Re: [PATCH] Fix minor nits in gimple-ssa-sprintf.c (PR tree-optimization/78586)

[Martin Sebor <msebor at gmail dot com>]

On 12/01/2016 09:24 AM, Martin Sebor wrote:
> > So, let's use another testcase, -O2 -W -Wall -fno-tree-vrp -fno-tree-ccp
> > and again UB in it:
> > volatile bool e;
> > volatile int x;
> >
> > int
> > main ()
> > {
> >   x = 123;
> >   *(char *)&e = x;
> >   bool f = e;
> >   x = __builtin_snprintf (0, 0, "%d", f);
> > }
> >
> > This will store 1 into x, while without -fprintf-return-value it would
> > store
> > 3.
>
> Great, that's what I was looking for.  I turned it into the following
> test case.  Let me try to massage it into a compile-only test suitable
> for the test suite and commit it later today.

I hadn't looked at the test case carefully enough when I said
the above.  Now that I have I see it fails even with your patch
with VRP enabled) and I don't think that's a problem in GCC but
rather in the test (the undefined behavior you mentioned).  So
I won't be adding it after all.  I don't think it's appropriate
to exercise behavior that we cannot or will not (or should not)
guarantee.  If you have a different test case does show
the problem under these constraints I'm still interested.

Martin

> volatile bool e;
> volatile int x;
>
> #define FMT "%d"
> const char *fmt = FMT;
>
> int
> main ()
> {
>   x = 123;
>   *(char *)&e = x;
>   bool f = e;
>
>   int n1 = __builtin_snprintf (0, 0, FMT, f);
>   int n2 = __builtin_snprintf (0, 0, fmt, f);
>
>   __builtin_printf ("%i == %i\n", n1, n2);
>   if (n1 != n2)
>     __builtin_abort ();
> }
>
> Martin