在SSE寄存器中設置最後或前n位

Question

如何創建一個，其中n最高有效位已設置（在整個向量中）？我需要這個來掩蓋與計算相關的緩衝區的部分。如果可能，解決方案應該沒有分支，但這似乎很難實現

我該怎麼做？

Answer 1

您可以使用this question中的一種方法生成MS n 字節設置爲全1的掩碼。然後你只需要解決任何剩餘的位，當n不是8

多我建議嘗試這樣的事：

- init vector A = all (8 bit) elements to the residual mask of n % 8 bits 
- init vector B = mask of n/8 bytes using one of the above-mentioned methods 
- init vector C = mask of (n + 7)/8 bytes using one of the above-mentioned methods 
- result = A | B & C 

因此，例如，如果n = 36：

A = f0 f0 f0 f0 f0 f0 f0 f0 f0 f0 f0 f0 f0 f0 f0 f0 
B = ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00 
C = ff ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 
==> ff ff ff ff f0 00 00 00 00 00 00 00 00 00 00 00 

根據需要，這將是無分支的，但它可能是大約10條指令的順序。可能有更有效的方法，但我需要給這個更多的想法。

Answer 2

我將此添加爲第二個答案並留下歷史興趣的第一個答案。它看起來像你可以做一些更有效的與_mm_slli_epi64：

#include <emmintrin.h> 
#include <stdio.h> 

__m128i bit_mask(int n) 
{ 
    __m128i v0 = _mm_set_epi64x(-1, -(n > 64)); // AND mask 
    __m128i v1 = _mm_set_epi64x(-(n > 64), 0); // OR mask 
    __m128i v2 = _mm_slli_epi64(_mm_set1_epi64x(-1), (128 - n) & 63); 
    v2 = _mm_and_si128(v2, v0); 
    v2 = _mm_or_si128(v2, v1); 
    return v2; 
} 

int main(int argc, char *argv[]) 
{ 
    int n = 36; 

    if (argc > 1) n = atoi(argv[1]); 

    printf("bit_mask(%3d) = %02vx\n", n, bit_mask(n)); 

    return 0; 
} 

測試：

$ gcc -Wall -msse2 sse_bit_mask.c 
$ for n in 1 2 3 63 64 65 127 128 ; do ./a.out $n ; done 
bit_mask( 1) = 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 80 
bit_mask( 2) = 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 c0 
bit_mask( 3) = 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 e0 
bit_mask(63) = 00 00 00 00 00 00 00 00 fe ff ff ff ff ff ff ff 
bit_mask(64) = 00 00 00 00 00 00 00 00 ff ff ff ff ff ff ff ff 
bit_mask(65) = 00 00 00 00 00 00 00 80 ff ff ff ff ff ff ff ff 
bit_mask(127) = fe ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 
bit_mask(128) = ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff