Author: lebedevri Date: Fri Sep 6 07:18:49 2019 New Revision: 371206 URL: http://llvm.org/viewvc/llvm-project?rev=371206&view=rev Log: [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 Modified: cfe/trunk/lib/CodeGen/CGExprScalar.cpp Modified: cfe/trunk/lib/CodeGen/CGExprScalar.cpp URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGExprScalar.cpp?rev=371206&r1=371205&r2=371206&view=diff ============================================================================== --- cfe/trunk/lib/CodeGen/CGExprScalar.cpp (original) +++ cfe/trunk/lib/CodeGen/CGExprScalar.cpp Fri Sep 6 07:18:49 2019 @@ -4643,19 +4643,29 @@ Value *CodeGenFunction::EmitCheckedInBou 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. _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
