我应该计算__m128i寄存器的设置位数。 特别是，我应该使用以下方法编写两个能够计算寄存器位数的函数。
是否有可以全部或部分执行上述操作的内在功能？
答案 0 :(得分：24)
以下是我在旧项目中使用的一些代码（there is a research paper about it）。下面的函数popcnt8计算每个字节中设置的位数。
仅限SSE2版本（基于Hacker's Delight book中的算法3）：
static const __m128i popcount_mask1 = _mm_set1_epi8(0x77);
static const __m128i popcount_mask2 = _mm_set1_epi8(0x0F);
static inline __m128i popcnt8(__m128i x) {
__m128i n;
// Count bits in each 4-bit field.
n = _mm_srli_epi64(x, 1);
n = _mm_and_si128(popcount_mask1, n);
x = _mm_sub_epi8(x, n);
n = _mm_srli_epi64(n, 1);
n = _mm_and_si128(popcount_mask1, n);
x = _mm_sub_epi8(x, n);
n = _mm_srli_epi64(n, 1);
n = _mm_and_si128(popcount_mask1, n);
x = _mm_sub_epi8(x, n);
x = _mm_add_epi8(x, _mm_srli_epi16(x, 4));
x = _mm_and_si128(popcount_mask2, x);
return x;
}
SSSE3版本（由于Wojciech Mula）：
static const __m128i popcount_mask = _mm_set1_epi8(0x0F);
static const __m128i popcount_table = _mm_setr_epi8(0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4);
static inline __m128i popcnt8(__m128i n) {
const __m128i pcnt0 = _mm_shuffle_epi8(popcount_table, _mm_and_si128(n, popcount_mask));
const __m128i pcnt1 = _mm_shuffle_epi8(popcount_table, _mm_and_si128(_mm_srli_epi16(n, 4), popcount_mask));
return _mm_add_epi8(pcnt0, pcnt1);
}
XOP版本（相当于SSSE3，但使用XOP指令，这些指令在AMD Bulldozer上更快）
static const __m128i popcount_mask = _mm_set1_epi8(0x0F);
static const __m128i popcount_table = _mm_setr_epi8(0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4);
static const __m128i popcount_shift = _mm_set1_epi8(-4);
static inline __m128i popcount8(__m128i n) {
const __m128i pcnt0 = _mm_perm_epi8(popcount_table, popcount_table, _mm_and_si128(n, popcount_mask));
const __m128i pcnt1 = _mm_perm_epi8(popcount_table, popcount_table, _mm_shl_epi8(n, popcount_shift));
return _mm_add_epi8(pcnt0, pcnt1);
}
下面的函数popcnt64计算SSE寄存器的低位和高位64位中的位数：
SSE2版本：
static inline __m128i popcnt64(__m128i n) {
const __m128i cnt8 = popcnt8(n);
return _mm_sad_epu8(cnt8, _mm_setzero_si128());
}
XOP版本：
static inline __m128i popcnt64(__m128i n) {
const __m128i cnt8 = popcnt8(n);
return _mm_haddq_epi8(cnt8);
}
最后，下面的函数popcnt128计算整个128位寄存器中的位数：
static inline int popcnt128(__m128i n) {
const __m128i cnt64 = popcnt64(n);
const __m128i cnt64_hi = _mm_unpackhi_epi64(cnt64, cnt64);
const __m128i cnt128 = _mm_add_epi32(cnt64, cnt64_hi);
return _mm_cvtsi128_si32(cnt128);
}
但是，实现popcnt128的更有效方法是使用硬件POPCNT指令（在支持它的处理器上）：
static inline int popcnt128(__m128i n) {
const __m128i n_hi = _mm_unpackhi_epi64(n, n);
#ifdef _MSC_VER
return __popcnt64(_mm_cvtsi128_si64(n)) + __popcnt64(_mm_cvtsi128_si64(n_hi));
#else
return __popcntq(_mm_cvtsi128_si64(n)) + __popcntq(_mm_cvtsi128_si64(n_hi));
#endif
}
答案 1 :(得分：1)
以下是基于Bit Twiddling Hacks - Counting Set Bits in Parallel的版本，其命名与其他内部函数类似，以及16个32位和64位向量的一些额外函数
#include "immintrin.h"
/* bit masks: 0x55 = 01010101, 0x33 = 00110011, 0x0f = 00001111 */
static const __m128i m1 = {0x5555555555555555ULL,0x5555555555555555ULL};
static const __m128i m2 = {0x3333333333333333ULL,0x3333333333333333ULL};
static const __m128i m3 = {0x0f0f0f0f0f0f0f0fULL,0x0f0f0f0f0f0f0f0fULL};
static const __m128i m4 = {0x001f001f001f001fULL,0x001f001f001f001fULL};
static const __m128i m5 = {0x0000003f0000003fULL,0x0000003f0000003fULL};
__m128i _mm_popcnt_epi8(__m128i x) {
/* Note: if we returned x here it would be like _mm_popcnt_epi1(x) */
__m128i y;
/* add even and odd bits*/
y = _mm_srli_epi64(x,1); //put even bits in odd place
y = _mm_and_si128(y,m1); //mask out the even bits (0x55)
x = _mm_subs_epu8(x,y); //shortcut to mask even bits and add
/* if we just returned x here it would be like _mm_popcnt_epi2(x) */
/* now add the half nibbles */
y = _mm_srli_epi64 (x,2); //move half nibbles in place to add
y = _mm_and_si128(y,m2); //mask off the extra half nibbles (0x0f)
x = _mm_and_si128(x,m2); //ditto
x = _mm_adds_epu8(x,y); //totals are a maximum of 5 bits (0x1f)
/* if we just returned x here it would be like _mm_popcnt_epi4(x) */
/* now add the nibbles */
y = _mm_srli_epi64(x,4); //move nibbles in place to add
x = _mm_adds_epu8(x,y); //totals are a maximum of 6 bits (0x3f)
x = _mm_and_si128(x,m3); //mask off the extra bits
return x;
}
__m128i _mm_popcnt_epi16(__m128i x) {
__m128i y;
x = _mm_popcnt_epi8(x); //get byte popcount
y = _mm_srli_si128(x,1); //copy even bytes for adding
x = _mm_add_epi16(x,y); //add even bytes into the odd bytes
return _mm_and_si128(x,m4);//mask off the even byte and return
}
__m128i _mm_popcnt_epi32(__m128i x) {
__m128i y;
x = _mm_popcnt_epi16(x); //get word popcount
y = _mm_srli_si128(x,2); //copy even words for adding
x = _mm_add_epi32(x,y); //add even words into odd words
return _mm_and_si128(x,m5);//mask off the even words and return
}
__m128i _mm_popcnt_epi64(__m128i x){
/* _mm_sad_epu8() is weird
It takes the absolute difference of bytes between 2 __m128i
then horizontal adds the lower and upper 8 differences
and stores the sums in the lower and upper 64 bits
*/
return _mm_sad_epu8(_mm_popcnt_epi8(x),(__m128i){0});
}
int _mm_popcnt_si128(__m128i x){
x = _mm_popcnt_epi64(x);
__m128i y = _mm_srli_si128(x,8);
return _mm_add_epi64(x,y)[0];
//alternative: __builtin_popcntll(x[0])+__builtin_popcntll(x[1]);
}
答案 2 :(得分：0)
如第一条评论中所述，gcc 3.4+可以轻松访问（希望是最佳的）内置
int __builtin_popcount (unsigned int x) /* Returns the number of 1-bits in x. */
如上所述： http://gcc.gnu.org/onlinedocs/gcc-3.4.3/gcc/Other-Builtins.html#Other%20Builtins
没有完全回答128位的问题，但对我登陆这里的问题给出了一个很好的答案：）
答案 3 :(得分：-3)
编辑：我想我不明白OP正在寻找什么，但我保留了我的答案，以防它对其他任何人有用。
C提供了一些不错的按位操作。
这是用于计算整数中设置的位数的代码：
countBitsSet(int toCount)
{
int numBitsSet = 0;
while(toCount != 0)
{
count += toCount % 2;
toCount = toCount >> 1;
}
return numBitsSet;
}
说明：
toCount % 2
返回整数中的最后一位。 （除以2并检查余数）。我们将其添加到总计数中，然后将toCount值的位移一。应该继续此操作，直到toCount中没有设置更多位（当toCount等于0时）
要计算特定字节中的位数，您需要使用掩码。这是一个例子：
countBitsInByte(int toCount, int byteNumber)
{
int mask = 0x000F << byteNumber * 8
return countBitsSet(toCount & mask)
}
让我们说在我们的系统中，我们认为字节0是小端系统中最不重要的字节。我们想创建一个新的toCount，通过屏蔽掉设置为0的位来传递给我们之前的countBitsSet函数。我们通过将一个字节（由字母F表示）移位到我们想要的位置（byteNumber *）来实现这一点。 8个字节中的8位）并使用我们的toCount变量执行按位AND运算。