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

[Martin Sebor]

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