Is there a way to auto-generate MMX instructions (not SSE) with gcc

Question

It appears gcc will happily auto-vectorize simple examples, and emit SSE instructions. Is there any way to emit MMX instructions only?

For example if I try the following example on Godbolt:

int sumint(int *arr) {
    int sum = 0;
    for (int i=0 ; i<2048 ; i++){
        sum += arr[i];
    }
    return sum;
}

compiling on GCC 9.2 with -mmmx -O3 -m32 -msse2, I get

sumint:
        mov     eax, DWORD PTR [esp+4]
        pxor    xmm0, xmm0
        lea     edx, [eax+8192]
.L2:
        movdqu  xmm2, XMMWORD PTR [eax]
        add     eax, 16
        paddd   xmm0, xmm2
        cmp     edx, eax
        jne     .L2
        movdqa  xmm1, xmm0
        psrldq  xmm1, 8
        paddd   xmm0, xmm1
        movdqa  xmm1, xmm0
        psrldq  xmm1, 4
        paddd   xmm0, xmm1
        movd    eax, xmm0
        ret

But without sse (i.e. -mmmx -O3 -m32 -mno-sse2), it falls back to only using general registers, and no mmx instructions:

sumint:
        mov     eax, DWORD PTR [esp+4]
        xor     edx, edx
        lea     ecx, [eax+8192]
.L2:
        add     edx, DWORD PTR [eax]
        add     eax, 4
        cmp     eax, ecx
        jne     .L2
        mov     eax, edx
        ret

I wanted to run some Benchmarks, comparing the effect of running with just x87-fpu, MMX, SSE and SSE2, but if gcc won't emit MMX instructions, then there won't be any difference between compiling for x87 and x87+mmx.

Answer 1

GCC can't autovectorize using MMX or 3DNow! because it lacks the ability to properly insert EMMS/FEMMS. You have to use ICC for MMX. See https://gcc.gnu.org/ml/gcc-patches/2004-12/msg01955.html