[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")

[Sanjoy Das via llvm-dev]

On Mon, Feb 29, 2016 at 10:42 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
> On Feb 29, 2016, at 10:38 AM, Xinliang David Li via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
> ok thanks.  A more reduced test case can show different behavior between O2
> and O0.
>
> Say we have
>
> unsigned maybe_divide (unsigned *ptr) {
>    int flag = false;
>    unsigned val = 500/ptr[0];
>    if (flag)
>        return val;
>    return (unsigned)(intptr_t)ptr);
> }
>
> int main() {
>    unsigned g = 0;
>     return maybe_divide(&g);
> }
>
>
> At O2, it runs fine, but at O0 it core dumps.
>
>
> I believe this program has a divide by zero and is not correct.
> By luck the optimization removes the faulty instruction, which does not mean
> the program is well formed.
>
> IMO this is different from Sanjoy's example, where a wrong optimization
> introduces the error.

Yup; in my example there actually is no place where the program
divides by zero.  Both the call sites that call maybe_divide store a
non-zero value to ptr[0] before calling maybe_divide.

-- Sanjoy