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BEAM has had a `swap` instruction for several releases, but it was not
known to the compiler. The loader would translate a sequence of three
`move` instructions to the `swap` instructions, but only when it was
possible to determine that it would be safe.
By making `swap` known to the compiler, it can be applied in more
situations since it is easier for the compiler than for the loader
to ensure that the usage is safe, and the loader shenanigans can be
eliminated.
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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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