c++ - 如何在 clang++ 中禁用矢量化？

Question

考虑以下小型搜索功能:

template <uint32_t N>
int32_t countsearch(const uint32_t *base, uint32_t needle) {
    uint32_t count = 0;
    #pragma clang loop vectorize(disable)
    for (const uint32_t *probe = base; probe < base + N; probe++) {
        if (*probe < needle)
            count++;
    }
    return count;
}

在 -O2 或更高级别，clang vectorizes this search ，例如，。产生这样的代码(对于 10 个元素):

int countsearch<10u>(unsigned int const*, unsigned int):            # @int countsearch<10u>(unsigned int const*, unsigned int)
        vmovd   xmm0, esi
        vpbroadcastd    ymm0, xmm0
        vpbroadcastd    ymm1, dword ptr [rip + .LCPI0_0] # ymm1 = [2147483648,2147483648,2147483648,2147483648,2147483648,2147483648,2147483648,2147483648]
        vpxor   ymm2, ymm1, ymmword ptr [rdi]
        vpxor   ymm0, ymm0, ymm1
        vpcmpgtd        ymm0, ymm0, ymm2
        cmp     dword ptr [rdi + 32], esi
        vpsrld  ymm1, ymm0, 31
        vextracti128    xmm1, ymm1, 1
        vpsubd  ymm0, ymm1, ymm0
        vpshufd xmm1, xmm0, 78          # xmm1 = xmm0[2,3,0,1]
        vpaddd  ymm0, ymm0, ymm1
        vphaddd ymm0, ymm0, ymm0
        vmovd   eax, xmm0
        adc     eax, 0
        cmp     dword ptr [rdi + 36], esi
        adc     eax, 0
        vzeroupper
        ret

如何在命令行或在代码中使用 #pragma 禁用此矢量化？

我尝试了以下命令行参数，没有一个能阻止向量化:

-disable-loop-vectorization 
-disable-vectorization
-fno-vectorize 
-fno-tree-vectorize

我还尝试了 #pragma clang loop vectorize(disable) 如您在上面的代码中看到的那样，但没有成功。

Answer 1

关闭 SLP Vectorization :

clang++ -O2 -fno-slp-vectorize

Godbolt Link