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The type annotations inserted by beam_ssa_type and beam_ssa_bsm
would inadvertently disable stack trimming, as unknown instructions
are considered unsafe.
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Also rename a few functions in attempt to make it clearer.
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Recognize more safe labels to enable stack trimming in more
circumstances.
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Eliminate the use of beam_utils:is_not_used/3 by implementing a simple
is_not_used() function in beam_trim itself. The new version actually
makes trimming possible in more circumstances, because
beam_utils:is_not_used/3 was too conservative for the purpose of stack
trimming (it was previously used for optimizations where it was
necessary to be more conservative).
Alternatives considered: I tried to implement stack trimming in
beam_ssa_codegen but it turned out to be a total mess. Not
surprisingly, it turns out that an optimization that renumbers
Y registers is hard to do on an intermediate representation that
still use variables instead of BEAM registers.
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This will help investigation of compiler bugs.
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When beam_utils was first written, it did not have the functions
for testing whether a register was not used. Those were added
later, in sort of a hacky way.
Also, is_killed*() and is_not_used*() for Y registers would
return the same answer. Fix that to make the API more consistent
(an Y register can only be killed by a deallocate/1 instruction).
We will need to change beam_trim to call beam_utils:is_not_used/3
instead of beam_utils:is_killed/3.
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Somewhat reduce the code bloat by eliminating special cases.
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Somewhat reduce code bloat.
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Rewrite the five binary creation instructions to a bs_init
instruction, in order to somewhat reduce code bloat.
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We can remove some code bloat by handling the special instructions
as BIF instructions in the optimization passes. Also note that
bs_utf*_size was not handled by beam_utils:check_liveness/3
(meaning the conservative answer instead of the correct answer
would be returned).
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Seven bs_put_* instructions can be combined into one generic bs_put
instruction to avoid some code bloat. That will also improve some
optimizations (such as beam_trim) that did not handle all bs_put*
variants.
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