[PATCH] D38273: [X86] Don't select (cmp (and, imm), 0) to testw

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

X86ISelDAGToDAG tries to analyze ANDs compared with 0 to optimize to narrower immediates using subregisters.

I don't think we should be optimizing to 16-bit test instructions. It goes against our normal behavior of promoting i16 operations to i32. It only saves one byte due to the need to add a 0x66 prefix. I think it would also be subject to a length changing prefix penalty in the decoders on Intel CPUs.


https://reviews.llvm.org/D38273

Files:
  lib/Target/X86/X86ISelDAGToDAG.cpp
  test/CodeGen/X86/tbm_patterns.ll
  test/CodeGen/X86/test-shrink-bug.ll
  test/CodeGen/X86/test-shrink.ll

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