[InstCombine] morph an existing instruction instead of creating a new one

One potential way to make InstCombine (very slightly?) faster is to recycle instructions 
when possible instead of creating new ones. It's not explicitly stated AFAIK, but we don't
consider this an "InstSimplify". We could, however, make a new layer to house transforms 
like this if that makes InstCombine more manageable (just throwing out an idea; not sure 
how much opportunity is actually here).

Differential Revision: https://reviews.llvm.org/D31863

llvm-svn: 300067

1 files changed, 6 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index ea8c0a6d38b..99a983ab474 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2408,13 +2408,12 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
     }
   }
 
-  if (Constant *RHS = dyn_cast<Constant>(Op1)) {
-    if (RHS->isAllOnesValue() && Op0->hasOneUse())
-      // xor (cmp A, B), true = not (cmp A, B) = !cmp A, B
-      if (CmpInst *CI = dyn_cast<CmpInst>(Op0))
-        return CmpInst::Create(CI->getOpcode(),
-                               CI->getInversePredicate(),
-                               CI->getOperand(0), CI->getOperand(1));
+  // xor (cmp A, B), true = not (cmp A, B) = !cmp A, B
+  ICmpInst::Predicate Pred;
+  if (match(Op0, m_OneUse(m_Cmp(Pred, m_Value(), m_Value()))) &&
+      match(Op1, m_AllOnes())) {
+    cast<CmpInst>(Op0)->setPredicate(CmpInst::getInversePredicate(Pred));
+    return replaceInstUsesWith(I, Op0);
   }
 
   if (ConstantInt *RHSC = dyn_cast<ConstantInt>(Op1)) {