[PATCH] D78880: [InstCombine] use select-of-constants with set/clear bit mask patterns

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: lebedev.ri, xbolva00, dmgreen.
Herald added subscribers: hiraditya, mcrosier.
Herald added a project: LLVM.

Cond ? (X & ~C) : (X | C) --> (X & ~C) | (Cond ? 0 : C)
Cond ? (X | C) : (X & ~C) --> (X & ~C) | (Cond ? C : 0)

The select-of-constants form results in better codegen. There's an existing test diff that shows a transform that results in an extra IR instruction, but that's an existing problem.

This is motivated by code seen in LLVM itself - see PR37581:
https://bugs.llvm.org/show_bug.cgi?id=37581

  define i8 @src(i8 %x, i8 %C, i1 %b)  {
    %notC = xor i8 %C, -1
    %and = and i8 %x, %notC
    %or = or i8 %x, %C
    %cond = select i1 %b, i8 %or, i8 %and
    ret i8 %cond
  }
  
  define i8 @tgt(i8 %x, i8 %C, i1 %b)  {
    %notC = xor i8 %C, -1
    %and = and i8 %x, %notC
    %mul = select i1 %b, i8 %C, i8 0
    %or = or i8 %mul, %and
    ret i8 %or
  }

http://volta.cs.utah.edu:8080/z/Vt2WVm


https://reviews.llvm.org/D78880

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/cast.ll
  llvm/test/Transforms/InstCombine/select-with-bitwise-ops.ll

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