我应该计算一个__m128i寄存器内设置为1的位数。
特别地,我应编写两个函数,能够使用以下方式计数寄存器中的位数:
- 寄存器中设置为1的位的总数。
- 每个字节中设置为1的位的数量。
是否有内置函数可以完全或部分地执行上述操作?
我应该计算一个__m128i寄存器内设置为1的位数。
特别地,我应编写两个函数,能够使用以下方式计数寄存器中的位数:
是否有内置函数可以完全或部分地执行上述操作?
popcnt8
函数计算每个字节中设置的位数。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Wojciech Mula提供):
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,但使用的是XOP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 popcnt64
函数计算 SSE 寄存器低 64 位和高 64 位的比特数:
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);
}
popcnt128
函数计算整个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);
}
然而,实现popcnt128
的更高效方法是使用硬件POPCNT指令(在支持该指令的处理器上):
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
}
#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]);
}
paddusb
在所有方面的性能与paddb
相同,因此没有性能原因来避免使用饱和加法。这只是令人惊讶。) - Peter Cordes正如第一条评论所说,gcc 3.4+通过
提供了对(希望是最优的)内置函数的简单访问。int __builtin_popcount (unsigned int x) /* Returns the number of 1-bits in x. */
编辑:我想我没有理解 OP 寻找什么,但我会保留我的答案,以防其他人偶然发现它有用。
C 语言提供了一些不错的位运算操作。
以下是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;
}
解释:
toCount % 2
返回我们整数中的最后一位(通过除以二并检查余数)。我们将此添加到我们的总计中,然后将我们的 toCount 值的位移一位。应该继续执行此操作,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,您将需要使用掩码。以下是一个示例:
countBitsInByte(int toCount, int byteNumber)
{
int mask = 0x000F << byteNumber * 8
return countBitsSet(toCount & mask)
}
POPCNT
),问题是关于在128位SSE(XMM)寄存器中计算设置位数,而不是一个int
。 - Paul R>>
却是逻辑移位。但在实践中,人们真正想使用的所有编译器都会为有符号类型提供算术右移,因此如果toCount
为负数,则您的函数将成为一个无限循环。而有符号的%2
比&1
需要更多的工作,因为它必须为负奇数产生-1
。但是(在正常的编译器上),如果toCount
为负数,则您的函数永远不会返回,因此该问题被隐藏了... - Peter Cordes