1 files changed, 15 insertions, 1 deletions
diff --git a/lib/Transforms/Scalar/EarlyCSE.cpp b/lib/Transforms/Scalar/EarlyCSE.cpp
index 7fd77a082b82..c5c9b2c185d6 100644
--- a/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -562,13 +562,27 @@ bool EarlyCSE::isSameMemGeneration(unsigned EarlierGeneration,
   if (!MSSA)
     return false;
 
+  // If MemorySSA has determined that one of EarlierInst or LaterInst does not
+  // read/write memory, then we can safely return true here.
+  // FIXME: We could be more aggressive when checking doesNotAccessMemory(),
+  // onlyReadsMemory(), mayReadFromMemory(), and mayWriteToMemory() in this pass
+  // by also checking the MemorySSA MemoryAccess on the instruction.  Initial
+  // experiments suggest this isn't worthwhile, at least for C/C++ code compiled
+  // with the default optimization pipeline.
+  auto *EarlierMA = MSSA->getMemoryAccess(EarlierInst);
+  if (!EarlierMA)
+    return true;
+  auto *LaterMA = MSSA->getMemoryAccess(LaterInst);
+  if (!LaterMA)
+    return true;
+
   // Since we know LaterDef dominates LaterInst and EarlierInst dominates
   // LaterInst, if LaterDef dominates EarlierInst then it can't occur between
   // EarlierInst and LaterInst and neither can any other write that potentially
   // clobbers LaterInst.
   MemoryAccess *LaterDef =
       MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
-  return MSSA->dominates(LaterDef, MSSA->getMemoryAccess(EarlierInst));
+  return MSSA->dominates(LaterDef, EarlierMA);
 }
 
 bool EarlyCSE::processNode(DomTreeNode *Node) {