[Bug middle-end/87276] New: Buggy code with -O2 in trunk revision 264170: MPFR test tstrtofr fails

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87276

            Bug ID: 87276
           Summary: Buggy code with -O2 in trunk revision 264170: MPFR
                    test tstrtofr fails
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

On Debian x86_64, I get the following failure with

gcc (Debian 20180908-1) 9.0.0 20180908 (experimental) [trunk revision 264170]

This is a regression, as there are no issues with either GCC 8 or with

gcc (Debian 20180822-1) 9.0.0 20180822 (experimental) [trunk revision 263760]

When building the MPFR trunk revision 13165 with

  ./configure --enable-assert=full CC=gcc-snapshot CFLAGS="-O2"

(no issues with -O or with -O2 -fsanitize=undefined -fno-sanitize-recover), I
get a failure in tstrtofr:

Check overflow failed (2) with:
 s=''
 x=0

I've modified MPFR's strtofr.c to the following (i.e. adding printf's):

[...]
      long read_exp = strtol (str + 1, &endptr, 10);
      if (endptr != str+1)
        str = endptr;
      sum =
        read_exp < MPFR_EXP_MIN ? (str = endptr, MPFR_EXP_MIN) :
        read_exp > MPFR_EXP_MAX ? (str = endptr, MPFR_EXP_MAX) :
        (mpfr_exp_t) read_exp;
      printf ("%ld %d\n", sum, res);
      printf ("%ld\n", pstr->exp_base);
      MPFR_SADD_OVERFLOW (sum, sum, pstr->exp_base,
                          mpfr_exp_t, mpfr_uexp_t,
                          MPFR_EXP_MIN, MPFR_EXP_MAX,
                          res = 2, res = 3);
      // printf ("%ld %d\n", sum, res);
[...]

and reduced tstrtofr.c to:

#include "mpfr-test.h"

static void
check_overflow (void)
{
  mpfr_t x;
  char *s;

  mpfr_init (x);

  mpfr_strtofr (x, "123456789E9223372036854775807", &s, 0, MPFR_RNDN);
  if (s[0] != 0 || !MPFR_IS_INF (x) || !MPFR_IS_POS (x) )
    {
      printf ("Check overflow failed (2) with:\n s='%s'\n x=", s);
      mpfr_dump (x);
      exit (1);
    }
  mpfr_clear (x);
}

int
main (int argc, char *argv[])
{
  tests_start_mpfr ();

  check_overflow ();

  tests_end_mpfr ();
  return 0;
}

I get the following failure:

9223372036854775807 1
9
Check overflow failed (2) with:
 s=''
 x=0

Then, if I uncomment the last printf line, the failure disappears, and I get:

9223372036854775807 1
9
9223372036854775807 2