + ERTS_HOLE_CHECK(c_p); + if (ERTS_LIKELY(is_value(result))) { + $REFRESH_GEN_DEST(); + $Dst = result; + $NEXT0(); + } + ASSERT(c_p->freason != BADMATCH || is_value(c_p->fvalue)); + goto find_func_info; + } + +We call this fragment `execute`. It will handle the three remaining +operands (`W t d`). There will only be one copy of this fragment. + +Now that we have defined the fragments, we need to inform +**beam\_makeops** how they should be connected: + + i_increment := increment.fetch.execute; + +To the left of the `:=` is the name of the specific instruction that +should be implemented by the fragments, in this case `i_increment`. +To the right of `:=` is the name of the group with the fragments, +followed by a `.`. Then the name of the fragments in the group are +listed in the order they should be executed. Note that the `head` +fragment is not listed. + +The line ends in `;` (to avoid messing up the indentation in Emacs). + +(Note that in practice the `:=` line is usually placed before the +fragments.) + +The generated code looks like this: + + { + Eterm increment_reg_val; + OpCase(i_increment_rWtd): + { + increment_reg_val = r(0); + } + goto increment__execute; + + OpCase(i_increment_xWtd): + { + increment_reg_val = xb(BeamExtraData(I[0])); + } + goto increment__execute; + + OpCase(i_increment_yWtd): + { + increment_reg_val = yb(BeamExtraData(I[0])); + } + goto increment__execute; + + increment__execute: + { + // Here follows the code from increment.execute() + . + . + . + } + +##### Some notes about combined instructions ##### + +The operands that are different must be at +the beginning of the instruction. All operands in the last +fragment must have the same operands in all variants of +the specific instruction. + +As an example, the following specific instructions cannot be +implemented as a combined instruction: + + i_times j? t x x d + i_times j? t x y d + i_times j? t s s d + +We would have to change the order of the operands so that the +two operands that are different are placed first: + + i_times x x j? t d + i_times x y j? t d + i_times s s j? t d + +We can then define: + + i_times := times.fetch.execute; + + times.head { + Eterm op1, op2; + } + + times.fetch(Src1, Src2) { + op1 = $Src1; + op2 = $Src2; + } + + times.execute(Fail, Live, Dst) { + // Multiply op1 and op2. + . + . + . + } + +Several instructions can share a group. As an example, the following +instructions have different names, but in the end they all create a +binary. The last two operands are common for all of them: + + i_bs_init_fail xy j? t? x + i_bs_init_fail_heap s I j? t? x + i_bs_init W t? x + i_bs_init_heap W I t? x + +The instructions are defined like this (formatted with extra +spaces for clarity): + + i_bs_init_fail_heap := bs_init . fail_heap . verify . execute; + i_bs_init_fail := bs_init . fail . verify . execute; + i_bs_init := bs_init . . plain . execute; + i_bs_init_heap := bs_init . heap . execute; + +Note that the first two instruction have three fragments, while the +other two only have two fragments. Here are the fragments: + + bs_init_bits.head() { + Eterm num_bits_term; + Uint num_bits; + Uint alloc; + } + + bs_init_bits.plain(NumBits) { + num_bits = $NumBits; + alloc = 0; + } + + bs_init_bits.heap(NumBits, Alloc) { + num_bits = $NumBits; + alloc = $Alloc; + } + + bs_init_bits.fail(NumBitsTerm) { + num_bits_term = $NumBitsTerm; + alloc = 0; + } + + bs_init_bits.fail_heap(NumBitsTerm, Alloc) { + num_bits_term = $NumBitsTerm; + alloc = $Alloc; + } + + bs_init_bits.verify(Fail) { + // Verify the num_bits_term, fail using $FAIL + // if there is a problem. + . + . + . + } + + bs_init_bits.execute(Live, Dst) { + // Long complicated code to a create a binary. + . + . + . + } + +The full definitions of those instructions can be found in `bs_instrs.tab`. +The generated code can be found in `beam_warm.h`. diff --git a/erts/emulator/internal_doc/figures/.gitignore b/erts/emulator/internal_doc/figures/.gitignore new file mode 100644 index 0000000000..c2813ac866 --- /dev/null +++ b/erts/emulator/internal_doc/figures/.gitignore @@ -0,0 +1 @@ +*.eps
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