SimplifyInstruction does not imply DCE

We cannot remove an instruction with no uses just because
SimplifyInstruction succeeds.  It may have side effects.

llvm-svn: 273711

1 files changed, 9 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 6e1ac2c..d22f5c6 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1885,14 +1885,19 @@ static bool FoldCondBranchOnPHI(BranchInst *BI, const DataLayout &DL) {
 
       // Check for trivial simplification.
       if (Value *V = SimplifyInstruction(N, DL)) {
-        TranslateMap[&*BBI] = V;
-        delete N; // Instruction folded away, don't need actual inst
+        if (!BBI->use_empty())
+          TranslateMap[&*BBI] = V;
+        if (!N->mayHaveSideEffects()) {
+          delete N; // Instruction folded away, don't need actual inst
+          N = nullptr;
+        }
       } else {
-        // Insert the new instruction into its new home.
-        EdgeBB->getInstList().insert(InsertPt, N);
         if (!BBI->use_empty())
           TranslateMap[&*BBI] = N;
       }
+      // Insert the new instruction into its new home.
+      if (N)
+        EdgeBB->getInstList().insert(InsertPt, N);
     }
 
     // Loop over all of the edges from PredBB to BB, changing them to branch