[SCCP] Update condition to avoid overflow.

Summary:
Update condition to remove addition that may cause an overflow.
Resolves PR42814.

Reviewers: sanjoy, RKSimon

Subscribers: jlebar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65417

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367461 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ConstantFolding.cpp

@@ -544,7 +544,7 @@ Constant *FoldReinterpretLoadFromConstPtr(Constant *C, Type *LoadTy,
   int64_t InitializerSize = DL.getTypeAllocSize(GV->getInitializer()->getType());
 
   // If we're not accessing anything in this constant, the result is undefined.
-  if (Offset + BytesLoaded <= 0)
+  if (Offset <= -1 * static_cast<int64_t>(BytesLoaded))
     return UndefValue::get(IntType);
 
   // If we're not accessing anything in this constant, the result is undefined.

test/Transforms/SCCP/ubsan_overflow.ll

@@ -0,0 +1,13 @@
+; RUN: opt -sccp -S %s | FileCheck %s
+
+@0 = private unnamed_addr constant [16 x i8] c"\01\00\00\00\01\01\00\00\01\01\01\00\01\01\01\01"
+
+; CHECK-LABEL: @foo
+define i8 @foo() {
+entry:
+  %0 = add nuw nsw i64 0, -1
+  %1 = lshr i64 %0, 1
+  %2 = getelementptr inbounds [4 x [4 x i8]], [4 x [4 x i8]]* bitcast ([16 x i8]* @0 to [4 x [4 x i8]]*), i64 0, i64 0, i64 %1
+  %3 = load i8, i8* %2, align 1
+  ret i8 %3
+}