1 files changed, 26 insertions, 28 deletions

diff --git a/lib/Transforms/Scalar/DeadStoreElimination.cpp b/lib/Transforms/Scalar/DeadStoreElimination.cpp
index 867a06ad202d..53e46400dca8 100644
--- a/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -340,24 +340,35 @@ static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
   // Okay, we have stores to two completely different pointers.  Try to
   // decompose the pointer into a "base + constant_offset" form.  If the base
   // pointers are equal, then we can reason about the two stores.
-  int64_t Off1 = 0, Off2 = 0;
-  const Value *BP1 = GetPointerBaseWithConstantOffset(P1, Off1, TD);
-  const Value *BP2 = GetPointerBaseWithConstantOffset(P2, Off2, TD);
+  int64_t EarlierOff = 0, LaterOff = 0;
+  const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
+  const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
   
   // If the base pointers still differ, we have two completely different stores.
   if (BP1 != BP2)
     return false;
-  
-  // Otherwise, we might have a situation like:
-  //  store i16 -> P + 1 Byte
-  //  store i32 -> P
-  // In this case, we see if the later store completely overlaps all bytes
-  // stored by the previous store.
-  if (Off1 < Off2 ||                       // Earlier starts before Later.
-      Off1+Earlier.Size > Off2+Later.Size) // Earlier goes beyond Later.
-    return false;
-  // Otherwise, we have complete overlap.
-  return true;
+
+  // The later store completely overlaps the earlier store if:
+  // 
+  // 1. Both start at the same offset and the later one's size is greater than
+  //    or equal to the earlier one's, or
+  //
+  //      |--earlier--|
+  //      |--   later   --|
+  //      
+  // 2. The earlier store has an offset greater than the later offset, but which
+  //    still lies completely within the later store.
+  //
+  //        |--earlier--|
+  //    |-----  later  ------|
+  //
+  // We have to be careful here as *Off is signed while *.Size is unsigned.
+  if (EarlierOff >= LaterOff &&
+      uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
+    return true;
+
+  // Otherwise, they don't completely overlap.
+  return false;
 }
 
 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
@@ -474,7 +485,7 @@ bool DSE::runOnBasicBlock(BasicBlock &BB) {
       // away the store and we bail out.  However, if we depend on on something
       // that overwrites the memory location we *can* potentially optimize it.
       //
-      // Find out what memory location the dependant instruction stores.
+      // Find out what memory location the dependent instruction stores.
       Instruction *DepWrite = InstDep.getInst();
       AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
       // If we didn't get a useful location, or if it isn't a size, bail out.
@@ -631,28 +642,15 @@ bool DSE::handleEndBlock(BasicBlock &BB) {
       if (AA->doesNotAccessMemory(CS))
         continue;
       
-      unsigned NumModRef = 0, NumOther = 0;
-      
       // If the call might load from any of our allocas, then any store above
       // the call is live.
       SmallVector<Value*, 8> LiveAllocas;
       for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
            E = DeadStackObjects.end(); I != E; ++I) {
-        // If we detect that our AA is imprecise, it's not worth it to scan the
-        // rest of the DeadPointers set.  Just assume that the AA will return
-        // ModRef for everything, and go ahead and bail out.
-        if (NumModRef >= 16 && NumOther == 0)
-          return MadeChange;
-
         // See if the call site touches it.
         AliasAnalysis::ModRefResult A = 
           AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
         
-        if (A == AliasAnalysis::ModRef)
-          ++NumModRef;
-        else
-          ++NumOther;
-        
         if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
           LiveAllocas.push_back(*I);
       }