[Bug c/86835] New: [8/9 Regression] Bogus "is used uninitialized" warning with -ffast-math

["bunk at stusta dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86835

            Bug ID: 86835
           Summary: [8/9 Regression] Bogus "is used uninitialized" warning
                    with -ffast-math
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bunk at stusta dot de
  Target Milestone: ---

From https://bugs.debian.org/897876

$ cat test.c
#include <math.h>

void
evallogisticML(const double *x, const int n,
               double *params, double *fvec, double **fjac)
{
    int            i;
    double         a, b, amxb, /* exp_xmab,*/ cosh_amxb, nd, xi, diff;

    a = params[0];
    b = params[1];
    nd = (double) n;

    fvec[0] = fvec[1] = fjac[0][0] = fjac[1][0] = fjac[0][1] = fjac[1][1] =
0.0;
    for (i = 0; i < n; ++i)
    {
        xi = x[i];
        /* xmab = (x[i] - a) / b; */
        diff = a - xi;
        amxb = diff / b;
        cosh_amxb = cosh(amxb);

        fvec[0] += tanh(amxb/2.0); /* good */
        fvec[1] += diff * tanh(amxb/2.0); /* good */

        fjac[0][0] += (1.0 / (1.0 + cosh(amxb))); /* good */
        fjac[0][1] += (diff / (1.0 + cosh_amxb)); /* good */
        fjac[1][0] += (diff + b * sinh(amxb)) / (1.0 + cosh_amxb); /* good */
        fjac[1][1] += diff * diff / (1.0 + cosh_amxb); /* good */
    }

    fvec[0] = fvec[0]; /* good */
    fvec[1] -= (b * nd); /* good */

    fjac[0][0] /= b; /* good */
    fjac[0][1] /= b*b; /* good */
    fjac[1][0] /= b; /* good */
    fjac[1][1] = -fjac[1][1]/(b*b) - nd; /* good */
}
$ gcc-7 -O1 -ffast-math -Wall -c test.c
$ gcc-8 -O1 -ffast-math -Wall -c test.c
test.c: In function 'evallogisticML':
test.c:4:1: warning: 'reciptmp.6' is used uninitialized in this function
[-Wuninitialized]
 evallogisticML(const double *x, const int n,
 ^~~~~~~~~~~~~~
$ /usr/lib/gcc-snapshot/bin/gcc -O1 -ffast-math -Wall -c test.c
test.c: In function 'evallogisticML':
test.c:4:1: warning: 'reciptmp.6' is used uninitialized in this function
[-Wuninitialized]
 evallogisticML(const double *x, const int n,
 ^~~~~~~~~~~~~~
$ gcc-8 --version
gcc-8 (Debian 8.2.0-2) 8.2.0
...
$ /usr/lib/gcc-snapshot/bin/gcc --version
gcc (Debian 20180721-1) 9.0.0 20180721 (experimental) [trunk revision 262917]


8.2.0-2 is r263045 from the gcc-8-branch.