Sanjay Patel 682f0b366b [InstCombine] use select-of-constants with set/clear bit mask patterns ...

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

Differential Revision: https://reviews.llvm.org/D78880

2020-05-03 09:44:43 -04:00

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