[PATCH] D63993: [InstCombine] (Y + ~X) + 1 --> Y - X fold (PR42459)

[Sanjay Patel via Phabricator via llvm-commits]

spatel accepted this revision.
spatel added a comment.
This revision is now accepted and ready to land.

LGTM



================
Comment at: lib/Transforms/InstCombine/InstCombineAddSub.cpp:1207-1210
   // (A + 1) + ~B --> A - B
   // ~B + (A + 1) --> A - B
   if (match(&I, m_c_BinOp(m_Add(m_Value(A), m_One()), m_Not(m_Value(B)))))
     return BinaryOperator::CreateSub(A, B);
----------------
lebedev.ri wrote:
> spatel wrote:
> > Here's the sibling matcher. The -reassociation pass tries to create this form (move constant operands up in a chain of reassociative ops).
> Yep, found it once i understood that the fold *was* being done by instcombine after all (after adding commutative tests), and moved the fold here.
> Though i don't understand the `-print-after-all` output, `-reassociation` does not run there..
The descriptive text for -reassociation is "Reassociate expressions", so that might be obscuring your search, but I see the expected diff:
```
*** IR Dump After Simplify the CFG ***
; Function Attrs: norecurse nounwind readnone
define i32 @t0(i32 %x, i32 %y) local_unnamed_addr #0 {
  %o0 = xor i32 %x, -1
  %o1 = add i32 %o0, %y
  %o2 = add i32 %o1, 1
  ret i32 %o2
}
*** IR Dump After Reassociate expressions ***
; Function Attrs: norecurse nounwind readnone
define i32 @t0(i32 %x, i32 %y) local_unnamed_addr #0 {
  %o0 = xor i32 %x, -1
  %o1 = add i32 %o0, 1
  %o2 = add i32 %o1, %y
  ret i32 %o2
}

```


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D63993/new/

https://reviews.llvm.org/D63993