[PATCH] D132960: [InstCombine] Transform small unaligned memcmp calls used in zero equality tests

[Martin Sebor via Phabricator via llvm-commits]

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The `memcmp` transformation of calls used in equality tests with zero is restricted to arrays of a power-of-two alignment and size, even though the middle end is capable of emitting efficient code for arrays that don't satisfy this restriction.  As a result, some `memcmp` calls result in less than optimal code for strictly aligned targets even with the subsequent optimization in `ExpandMemCmp.cpp`.

The proposed change relaxes the restriction in the simplifier, letting even calls with unaligned arrays whose size is not a power of two expand to a series of loads followed by a single comparison (up to the maximum scalar register size).  This might come with a small increase in the size of the emitted code, but in my testing with targets like `mips64`, `riscv64` and `sparcv9` the increase didn't seem excessive.  The change might also results in slightly different code for targets that don't penalize unaligned accesses (because it preempts the transformation in `ExpandMemCmp.cpp`) but based on inspection the resulting object code appears comparably optimal.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D132960

Files:
  llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
  llvm/test/Transforms/InstCombine/memcmp-9.ll
  llvm/test/Transforms/InstCombine/memcmp-constant-fold.ll
  llvm/test/Transforms/InstCombine/simplify-libcalls.ll
  llvm/test/Transforms/InstCombine/strcmp-memcmp.ll

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