[all-commits] [llvm/llvm-project] ec06b3: [InstCombine] canonicalize 'not' ops before logica...

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  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: ec06b381304140b2553cfdfae5a063f39c5c59ff
      https://github.com/llvm/llvm-project/commit/ec06b381304140b2553cfdfae5a063f39c5c59ff
  Author: Sanjay Patel <spatel at rotateright.com>
  Date:   2020-08-22 (Sat, 22 Aug 2020)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
    M llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
    M llvm/test/Transforms/InstCombine/2010-11-01-lshr-mask.ll
    M llvm/test/Transforms/InstCombine/and-xor-merge.ll
    M llvm/test/Transforms/InstCombine/compare-signs.ll
    M llvm/test/Transforms/InstCombine/icmp.ll
    M llvm/test/Transforms/InstCombine/xor.ll

  Log Message:
  -----------
  [InstCombine] canonicalize 'not' ops before logical shifts

This reverses the existing transform that would uniformly canonicalize any 'xor' after any shift. In the case of logical shifts, that turns a 'not' into an arbitrary 'xor' with constant, and that's probably not as good for analysis, SCEV, or codegen.

The SCEV motivating case is discussed in:
http://bugs.llvm.org/PR47136

There's an analysis motivating case at:
http://bugs.llvm.org/PR38781

I did draft a patch that would do the same for 'ashr' but that's questionable because it's just swapping the position of a 'not' and uncovers at least 2 missing folds that we would probably need to deal with as preliminary steps.

Alive proofs:
https://rise4fun.com/Alive/BBV

  Name: shift right of 'not'
  Pre: C2 == (-1 u>> C1)
  %a = lshr i8 %x, C1
  %r = xor i8 %a, C2
  =>
  %n = xor i8 %x, -1
  %r = lshr i8 %n, C1

  Name: shift left of 'not'
  Pre: C2 == (-1 << C1)
  %a = shl i8 %x, C1
  %r = xor i8 %a, C2
  =>
  %n = xor i8 %x, -1
  %r = shl i8 %n, C1

  Name: ashr of 'not'
  %a = ashr i8 %x, C1
  %r = xor i8 %a, -1
  =>
  %n = xor i8 %x, -1
  %r = ashr i8 %n, C1

Differential Revision: https://reviews.llvm.org/D86243