Attachment 249266 Details for Bug 277783 – Example reproducing the fma() zero sign issue

Example reproducing the fma() zero sign issue

x.c (text/plain), 1.01 KB, created by Victor Stinner on 2024-03-18 13:04:09 UTC

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Creator: Victor Stinner

Created: 2024-03-18 13:04:09 UTC

Size: 1.01 KB

patch

obsolete

>#include <math.h>
>#include <string.h>
>#include <stdio.h>
>
>void set_double(double *x, const char *bytes)
>{
>    memcpy(x, bytes, sizeof(*x));
>}
>
>int main(int argc, char **argv)
>{
>    double x = 0; //float.fromhex('0x1p-500')
>    double y = 0; //float.fromhex('0x1p-550')
>    double z = 0; //float.fromhex('0x1p-1000')
>#if 1
>    // condition always true, but trick the compiler, otherwise it will not
>    // call fma() at runtime, but just compute the result at build time.
>    if (argc) {
>        set_double(&x, "\x00\x00\x00\x00\x00\x00\xb0\x20");
>    }
>    else {
>        x = 123;
>    }
>#else
>    set_double(&x, "\x00\x00\x00\x00\x00\x00\xb0\x20");
>#endif
>    set_double(&y, "\x00\x00\x00\x00\x00\x00\x90\x1d");
>    set_double(&z, "\x00\x00\x00\x00\x00\x00p\x01");
>
>    printf("x %g\n", x);
>    printf("y %g\n", y);
>    printf("z %g\n", z);
>
>    double a, b, c, r;
>    a = x-y;
>    b = x+y;
>    c = -z;
>    r = fma(a, b, c);
>    printf("fma(x-y, x+y, -z):\n");
>    printf("fma(%+g, %+g, %+g) = %+g\n", a, b, c, r);
>    return 0;
>}

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Attachments on bug 277783: 249266 | 251421 | 251488 | 251489