baby steps toward fixing some problems with inbound GEPs that overflow, as discussed 2 months ago or so.

Make sure we do not emit index computations with NSW flags so that we dont get an undef value if the GEP overflows

llvm-svn: 160589

llvm/include/llvm/Transforms/Utils/Local.h

@@ -168,15 +168,18 @@ static inline unsigned getKnownAlignment(Value *V, const TargetData *TD = 0) {
 /// EmitGEPOffset - Given a getelementptr instruction/constantexpr, emit the
 /// code necessary to compute the offset from the base pointer (without adding
 /// in the base pointer).  Return the result as a signed integer of intptr size.
+/// When NoAssumptions is true, no assumptions about index computation not
+/// overflowing is made.
 template<typename IRBuilderTy>
-Value *EmitGEPOffset(IRBuilderTy *Builder, const TargetData &TD, User *GEP) {
+Value *EmitGEPOffset(IRBuilderTy *Builder, const TargetData &TD, User *GEP,
+                     bool NoAssumptions = false) {
   gep_type_iterator GTI = gep_type_begin(GEP);
   Type *IntPtrTy = TD.getIntPtrType(GEP->getContext());
   Value *Result = Constant::getNullValue(IntPtrTy);
 
   // If the GEP is inbounds, we know that none of the addressing operations will
   // overflow in an unsigned sense.
-  bool isInBounds = cast<GEPOperator>(GEP)->isInBounds();
+  bool isInBounds = cast<GEPOperator>(GEP)->isInBounds() && !NoAssumptions;
 
   // Build a mask for high order bits.
   unsigned IntPtrWidth = TD.getPointerSizeInBits();

llvm/lib/Analysis/MemoryBuiltins.cpp

@@ -645,7 +645,7 @@ ObjectSizeOffsetEvaluator::visitGEPOperator(GEPOperator &GEP) {
   if (!bothKnown(PtrData))
     return unknown();
 
-  Value *Offset = EmitGEPOffset(&Builder, *TD, &GEP);
+  Value *Offset = EmitGEPOffset(&Builder, *TD, &GEP, /*NoAssumptions=*/true);
   Offset = Builder.CreateAdd(PtrData.second, Offset);
   return std::make_pair(PtrData.first, Offset);
 }

llvm/test/Instrumentation/BoundsChecking/simple.ll

@@ -116,3 +116,13 @@ define void @f11(i128* byval %x) nounwind {
   %3 = load i8* %2, align 4
   ret void
 }
+
+; CHECK: @f12
+define i64 @f12(i64 %x, i64 %y) nounwind {
+  %1 = tail call i8* @calloc(i64 1, i64 %x)
+; CHECK: mul i64 %y, 8
+  %2 = bitcast i8* %1 to i64*
+  %3 = getelementptr inbounds i64* %2, i64 %y
+  %4 = load i64* %3, align 8
+  ret i64 %4
+}