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Use S operands instead of s operands for a slight speed increase
and reduction in code size of process_main(). Use micro instructions
for frequently executed instructions.
While at it, use safe multiplication in gen_get_integer() in
beam_load.c.
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Starting in OTP 19 (in commit 9504c0dd71d0), the compiler emits
a test_unit instruction instead of a skip instruction at the end
of binary. We can do the same replacement in the loader to get
rid of the i_bs_skip_bits_all2 instruction.
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The new compiler required adding support for Y register for all
binary matching instructions. That was (intentionally) done in a
naive way that simplicated duplicated the entire body of each
instruction.
Now it's time to be less naive. Rewrite the binary matching
instructions using micro instructions. Because some of the binary
instructions are huge, that will significantly decrease the size of
process_main().
When compiling with clang, a huge process_main() would mess up
profile-guide optimization resulting in a significant performance
degradation. On my Mac, profile-guide optimzation would decrease
the estone benchmark by 100K estones (about 20 percent). This commit
gives me back the lost estones.
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Mark the obsoleted instructions bs_start_match2, bs_save2, bs_restore2, and
bs_context_to_binary as cold.
Remove support of a Y operand for bs_save2 and bs_restore2.
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get_tuple_element with an Y register has become more frequent with
the new compiler.
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The compiler used to generate "move Literal y(Y)" instructions very
rarely. Therefore, there was a transformation to avoid having a "move
c y" instruction.
With the new compiler, "move Literal y(Y)" instructions are relatively frequent, so we will need a "move c y" instruction.
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The is_nonempty_list test is very frequently followed by
get_tl, and frequently followed by get_hd.
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It turns out that the combination of is_nonempty_list
and test_heap is no longer frequent.
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The test_arity instruction is often followed by get_tuple_element.
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`swap x y` is rarely or never used. I found a single use of
`swap_temp x y x` in the sample of modules compiled by
`scripts/diffable`.
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Of the `move_dup` instructions, only `move_dup x x x` was
frequently used. Remove the other register combinations.
With those instruction `move_dup` instructions removed, it
is necessary to add new predicates to avoid unsafe translation
to `move_shift` and `move2_par`.
Also add additional transformations to transform more `move`
instructions into `move2_par`. The existing transformation
would require the `move` instructions to be in the "right"
order in order to be transformed.
Remove `move3 x y x y x y` because it turns out to be rarely
executed.
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The is_function2 instruction is executed surprisingly
frequently when running dialyzer or the compiler. It
cannot hurt to optimize it a little.
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Use microops for BIFs
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The guard BIF `length/1` would calculate the length of the list in one
go without yielding, even if the list was were long. To make it even
worse, the call to `length/1` would only cost a single reduction.
This commit reimplements `length/1` so that it eats a number of
reductions proportional to the length of the list, and yields if the
available reductions run out.
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This allows bif1/2/3 to share the main part of the code.
The price is that we always need to copy all three temporary registers
when error handling in bodies, but that should be infrequent.
Additionally it makes it a bit harder to read the disasembly since now
the arguments to BIFs are in the reverse order.
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Summary: This commit simplifies the implementation of the "GC BIFs" so
that they no longer need to do a garbage collection, removing duplicate
code for all GC BIFs in the runtime system, as well as potentially
reducing the size of the loaded BEAM code by using shorter
instructions calling those BIFs.
A GC BIF is a guard BIF that will do a garbage
collection if it needs to build anything on the heap.
For example, `abs/1` is a GC BIF because it might need to
allocate space on the heap (if the result is a floating point
number or the resulting integer is a bignum).
Before R12, a guard BIF (such as `abs/1`) that need to allocate
heap space would allocate outside of process's main heap, in
a heap fragment.
GC BIFs were introduced in R12B to support literals. During garbage
collection it become necessary to quickly test whether a term was
a literal. To make the check simple, guards BIFs were no longer
allowed to create heap fragments. Instead GC BIFs were introduced.
In OTP 19, the implementation of literals was changed to support
storing messages in heap fragments outside of the main heap for a
process. That change again made it allowed for guard BIFs to create
heap fragments when they need to build terms on the heap.
It would even be possible for the guard BIFs to build directly
on the main heap if there is room there, because the compiler
assumes that a new `test_heap/2` instruction must be emitted
when building anything after calling a GC BIF. (We don't do that
in this commit; see below.)
This commit simplifies the implementation of the GC BIFs in
the runtime system.
Each GC BIF had a dual implementation: one that was used when the GC
BIF was called directly and one used when it was called via
`apply/3`. For example, `abs/1` was implemented in `abs_1()` and
`erts_gc_abs_1()`. This commit removes the GC version of each BIF. The
other version that allocates heap space using `HAlloc()` is updated to
use the new `HeapFragOnlyAlloc()` macro that will allocate heap
space in a heap fragment outside of the main heap.
Because the BIFs will allocate outside of the main heap, the same
`bif` instructions used by nonbuilding BIFs can be used for the
(former) GC BIFs. Those instructions don't use the macros that save
and restore the heap and stack pointers (SWAPOUT/SWAPIN). If the
former GC BIFs would build on the main heap, either new instructions
would be needed, or SWAPOUT/SWAPIN instructions would need to be added
to the `bif` instructions.
Instructions that call the former GC BIFs don't need the operand
that specifies the number of live X registers. Therefore, the
instructions that call the BIFs are usually one word shorter.
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This commit improves the bit-syntax match optimization pass,
leveraging the new SSA intermediate format to perform much more
aggressive optimizations. Some highlights:
* Watch contexts can be reused even after being passed to a
function or being used in a try block.
* Sub-binaries are no longer eagerly extracted, making it far
easier to keep "happy paths" free from binary creation.
* Trivial wrapper functions no longer disable context reuse.
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The upcoming beam_ssa_bsm pass allows match contexts to be used
across function calls that take said context as an argument, which
means it's fairly common for them to end up in Y registers.
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Sometimes when building a tuple, there is no way to avoid an
extra `move` instruction. Consider this code:
make_tuple(A) -> {ok,A}.
The corresponding BEAM code looks like this:
{test_heap,3,1}.
{put_tuple,2,{x,1}}.
{put,{atom,ok}}.
{put,{x,0}}.
{move,{x,1},{x,0}}.
return.
To avoid overwriting the source register `{x,0}`, a `move`
instruction is necessary.
The problem doesn't exist when building a list:
%% build_list(A) -> [A].
{test_heap,2,1}.
{put_list,{x,0},nil,{x,0}}.
return.
Introduce a new `put_tuple2` instruction that builds a tuple in a
single instruction, so that the `move` instruction can be eliminated:
%% make_tuple(A) -> {ok,A}.
{test_heap,3,1}.
{put_tuple2,{x,0},{list,[{atom,ok},{x,0}]}}.
return.
Note that the BEAM loader already combines `put_tuple` and `put`
instructions into an internal instruction similar to `put_tuple2`.
Therefore the introduction of the new instruction will not speed up
execution of tuple building itself, but it will be less work for
the loader to load the new instruction.
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* maint:
ops.tab: Fix potentially unsafe optimization of raise/2
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The operands for the raise/2 instruction are almost always in x(2) and
x(1). Therefore the loader translates the raise/2 instruction to an
i_raise/0 instruction which uses the values in x(2) and x(1). If the
operands happens to be in other registers, the loader inserts move/2
instruction to move them to x(2) and x(1).
The problem is that x(3) is used as a temporary register when
generating the move/2 instructions. That is unsafe if the
Value operand for raise/2 is x(3).
Thus:
raise x(0) x(3)
will be translated to:
move x(0) x(3)
move x(3) x(1)
move x(3) x(2)
i_raise
The Trace will be written to both x(2) and x(1).
The current compiler will never use x(3) for the Value operand,
so there is no need to patch previous releases. But a future compiler
version might allocate registers differently.
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The new code generator will use Y registers as a destination for
binary construction and matching instructions. v3_codegen would
always first store terms in an X register and it would be the
responsibility of the optimization passes to optimize the extra
moves.
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When matching on a literal map, the map is placed into the general
scratch register first. This is fine in isolation, but when the
key to be matched was in a Y register it would also be placed in
the scratch register, overwriting the map and crashing the
emulator.
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Compile external fun expressions to literals
OTP-15003
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The expressions fun M:F/A, when all elements are literals are also
treated as a literal. Since they have consistent representation and
don't depend on the code currently loaded in the VM, this is safe.
This can provide significant performance improvements in code using such
functions extensively - a full function call to erlang:make_fun/3 is
replaced by a single move instruction and no register shuffling or
saving registers to stack is necessary. Additionally, compound data
types that contain such external functions as elements can be treated as
literals too.
The commit also changes the representation of external funs to be a
valid Erlang syntax and adds support for literal external funs to core
Erlang.
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A get_map_elements instruction that has a literal map operand
would never be translated to a i_get_map_element instruction.
That would be a problem for the following instruction:
get_map_elements Fail #{} {x,0}, {x,1}
Since the key is not a literal, get_map_element must be used,
since get_map_elements requires that a hash value can be
calculated for each element.
When the instruction is translated to i_get_map_element, the
hash value will be set to 0 and an assertion will trigger in
the debug build.
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Instructions that produce more than one result complicate
optimizations. get_list/3 is one of two instructions that
produce multiple results (get_map_elements/3 is the other).
Introduce the get_hd/2 and get_tl/2 instructions
that return the head and tail of a cons cell, respectively,
and use it internally in all optimization passes.
For efficiency, we still want to use get_list/3 if both
head and tail are used, so we will translate matching pairs
of get_hd and get_tl back to get_list instructions.
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Consider the following function:
function({function,Name,Arity,CLabel,Is0}, Lc0) ->
try
%% Optimize the code for the function.
catch
Class:Error:Stack ->
io:format("Function: ~w/~w\n", [Name,Arity]),
erlang:raise(Class, Error, Stack)
end.
The stacktrace is retrieved, but it is only used in the call
to erlang:raise/3. There is no need to build a stacktrace
in this function. We can avoid the building if we introduce
an instruction called raw_raise/3 that works exactly like
the erlang:raise/3 BIF except that its third argument must
be a raw stacktrace.
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Add syntax in try/catch to retrieve the stacktrace directly
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This commit adds a new syntax for retrieving the stacktrace
without calling erlang:get_stacktrace/0. That allow us to
deprecate erlang:get_stacktrace/0 and ultimately remove it.
The problem with erlang:get_stacktrace/0 is that it can keep huge
terms in a process for an indefinite time after an exception. The
stacktrace can be huge after a 'function_clause' exception or a failed
call to a BIF or operator, because the arguments for the call will be
included in the stacktrace. For example:
1> catch abs(lists:seq(1, 1000)).
{'EXIT',{badarg,[{erlang,abs,
[[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20|...]],
[]},
{erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,674}]},
{erl_eval,expr,5,[{file,"erl_eval.erl"},{line,431}]},
{shell,exprs,7,[{file,"shell.erl"},{line,687}]},
{shell,eval_exprs,7,[{file,"shell.erl"},{line,642}]},
{shell,eval_loop,3,[{file,"shell.erl"},{line,627}]}]}}
2> erlang:get_stacktrace().
[{erlang,abs,
[[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,
23,24|...]],
[]},
{erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,674}]},
{erl_eval,expr,5,[{file,"erl_eval.erl"},{line,431}]},
{shell,exprs,7,[{file,"shell.erl"},{line,687}]},
{shell,eval_exprs,7,[{file,"shell.erl"},{line,642}]},
{shell,eval_loop,3,[{file,"shell.erl"},{line,627}]}]
3>
We can extend the syntax for clauses in try/catch to optionally bind
the stacktrace to a variable.
Here is an example using the current syntax:
try
Expr
catch C:E ->
Stk = erlang:get_stacktrace(),
.
.
.
In the new syntax, it would look like:
try
Expr
catch
C:E:Stk ->
.
.
.
Only a variable (not a pattern) is allowed in the stacktrace position,
to discourage matching of the stacktrace. (Matching would also be
expensive, because the raw format of the stacktrace would have to be
converted to the cooked form before matching.)
Note that:
try
Expr
catch E ->
.
.
.
is a shorthand for:
try
Expr
catch throw:E ->
.
.
.
If the stacktrace is to be retrieved for a throw, the 'throw:'
prefix must be explicitly included:
try
Expr
catch throw:E:Stk ->
.
.
.
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X register 0 used to be mapped to a hardware register, and therefore
faster than the other registers. Because of that, the compiler
tried to use x(0) as much as possible as a temporary register.
That was changed a few releases ago. X register 0 is now placed
in the array of all X registers and has no special speed
advantage compared to the other registers.
Remove the code in the compiler that attempts to use x(0) as
much as possible. As a result, the following type of instruction
will be much less frequent:
{put_list,Src,{x,0},{x,0}}
Instead, the following type of instruction will be more frequent:
{put_list,Src,{x,X},{x,X}}
(Where X is an arbitrary X register.)
Update the runtime system to specialize that kind of put_list
instruction.
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When a ref is created before performing a receive that will only
receive message containing that ref, there is a compiler optimization
to avoid scanning messages that can't possible contain the newly
created ref.
Magnus Lång pointed out that the implementation of the optimization
is flawed. Exceptions or recursive calls could cause the receive
operation to scan the receive queue from a position beyond the expected
message (that is, the message containing the ref would never be
matched out). See the receive_opt_exception/1 and receive_opt_recursion/1
test cases in receive_SUITE.
It turns out that we can simplify the implementation of the
optimization while fixing the bug (suggested by Magnus Lång). We
actually don't need the c_p->msg.mark field. It is enough to have
c_p->msg.saved_pos; if it is non-zero, it is a valid position in the
message qeueue. All we need to do is to ensure that we clear
c_p->msg.saved_pos when a receive is exited normally or abnormally.
We can clear c_p->msg.saved_pos in JOIN_MESSAGE(), since it is called
both when leaving a receive because a message matched and because there
was a timeout and the 'after' clause was executed. In addition, we
need to clear c_p->msg.saved_pos when an exception is caught.
https://bugs.erlang.org/browse/ERL-511
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Don't allow defining an specific operation more than once with
the exact same operands. Don't allow a specific operation to be
defined with different arities.
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The try_end and try_case instructions are implemented the same way
(try_case is translated to try_end by the loader).
We can do better than that. We know that try_case will only be executed
when an exception has been caught. Therefore, we know that x(0) is
the non-value and that x(1) through x(3) need to be shifted down to
x(0) through x(2). There is no need to test x(0) before shifting down.
try_end does not need the register shifting code at all.
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Introduce a syntax to mark an operand that is not always used when
an instrution is executed. Example of such operands are the fail
label for is_nil or the number of live registers for an
allocate instruction.
Use a question mark to annotate optional use:
is_nil f? xy
allocate t t?
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All other instructions that increment the stack pointer takes a 'Q'
operand.
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* bjorn/erts/relative-jumps:
Pack failure labels in i_select_val2 and i_select_tuple_arity2
Optimize i_select_tuple_arity2 and is_select_lins
Rewrite select_val_bins so that its labels can be packed
Pack sequences of trailing 'f' operands
Implement packing of 'f' and 'j'
Make sure that mask literals are 64 bits
Use relative failure labels
Add information about offset to common group start position
Remove JUMP_OFFSET
Refactor instructions to support relative jumps
Introduce a new trace_jump/1 instruction for tracing
Avoid using $Src more than once
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* bjorn/erts/improve-beam-ops:
Add built-in macros $ARG_POSITION() and $IS_PACKED()
Use 't' instead of 'I' bit syntax operands
Optimize operand type for match context in i_bs_get_integer
Change operand type from 's' to 'S' for a few instructions
Use the correct name of the parameter
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Optimise equality comparisons
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The number of live registers, unit and flags, and the number
of slots all fit comortably in 16 bits. This change will give
more opportunities for packing.
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The match context is always in an X register. Change the 's' operand
to 'x'.
While we are it, also change the operands for Live and FlagsAndUnit
to 't' (they will both fit comfortably in 16 bits).
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'S' is slightly more efficient. Using 'S' may also enable more
packing.
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