将__m256i的前N位或后N位设置为1，其余为0的有效方法答案

【问题标题】：Efficient way to set first N or last N bits of __m256i to 1, the rest to 0将__m256i的前N位或后N位设置为1，其余为0的有效方法
【发布时间】：2017-09-03 15:16:36
【问题描述】：

如何使用 AVX2 有效地设置为 1

第一个N位
最后一个N位

来自__m256i，将其余设置为0？

这些是位范围的尾部和头部的 2 个独立操作，当范围可能在 __m256i 值的中间开始和结束时。占满 __m256i 值的范围部分使用 all-0 或 all-1 掩码处理。

【问题讨论】：

标签： c++ bit-manipulation vectorization x86-64 avx2

【解决方案1】：

AVX2 移位指令 vpsllvd 和 vpsrlvd 具有移位计数的好特性大于或等于 32 导致 ymm 寄存器中的整数为零。换句话说：班次计数没有被掩盖，相反到 x86 标量移位指令的移位计数。

因此代码相当简单：

/*
gcc -O3 -m64 -Wall -mavx2 -march=broadwell avx2_bit_mask.c
*/
#include <immintrin.h>
#include <stdio.h>

__m256i bit_mask_avx2_msb(unsigned int n)      
{           
    __m256i ones       = _mm256_set1_epi32(-1);
    __m256i cnst32_256 = _mm256_set_epi32(32,64,96,128, 160,192,224,256);

    __m256i shift      = _mm256_set1_epi32(n);   
            shift      = _mm256_subs_epu16(cnst32_256,shift);  
                  return _mm256_sllv_epi32(ones,shift);         
}


__m256i bit_mask_avx2_lsb(unsigned int n)               
{           
    __m256i ones       = _mm256_set1_epi32(-1);
    __m256i cnst32_256 = _mm256_set_epi32(256,224,192,160, 128,96,64,32);

    __m256i shift      = _mm256_set1_epi32(n);   
            shift      = _mm256_subs_epu16(cnst32_256,shift);  
                  return _mm256_srlv_epi32(ones,shift);
}


int print_avx2_hex(__m256i ymm)
{
    long unsigned int x[4];
        _mm256_storeu_si256((__m256i*)x,ymm);
        printf("%016lX %016lX %016lX %016lX\n", x[3],x[2],x[1],x[0]);

    return 0;
}


int main()
{
    unsigned int i;

    for (i=0;i<259;i++){
        printf("bit_mask_avx2_lsb(%3d) ",i);
        print_avx2_hex(bit_mask_avx2_lsb(i));
    }
    printf("\n");

    for (i=0;i<259;i++){
        printf("bit_mask_avx2_msb(%3d) ",i);
        print_avx2_hex(bit_mask_avx2_msb(i));
    }
    printf("\n");


    return 0;
}

结果是：

$ ./a.out
bit_mask_avx2_lsb(  0) 0000000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_lsb(  1) 0000000000000000 0000000000000000 0000000000000000 0000000000000001
bit_mask_avx2_lsb(  2) 0000000000000000 0000000000000000 0000000000000000 0000000000000003
bit_mask_avx2_lsb(  3) 0000000000000000 0000000000000000 0000000000000000 0000000000000007
bit_mask_avx2_lsb(  4) 0000000000000000 0000000000000000 0000000000000000 000000000000000F
bit_mask_avx2_lsb(  5) 0000000000000000 0000000000000000 0000000000000000 000000000000001F
bit_mask_avx2_lsb(  6) 0000000000000000 0000000000000000 0000000000000000 000000000000003F
bit_mask_avx2_lsb(  7) 0000000000000000 0000000000000000 0000000000000000 000000000000007F
bit_mask_avx2_lsb(  8) 0000000000000000 0000000000000000 0000000000000000 00000000000000FF
bit_mask_avx2_lsb(  9) 0000000000000000 0000000000000000 0000000000000000 00000000000001FF
bit_mask_avx2_lsb( 10) 0000000000000000 0000000000000000 0000000000000000 00000000000003FF
bit_mask_avx2_lsb( 11) 0000000000000000 0000000000000000 0000000000000000 00000000000007FF
...
bit_mask_avx2_lsb(124) 0000000000000000 0000000000000000 0FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(125) 0000000000000000 0000000000000000 1FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(126) 0000000000000000 0000000000000000 3FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(127) 0000000000000000 0000000000000000 7FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(128) 0000000000000000 0000000000000000 FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(129) 0000000000000000 0000000000000001 FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(130) 0000000000000000 0000000000000003 FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(131) 0000000000000000 0000000000000007 FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(132) 0000000000000000 000000000000000F FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
...
bit_mask_avx2_lsb(248) 00FFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(249) 01FFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(250) 03FFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(251) 07FFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(252) 0FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(253) 1FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(254) 3FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(255) 7FFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(256) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(257) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_lsb(258) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF



bit_mask_avx2_msb(  0) 0000000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  1) 8000000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  2) C000000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  3) E000000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  4) F000000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  5) F800000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  6) FC00000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  7) FE00000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  8) FF00000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb(  9) FF80000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb( 10) FFC0000000000000 0000000000000000 0000000000000000 0000000000000000
bit_mask_avx2_msb( 11) FFE0000000000000 0000000000000000 0000000000000000 0000000000000000
...
bit_mask_avx2_msb(124) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFF0 0000000000000000 0000000000000000
bit_mask_avx2_msb(125) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFF8 0000000000000000 0000000000000000
bit_mask_avx2_msb(126) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFC 0000000000000000 0000000000000000
bit_mask_avx2_msb(127) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFE 0000000000000000 0000000000000000
bit_mask_avx2_msb(128) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF 0000000000000000 0000000000000000
bit_mask_avx2_msb(129) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF 8000000000000000 0000000000000000
bit_mask_avx2_msb(130) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF C000000000000000 0000000000000000
bit_mask_avx2_msb(131) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF E000000000000000 0000000000000000
bit_mask_avx2_msb(132) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF F000000000000000 0000000000000000
...
bit_mask_avx2_msb(248) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFF00
bit_mask_avx2_msb(249) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFF80
bit_mask_avx2_msb(250) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFC0
bit_mask_avx2_msb(251) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFE0
bit_mask_avx2_msb(252) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFF0
bit_mask_avx2_msb(253) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFF8
bit_mask_avx2_msb(254) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFC
bit_mask_avx2_msb(255) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFE
bit_mask_avx2_msb(256) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_msb(257) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF
bit_mask_avx2_msb(258) FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFF

对于值n，256n_mm256_subs_epu16()的16位饱和算法。对于n=65536，位掩码（输出值）为零。可以修改代码，使所有位都设置为 1 对于 256nINT_MAX 的范围。这可以通过替换来实现 shift = _mm256_subs_epu16(cnst32_256,shift); 与

    __m256i mask       = _mm256_cmpgt_epi32(cnst32_256,shift);
            shift      = _mm256_sub_epi32(cnst32_256,shift);
            shift      = _mm256_and_si256(shift,mask);

这三个内在函数或多或少模仿了_mm256_subs_epu32(cnst32_256,shift)，但它并不存在。

【讨论】：

非常酷。这仅比适用于字节掩码（而不是位掩码）的滑动窗口未对齐加载技术稍微贵一点。在某些情况下甚至可能更可取。