[Bug target/86393] GCC-8 appears to not detect AVX512 on iMac Pro 2018

[mrslevinsky at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86393

--- Comment #2 from Mikael Slevinsky <mrslevinsky at hotmail dot com> ---
I'm not so sure. Consider the following minimal test file:

#include <stdlib.h>
#include <stdio.h>

int main(void) {
        double * X, * Y, * Z;
        X = (double *) calloc(8, sizeof(double));
        Y = (double *) calloc(8, sizeof(double));
        Z = (double *) calloc(8, sizeof(double));

        for (int i = 0; i < 8; i++) {
                X[i] = (double) i;
                Y[i] = ((double) i)*((double) 2*i+1);
        }

        for (int i = 0; i < 8; i++)
                Z[i] = X[i] + Y[i];

        printf("\n");
        for (int i = 0; i < 8; i++) {
                printf("%1.2e  ", Z[i]);
        }
        printf("\n");

        return 0;
}


If I call it `testavx512.c`, then AVX512-like assembly is created, but it
errors when trying to create an executable:

dhcp-10-45-0-78:FastTransforms mikael$ gcc-8 -S -O3 -mavx512f testavx512.c -o
testavx512.s
dhcp-10-45-0-78:FastTransforms mikael$ gcc-8 -O3 -mavx512f testavx512.c -o
testavx512
/var/folders/dy/wbvgp8656tg6kwtz8sx_9z_m0000gn/T//cchgQrNi.s:45:2: error:
instruction requires: AVX-512 ISA
        vmovupd 0(%r13), %zmm0
        ^
/var/folders/dy/wbvgp8656tg6kwtz8sx_9z_m0000gn/T//cchgQrNi.s:46:2: error:
instruction requires: AVX-512 ISA
        vaddpd  (%r12), %zmm0, %zmm0
        ^
/var/folders/dy/wbvgp8656tg6kwtz8sx_9z_m0000gn/T//cchgQrNi.s:47:2: error:
instruction requires: AVX-512 ISA
        vmovupd %zmm0, (%rax)
        ^