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When matching a map, the compiler could fail to generate code that
would initialize all stack slots (Y registers) properly. Here is a
general outline of code that *could* cause this problem:
foo(Key, Map) ->
Res = case Map of
#{Key := Val} ->
%% Do something with Val here.
.
.
.
#{} ->
[]
end,
%% The stack slot for Val might not have been initialized
%% here if the key was not present in the map.
.
.
.
%% Use Res.
.
.
.
The code generator would wrongly assume that the map matching would
always initialize the stack slot, and if nothing else happened to
force that stack slot to be initialized, it would remain
uninitialized, which would likely crash the runtime system at the next
garbage collection.
`beam_validator` is supposed to find these kind of problems, but a bug
in `beam_validator` prevented it from detecting this problem.
https://bugs.erlang.org/browse/ERL-1017
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The following Core Erlang code could not be loaded:
'f'/1 = fun (_1) ->
case <_1> of
<~{'foo':='foo'}~> when 'true' ->
_1
end
Loading would fail with the following message:
beam/beam_load.c(2314): Error loading function example:f/1: op i_get_map_element_hash p x a u x:
no specific operation found
https://bugs.erlang.org/browse/ERL-955
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This commit lets the type optimization pass work across functions,
tracking return and argument types to eliminate redundant tests.
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fd682dd3b1dc corrected label generation for 'or', but forgot to
remove the old incorrect clause (that can no longer be reached).
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Code generation for 'or' with {z,0} destination could generate duplicate
new labels. The bug was introduced in eb571f8951bd.
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There could be `allocate_zero` instructions where `allocate` would
suffice or superfluous `init` instructions because all possible
initializations of Y registers were not taken into account.
While at it, also add some more comments.
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Minor cleanups and bug fixes of the compiler
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This has been superseded by bs_get_tail/3. Note that it is NOT
removed from the emulator or beam_disasm, as old modules are still
legal.
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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 only caller of bif_to_test/3 is beam_ssa_codegen.
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Continuing the simplification of beam_flatten, move the optimization
that eliminates a test_heap instruction following a binary construction
by incorporating the allocation of the heap space into the bs_init*
instruction itself.
This change does not change the generated code in any way.
Also remove beam_utils:combine_heap_needs/2, because beam_flatten
was the last user of it.
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The purpose of beam_flatten is to eliminate the blocks created by
beam_block, but it also does a few optimizations because at the
time the optimizations were added, beam_flatten was the most
convenient place.
Move the optimization that places `{move,Something,{x,0}}` before
`call` instructions from beam_flatten to beam_ssa_codegen.
This change will very slightly improve compilation times, and it
will also apply the optimization in more places. In particular,
a `{move,Literal,{x,0}}` would never be moved passed a `trim`
instruction before this change. Now it will.
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We chose to refer to variables through their var_name() because we
anticipated the need to annotate them, but it turned out we didn't
really need that, and many things become a lot cleaner if the
entire #b_var{} is used to represent variables.
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Omitting `kill` instructions before BIFs that throw exceptions
will reduce the code size. This optimization supersedes the same
optimizations in beam_dead.
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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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v3_codegen is replaced by three new passes:
* beam_kernel_to_ssa which translates the Kernel Erlang format
to a new SSA-based intermediate format.
* beam_ssa_pre_codegen which prepares the SSA-based format
for code generation, including register allocation. Registers
are allocated using the linear scan algorithm.
* beam_ssa_codegen which generates BEAM assembly code from the
SSA-based format.
It easier and more effective to optimize the SSA-based format before X
and Y registers have been assigned. The current optimization passes
constantly have to make sure no "holes" in the X register assignments
are created (that is, that no X register becomes undefined that an
allocation instruction depends on).
This commit also introduces the following optimizations:
* Replacing of tuple matching of records with the is_tagged_tuple
instruction. (Replacing beam_record.)
* Sinking of get_tuple_element instructions to just before the first
use of the extracted values. As well as potentially avoiding
extracting tuple elements when they are not actually used on all
executions paths, this optimization could also reduce the number
values that will need to be stored in Y registers. (Similar to
beam_reorder, but more effective.)
* Live optimizations, removing the definition of a variable that is
not subsequently used (provided that the operation has no side
effects), as well strength reduction of binary matching by replacing
the extraction of value from a binary with a skip instruction. (Used
to be done by beam_block, beam_utils, and v3_codegen.)
* Removal of redundant bs_restore2 instructions. (Formerly done
by beam_bs.)
* Type-based optimizations across branches. More effective than
the old beam_type pass that only did type-based optimizations in
basic blocks.
* Optimization of floating point instructions. (Formerly done
by beam_type.)
* Optimization of receive statements to introduce recv_mark and
recv_set instructions. More effective with far fewer restrictions
on what instructions are allowed between creating the reference
and entering the receive statement.
* Common subexpression elimination. (Formerly done by beam_block.)
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