[objc-arc] Track if we encountered an additive overflow while computing {TopDown,BottomUp}PathCounts and do nothing if it occured.

rdar://14590914

llvm-svn: 187941

lib/Transforms/ObjCARC/ObjCARCOpts.cpp

@@ -648,6 +648,8 @@ PtrState::Merge(const PtrState &Other, bool TopDown) {
 namespace {
   /// \brief Per-BasicBlock state.
   class BBState {
+    static const unsigned OverflowOccurredValue;
+
     /// The number of unique control paths from the entry which can reach this
     /// block.
     unsigned TopDownPathCount;
@@ -674,7 +676,7 @@ namespace {
     SmallVector<BasicBlock *, 2> Succs;
 
   public:
-    BBState() : TopDownPathCount(0), BottomUpPathCount(0) {}
+    BBState() : TopDownPathCount(0), BottomUpPathCount(0) { }
 
     typedef MapTy::iterator ptr_iterator;
     typedef MapTy::const_iterator ptr_const_iterator;
@@ -745,8 +747,9 @@ namespace {
     /// Returns true if overflow occured. Returns false if overflow did not
     /// occur.
     bool GetAllPathCountWithOverflow(unsigned &PathCount) const {
-      assert(TopDownPathCount != 0);
+      if (TopDownPathCount == OverflowOccurredValue ||
-      assert(BottomUpPathCount != 0);
+          BottomUpPathCount == OverflowOccurredValue)
+        return false;
       unsigned long long Product =
         (unsigned long long)TopDownPathCount*BottomUpPathCount;
       PathCount = Product;
@@ -766,6 +769,8 @@ namespace {
 
     bool isExit() const { return Succs.empty(); }
   };
+
+  const unsigned BBState::OverflowOccurredValue = -1;
 }
 
 void BBState::InitFromPred(const BBState &Other) {
@@ -781,13 +786,25 @@ void BBState::InitFromSucc(const BBState &Other) {
 /// The top-down traversal uses this to merge information about predecessors to
 /// form the initial state for a new block.
 void BBState::MergePred(const BBState &Other) {
+  if (TopDownPathCount == OverflowOccurredValue)
+    return;
+
   // Other.TopDownPathCount can be 0, in which case it is either dead or a
   // loop backedge. Loop backedges are special.
   TopDownPathCount += Other.TopDownPathCount;
 
+  // In order to be consistent, we clear the top down pointers when by adding
+  // TopDownPathCount becomes OverflowOccurredValue even though "true" overflow
+  // has not occured.
+  if (TopDownPathCount == OverflowOccurredValue) {
+    clearTopDownPointers();
+    return;
+  }
+
   // Check for overflow. If we have overflow, fall back to conservative
   // behavior.
   if (TopDownPathCount < Other.TopDownPathCount) {
+    TopDownPathCount = OverflowOccurredValue;
     clearTopDownPointers();
     return;
   }
@@ -813,13 +830,25 @@ void BBState::MergePred(const BBState &Other) {
 /// The bottom-up traversal uses this to merge information about successors to
 /// form the initial state for a new block.
 void BBState::MergeSucc(const BBState &Other) {
+  if (BottomUpPathCount == OverflowOccurredValue)
+    return;
+
   // Other.BottomUpPathCount can be 0, in which case it is either dead or a
   // loop backedge. Loop backedges are special.
   BottomUpPathCount += Other.BottomUpPathCount;
 
+  // In order to be consistent, we clear the top down pointers when by adding
+  // BottomUpPathCount becomes OverflowOccurredValue even though "true" overflow
+  // has not occured.
+  if (BottomUpPathCount == OverflowOccurredValue) {
+    clearBottomUpPointers();
+    return;
+  }
+
   // Check for overflow. If we have overflow, fall back to conservative
   // behavior.
   if (BottomUpPathCount < Other.BottomUpPathCount) {
+    BottomUpPathCount = OverflowOccurredValue;
     clearBottomUpPointers();
     return;
   }