Teach BasicAA about the LLVM IR rules that allow reading past the end of an object given sufficient alignment. Fixes PR12098.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151553 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 26 insertions, 9 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 4a55c219cb..23b89ddf75 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -84,42 +84,59 @@ static bool isEscapeSource(const Value *V) {
 
 /// getObjectSize - Return the size of the object specified by V, or
 /// UnknownSize if unknown.
-static uint64_t getObjectSize(const Value *V, const TargetData &TD) {
+static uint64_t getObjectSize(const Value *V, const TargetData &TD,
+                              bool RoundToAlign = false) {
   Type *AccessTy;
+  unsigned Align;
   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
     if (!GV->hasDefinitiveInitializer())
       return AliasAnalysis::UnknownSize;
     AccessTy = GV->getType()->getElementType();
+    Align = GV->getAlignment();
   } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
     if (!AI->isArrayAllocation())
       AccessTy = AI->getType()->getElementType();
     else
       return AliasAnalysis::UnknownSize;
+    Align = AI->getAlignment();
   } else if (const CallInst* CI = extractMallocCall(V)) {
-    if (!isArrayMalloc(V, &TD))
+    if (!RoundToAlign && !isArrayMalloc(V, &TD))
       // The size is the argument to the malloc call.
       if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
         return C->getZExtValue();
     return AliasAnalysis::UnknownSize;
   } else if (const Argument *A = dyn_cast<Argument>(V)) {
-    if (A->hasByValAttr())
+    if (A->hasByValAttr()) {
       AccessTy = cast<PointerType>(A->getType())->getElementType();
-    else
+      Align = A->getParamAlignment();
+    } else {
       return AliasAnalysis::UnknownSize;
+    }
   } else {
     return AliasAnalysis::UnknownSize;
   }
-  
-  if (AccessTy->isSized())
-    return TD.getTypeAllocSize(AccessTy);
-  return AliasAnalysis::UnknownSize;
+
+  if (!AccessTy->isSized())
+    return AliasAnalysis::UnknownSize;
+
+  uint64_t Size = TD.getTypeAllocSize(AccessTy);
+  if (RoundToAlign) {
+    if (!Align)
+      return AliasAnalysis::UnknownSize;
+    Size = RoundUpToAlignment(Size, Align);
+  }
+
+  return Size;
 }
 
 /// isObjectSmallerThan - Return true if we can prove that the object specified
 /// by V is smaller than Size.
 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
                                 const TargetData &TD) {
-  uint64_t ObjectSize = getObjectSize(V, TD);
+  // This function needs to use the aligned object size because we allow
+  // reads a bit past the end given sufficient alignment.
+  uint64_t ObjectSize = getObjectSize(V, TD, /*RoundToAlign*/true);
+  
   return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
 }