[cfe-dev] Integer overflow checking

[Eli Friedman]

2009/5/16 Martin Doucha <next_ghost at quick.cz>:
> +    if (CGF.getContext().getLangOptions().OverflowChecking &&
> ValTy->isIntegerType()) {
> +      std::string e = isInc ? "Integer increment overflow" : "Integer
> decrement overflow";
> +      unsigned IID = ValTy->isSignedIntegerType() ?
> +        llvm::Intrinsic::sadd_with_overflow :
> llvm::Intrinsic::uadd_with_overflow;
> +      PresumedLoc loc =
> CGF.getContext().getSourceManager().getPresumedLoc(E->getOperatorLoc());
> +      BinOpInfo Ops;
> +      Ops.Ty = ValTy;
> +      Ops.LHS = InVal;
> +      Ops.RHS = NextVal;
> +      NextVal = EmitOverflowInst(IID, e, loc, Ops);
> +    } else {
> +      NextVal = Builder.CreateAdd(InVal, NextVal, isInc ? "inc" : "dec");
> +    }
>   }

This is wrong: one, unsigned "overflow" is well-defined, and two,
CodeGen takes some shortcuts with inc/dec, so this will incorrectly
conclude that "short x = 0x7FFF; ++x;" overflows.

-Eli