1 files changed, 13 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 96d9adc718b..000bad19427 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1333,6 +1333,15 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
     if (!Op1)
       return nullptr;
 
+    // Don't fold a GEP into itself through a PHI node. This can only happen
+    // through the back-edge of a loop. Folding a GEP into itself means that
+    // the value of the previous iteration needs to be stored in the meantime,
+    // thus requiring an additional register variable to be live, but not
+    // actually achieving anything (the GEP still needs to be executed once per
+    // loop iteration).
+    if (Op1 == &GEP)
+      return nullptr;
+
     signed DI = -1;
 
     for (auto I = PN->op_begin()+1, E = PN->op_end(); I !=E; ++I) {
@@ -1340,6 +1349,10 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
       if (!Op2 || Op1->getNumOperands() != Op2->getNumOperands())
         return nullptr;
 
+      // As for Op1 above, don't try to fold a GEP into itself.
+      if (Op2 == &GEP)
+        return nullptr;
+
       // Keep track of the type as we walk the GEP.
       Type *CurTy = Op1->getOperand(0)->getType()->getScalarType();