relying on static_assert to test constexpr changes

[Martin Sebor]

On 04/28/2016 03:54 PM, Andrew Pinski wrote:
> On Thu, Apr 28, 2016 at 2:49 PM, Martin Sebor <msebor@gmail.com> wrote:
>> Many GCC tests for constexpr rely on static_assert to verify things
>> work correctly.  While testing some changes of my own, I was surprised
>> to find the static_asserts pass even though my changes did not (see
>> below).  It took me a while to realize that, and it took printing
>> the computed constant values via printf() to see they were wrong.
>> Even a runtime assert() didn't uncover the bug (see below).  After
>> thinking about it a bit, it makes sense that using the tool under
>> test to verify its own correctness isn't the most reliable way to
>> do it.  I've seen it before when testing other software, but it was
>> eye opening none-the-less to see it happen with a compiler.  Enough
>> that I think it's worthwhile to share it here.
>>
>> Martin
>>
>> $ cat builtin_constexpr.cpp && /home/msebor/build/gcc-70507/gcc/xgcc
>> -B/home/msebor/build/gcc-70507/gcc -Wall -Wextra -Wpedantic
>> builtin_constexpr.cpp && ./a.out
>> constexpr int f (int x, int y)
>> {
>>      int z = 0;
>>      __builtin_add_overflow (x, y, &z);
>>      return z;
>> }
>>
>> constexpr int z = f (12, 34);
>>
>> int main ()
>> {
>>    static_assert (z == (123 + 456), "z == (123 + 456)");   // passes
>>
>>    __builtin_printf ("z = %i\n", z);
>>
>>    if (z != (123 + 456))   // passes too
>>      __builtin_abort ();
>> }
>>
>> z = 0   << yet the output is zero!
>
> How about this doing something like this:
>
> int main ()
> {
>    int z1 = *(volatile int*)&z;
>
>    if (z1 != (123 + 456))   // passes too
>      __builtin_abort ();
> }
>
> Does the above pass now?

Yes, I think that should work.  I'm hoping for a solution that
doesn't require linking and running the tests.  The approach
you suggest and scanning the tree dump(s) for abort might be
the way to go.

Martin