Fix an issue where GVN was performing the return slot optimization when it was

not safe.  This is fixed by more aggressively checking that the return slot is
not used elsewhere in the function.

llvm-svn: 47544

1 files changed, 26 insertions, 8 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 650612144ad..4b7e82c927e 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -752,6 +752,8 @@ namespace {
     bool iterateOnFunction(Function &F);
     Value* CollapsePhi(PHINode* p);
     bool isSafeReplacement(PHINode* p, Instruction* inst);
+    bool valueHasOnlyOneUseAfter(Value* val, MemCpyInst* use,
+                                 Instruction* cutoff);
   };
   
   char GVN::ID = 0;
@@ -1055,22 +1057,32 @@ bool GVN::processLoad(LoadInst* L,
   return deletedLoad;
 }
 
-/// isReturnSlotOptznProfitable - Determine if performing a return slot 
-/// fusion with the slot dest is profitable
-static bool isReturnSlotOptznProfitable(Value* dest, MemCpyInst* cpy) {
-  // We currently consider it profitable if dest is otherwise dead.
-  SmallVector<User*, 8> useList(dest->use_begin(), dest->use_end());
+/// valueHasOnlyOneUse - Returns true if a value has only one use after the
+/// cutoff that is in the current same block and is the same as the use
+/// parameter.
+bool GVN::valueHasOnlyOneUseAfter(Value* val, MemCpyInst* use,
+                                  Instruction* cutoff) {
+  DominatorTree& DT = getAnalysis<DominatorTree>();
+  
+  SmallVector<User*, 8> useList(val->use_begin(), val->use_end());
   while (!useList.empty()) {
     User* UI = useList.back();
     
+    
     if (isa<GetElementPtrInst>(UI) || isa<BitCastInst>(UI)) {
       useList.pop_back();
       for (User::use_iterator I = UI->use_begin(), E = UI->use_end();
            I != E; ++I)
         useList.push_back(*I);
-    } else if (UI == cpy)
+    } else if (UI == use) {
       useList.pop_back();
-    else
+    } else if (Instruction* inst = dyn_cast<Instruction>(UI)) {
+      if (inst->getParent() == use->getParent() &&
+          (inst == cutoff || !DT.dominates(cutoff, inst))) {
+        useList.pop_back();
+      } else
+        return false;
+    } else
       return false;
   }
   
@@ -1123,8 +1135,14 @@ bool GVN::performReturnSlotOptzn(MemCpyInst* cpy, CallInst* C,
   if (TD.getTypeStoreSize(PT->getElementType()) != cpyLength->getZExtValue())
     return false;
   
+  // For safety, we must ensure that the output parameter of the call only has
+  // a single use, the memcpy.  Otherwise this can introduce an invalid
+  // transformation.
+  if (!valueHasOnlyOneUseAfter(CS.getArgument(0), cpy, C))
+    return false;
+  
   // We only perform the transformation if it will be profitable. 
-  if (!isReturnSlotOptznProfitable(cpyDest, cpy))
+  if (!valueHasOnlyOneUseAfter(cpyDest, cpy, C))
     return false;
   
   // In addition to knowing that the call does not access the return slot