[Bug tree-optimization/126856] New: [17 Regression] mpfr and mpc started failing tests recently

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126856

            Bug ID: 126856
           Summary: [17 Regression] mpfr and mpc started failing tests
                    recently
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
  Target Milestone: ---

Initially observed as mpfr test failure tset_d on mpfr-4.2.2 at
r17-3267-g7f549ea2b47a70, did not bisect. Extracted the example from
src/scale2.c
(https://gitlab.inria.fr/mpfr/mpfr/-/blob/4.2.2/src/scale2.c?ref_type=tags#L30)
as:

// $ cat bug.c
union mpfr_ieee_double_extract
{
  struct
    {
      unsigned int manl:32;
      unsigned int manh:20;
      unsigned int exp:11;
      unsigned int sig:1;
    } s;
  double d;
};

__attribute__((optimize(2)))
static double
mpfr_scale2_bad (double d, int exp)
{
    union mpfr_ieee_double_extract x;
    (! __builtin_constant_p (!!(-1073 <= exp && exp <= 1025) || !(-1073 <= exp
&& exp <= 1025)) || (-1073 <= exp && exp <= 1025) ? (void) 0 :
__builtin_unreachable());
    x.d = d;
    if (exp < -1021) __builtin_trap();
    else x.s.exp += exp;
    return x.d;
}
__attribute__((optimize(0)))
static double
mpfr_scale2_ok (double d, int exp)
{
    union mpfr_ieee_double_extract x;
    (! __builtin_constant_p (!!(-1073 <= exp && exp <= 1025) || !(-1073 <= exp
&& exp <= 1025)) || (-1073 <= exp && exp <= 1025) ? (void) 0 :
__builtin_unreachable());
    x.d = d;
    if (exp < -1021) __builtin_trap();
    else x.s.exp += exp;
    return x.d;
}
__attribute__((optimize(0)))
double
mpfr_scale2 (double d, int exp)
{
    d = 1.0;
    exp = 499;
    double o = mpfr_scale2_ok(d, exp);
    double b = mpfr_scale2_bad(d, exp);
    if (o != b) {
        __builtin_trap();
    }
    return b;
}
__attribute__((optimize(0)))
int main() {
    mpfr_scale2(1,1);
}

Crashing:

$ gcc/xgcc -Bgcc /mnt/archive/big/git/mpfr/bug.c -o bug && ./bug
Illegal instruction        (core dumped) ./bug

$ gcc/xgcc -Bgcc -v
Reading specs from gcc/specs
COLLECT_GCC=gcc/xgcc
COLLECT_LTO_WRAPPER=gcc/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /home/slyfox/dev/git/gcc/configure --disable-multilib
--disable-bootstrap --disable-lto --disable-libsanitizer --enable-languages=c
CFLAGS='-O1 -g0' CXXFLAGS='-O1 -g0' LDFLAGS='-O1 -g0'
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 17.0.0 20260813 (experimental) (GCC)