[NFC][InstCombine] '(X & (- Y)) - X' -> '- (X & (Y - 1))' fold (PR44448)

Name: (X & (- Y)) - X  ->  - (X & (Y - 1))  (PR44448)
  %negy = sub i8 0, %y
  %unbiasedx = and i8 %negy, %x
  %r = sub i8 %unbiasedx, %x
=>
  %ymask = add i8 %y, -1
  %xmasked = and i8 %ymask, %x
  %r = sub i8 0, %xmasked

https://rise4fun.com/Alive/OIpla

This decreases use count of %x, may allow us to
later hoist said negation even further,
and results in marginally nicer X86 codegen.

See
  https://bugs.llvm.org/show_bug.cgi?id=44448
  https://reviews.llvm.org/D71499

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index ad42f3dabfe..33803aaf359 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1879,6 +1879,16 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
           Y, Builder.CreateNot(Op1, Op1->getName() + ".not"));
   }
 
+  {
+    // (sub (and Op1, (neg X)), Op1) --> neg (and Op1, (add X, -1))
+    Value *X;
+    if (match(Op0, m_OneUse(m_c_And(m_Specific(Op1),
+                                    m_OneUse(m_Neg(m_Value(X))))))) {
+      return BinaryOperator::CreateNeg(Builder.CreateAnd(
+          Op1, Builder.CreateAdd(X, Constant::getAllOnesValue(I.getType()))));
+    }
+  }
+
   if (Op1->hasOneUse()) {
     Value *X = nullptr, *Y = nullptr, *Z = nullptr;
     Constant *C = nullptr;