[NFC][CodeGen][UBSan] EmitCheckedInBoundsGEP(): add some comments to pointer-overflow check
It's rather eye-twiching, some comments may help here..
This was originally part of https://reviews.llvm.org/D67122
llvm-svn: 371206
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 3d082de..1221527 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -4643,19 +4643,29 @@
llvm::Value *ValidGEP;
auto *NoOffsetOverflow = Builder.CreateNot(OffsetOverflows);
if (SignedIndices) {
+ // GEP is computed as `unsigned base + signed offset`, therefore:
+ // * If offset was positive, then the computed pointer can not be
+ // [unsigned] less than the base pointer, unless it overflowed.
+ // * If offset was negative, then the computed pointer can not be
+ // [unsigned] greater than the bas pointere, unless it overflowed.
auto *PosOrZeroValid = Builder.CreateICmpUGE(ComputedGEP, IntPtr);
auto *PosOrZeroOffset = Builder.CreateICmpSGE(TotalOffset, Zero);
llvm::Value *NegValid = Builder.CreateICmpULT(ComputedGEP, IntPtr);
- ValidGEP = Builder.CreateAnd(
- Builder.CreateSelect(PosOrZeroOffset, PosOrZeroValid, NegValid),
- NoOffsetOverflow);
- } else if (!SignedIndices && !IsSubtraction) {
- auto *PosOrZeroValid = Builder.CreateICmpUGE(ComputedGEP, IntPtr);
- ValidGEP = Builder.CreateAnd(PosOrZeroValid, NoOffsetOverflow);
+ ValidGEP = Builder.CreateSelect(PosOrZeroOffset, PosOrZeroValid, NegValid);
+ } else if (!IsSubtraction) {
+ // GEP is computed as `unsigned base + unsigned offset`, therefore the
+ // computed pointer can not be [unsigned] less than base pointer,
+ // unless there was an overflow.
+ // Equivalent to `@llvm.uadd.with.overflow(%base, %offset)`.
+ ValidGEP = Builder.CreateICmpUGE(ComputedGEP, IntPtr);
} else {
- auto *NegOrZeroValid = Builder.CreateICmpULE(ComputedGEP, IntPtr);
- ValidGEP = Builder.CreateAnd(NegOrZeroValid, NoOffsetOverflow);
+ // GEP is computed as `unsigned base - unsigned offset`, therefore the
+ // computed pointer can not be [unsigned] greater than base pointer,
+ // unless there was an overflow.
+ // Equivalent to `@llvm.usub.with.overflow(%base, sub(0, %offset))`.
+ ValidGEP = Builder.CreateICmpULE(ComputedGEP, IntPtr);
}
+ ValidGEP = Builder.CreateAnd(ValidGEP, NoOffsetOverflow);
llvm::Constant *StaticArgs[] = {EmitCheckSourceLocation(Loc)};
// Pass the computed GEP to the runtime to avoid emitting poisoned arguments.