[PATCH] D136582: [InstCombine] fold `sub + and` pattern with specific const value

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

Please pre-commit the baseline tests, so we will see the diffs here.

I'm still not sure if we want to add some more general transforms in addition to or instead of this fold.

Should we have the subtract version of this patch for consistency?
D130080 <https://reviews.llvm.org/D130080>

In case it was not clear, that was a transform I suggested earlier. Depending where it is added, I think the patch would look something like this:

  diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  index c3dabc2d4a07..e7ae1a9be39c 100644
  --- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  +++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  @@ -2035,6 +2035,18 @@ Instruction *InstCombinerImpl::visitAnd(BinaryOperator &I) {
                                   ConstantInt::getNullValue(Ty));
       }
   
  +    // If all bits affected by a sub are included in a high-bit-mask, do the
  +    // mask op before the adjusted sub. Example:
  +    // (0x0f - X) & 0xf8 --> 0x08 - (X & 0xf8)
  +    const APInt *SubC;
  +    if (C->isNegatedPowerOf2() &&
  +        match(Op0, m_OneUse(m_Sub(m_APInt(SubC), m_Value(X)))) &&
  +        (~*C).isSubsetOf(*SubC)) {
  +      Value *NewAnd = Builder.CreateAnd(X, *C);
  +      Constant *NewSubC = ConstantInt::get(Ty, *C & *SubC);
  +      return BinaryOperator::CreateSub(NewSubC, NewAnd);
  +    }
  +
       Constant *C1, *C2;
       const APInt *C3 = C;
       Value *X;

I didn't see any immediate failures in regression tests with that patch applied.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136582/new/

https://reviews.llvm.org/D136582