GVN: don't try to replace instruction with itself.

After some look-ahead PRE was added for GEPs, an instruction could end
up in the table of candidates before it was actually inspected. When
this happened the pass might decide it was the best candidate to
replace itself. This didn't go well.

Should fix PR25291

llvm-svn: 251145

1 files changed, 9 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index d91540cc26e..e2822e320a8 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2387,17 +2387,21 @@ bool GVN::processInstruction(Instruction *I) {
 
   // Perform fast-path value-number based elimination of values inherited from
   // dominators.
-  Value *repl = findLeader(I->getParent(), Num);
-  if (!repl) {
+  Value *Repl = findLeader(I->getParent(), Num);
+  if (!Repl) {
     // Failure, just remember this instance for future use.
     addToLeaderTable(Num, I, I->getParent());
     return false;
+  } else if (Repl == I) {
+    // If I was the result of a shortcut PRE, it might already be in the table
+    // and the best replacement for itself. Nothing to do.
+    return false;
   }
 
   // Remove it!
-  patchAndReplaceAllUsesWith(I, repl);
-  if (MD && repl->getType()->getScalarType()->isPointerTy())
-    MD->invalidateCachedPointerInfo(repl);
+  patchAndReplaceAllUsesWith(I, Repl);
+  if (MD && Repl->getType()->getScalarType()->isPointerTy())
+    MD->invalidateCachedPointerInfo(Repl);
   markInstructionForDeletion(I);
   return true;
 }