[Local] invertCondition: try modifying an existing ICmpInst

This avoids various cases where StructurizeCFG would otherwise insert an
xor i1 instruction, and it since it generally runs late in the pipeline,
instcombine does not clean up the xor-of-cmp pattern.

Differential Revision: https://reviews.llvm.org/D118478

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 9f33d2f..c457931 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -3343,6 +3343,14 @@ Value *llvm::invertCondition(Value *Condition) {
       if (I->getParent() == Parent && match(I, m_Not(m_Specific(Condition))))
         return I;
 
+  // Fourth: Modify an existing instruction
+  if (Condition->hasOneUse()) {
+    if (auto *CI = dyn_cast<CmpInst>(Condition)) {
+      CI->setPredicate(CI->getInversePredicate());
+      return Condition;
+    }
+  }
+
   // Last option: Create a new instruction
   auto *Inverted =
       BinaryOperator::CreateNot(Condition, Condition->getName() + ".inv");