[英]Saturated substraction - AVX or SSE4.2
我正在改進程序(C)的性能,我無法獲得更好的執行時間來改善最“昂貴”的循環。
如果元素大於零,我必須從unsigned long int數組的每個元素中減去1。
循環是:
unsigned long int * WorkerDataTime;
...
for (WorkerID=0;WorkerID<WorkersON;++WorkerID){
if(WorkerDataTime[WorkerID] > 0) WorkerDataTime[WorkerID]-=1;
}
我試試這個:
for (WorkerID=0;WorkerID<WorkersON;++WorkerID){
int rest = WorkerDataTime[WorkerID] > 0;
WorkerDataTime[WorkerID] = WorkerDataTime[WorkerID] - rest;
}
但執行時間類似。
問題: 是否有任何內向指令(SSE4.2,AVX ......)直接執行此操作? (我正在使用gcc 4.8.2)
我知道有可能使用char或short元素。 (_mm_subs_epi8和_mm_subs_epi16)我不能使用AVX2。
謝謝。
使用SSE4可以使用三條指令。 這是一個處理整個數組的代碼,遞減所有非零的無符號整數:
void clampedDecrement_SSE (__m128i * data, size_t count)
{
// processes 2 elements each, no checks for alignment done.
// count must be multiple of 2.
size_t i;
count /= 2;
__m128i zero = _mm_set1_epi32(0);
__m128i ones = _mm_set1_epi32(~0);
for (i=0; i<count; i++)
{
__m128i values, mask;
// load 2 64 bit integers:
values = _mm_load_si128 (data);
// compare against zero. Gives either 0 or ~0 (on match)
mask = _mm_cmpeq_epi64 (values, zero);
// negate above mask. Yields -1 for all non zero elements, 0 otherwise:
mask = _mm_xor_si128(mask, ones);
// now just add the mask for saturated unsigned decrement operation:
values = _mm_add_epi64(values, mask);
// and store the result back to memory:
_mm_store_si128(data,values);
data++;
}
}
使用AVX2,我們可以對此進行改進,並在時間處理4個元素:
void clampedDecrement (__m256i * data, size_t count)
{
// processes 4 elements each, no checks for alignment done.
// count must be multiple of 4.
size_t i;
count /= 4;
// we need some constants:
__m256i zero = _mm256_set1_epi32(0);
__m256i ones = _mm256_set1_epi32(~0);
for (i=0; i<count; i++)
{
__m256i values, mask;
// load 4 64 bit integers:
values = _mm256_load_si256 (data);
// compare against zero. Gives either 0 or ~0 (on match)
mask = _mm256_cmpeq_epi64 (values, zero);
// negate above mask. Yields -1 for all non zero elements, 0 otherwise:
mask = _mm256_xor_si256(mask, ones);
// now just add the mask for saturated unsigned decrement operation:
values = _mm256_add_epi64(values, mask);
// and store the result back to memory:
_mm256_store_si256(data,values);
data++;
}
}
編輯:添加了SSE代碼版本。
除非您的CPU具有XOP,否則沒有有效的方法來比較無符號的64位整數 。
我從Agner Fog的Vector Class Library中刪除了以下內容。 這顯示了如何比較無符號的64位整數。
static inline Vec2qb operator > (Vec2uq const & a, Vec2uq const & b) {
#ifdef __XOP__ // AMD XOP instruction set
return Vec2q(_mm_comgt_epu64(a,b));
#else // SSE2 instruction set
__m128i sign32 = _mm_set1_epi32(0x80000000); // sign bit of each dword
__m128i aflip = _mm_xor_si128(a,sign32); // a with sign bits flipped
__m128i bflip = _mm_xor_si128(b,sign32); // b with sign bits flipped
__m128i equal = _mm_cmpeq_epi32(a,b); // a == b, dwords
__m128i bigger = _mm_cmpgt_epi32(aflip,bflip); // a > b, dwords
__m128i biggerl = _mm_shuffle_epi32(bigger,0xA0); // a > b, low dwords copied to high dwords
__m128i eqbig = _mm_and_si128(equal,biggerl); // high part equal and low part bigger
__m128i hibig = _mm_or_si128(bigger,eqbig); // high part bigger or high part equal and low part bigger
__m128i big = _mm_shuffle_epi32(hibig,0xF5); // result copied to low part
return Vec2qb(Vec2q(big));
#endif
}
因此,如果CPU支持XOP,則應嘗試使用-mxop
編譯,並查看循環是否為矢量化。
編輯:如果GCC沒有像你想要的那樣向量化,你的CPU有XOP你可以做
for (WorkerID=0; WorkerID<WorkersON-1; workerID+=2){
__m128i v = _mm_loadu_si128((__m128i*)&WorkerDataTime[workerID]);
__m128i cmp = _mm_comgt_epu64(v, _mm_setzero_si128());
v = _mm_add_epi64(v,cmp);
_mm_storeu_si128((__m128i*)&WorkerDataTime[workerID], v);
}
for (;WorkerID<WorkersON;++WorkerID){
if(WorkerDataTime[WorkerID] > 0) WorkerDataTime[WorkerID]-=1;
}
使用-mxop
編譯並包含#include <x86intrin.h>
。
編輯:正如Nils Pipenbrinck指出的,如果你沒有XOP,你可以使用_mm_xor_si128
一次指令:
for (WorkerID=0; WorkerID<WorkersON-1; WorkerID+=2){
__m128i v = _mm_loadu_si128((__m128i*)&WorkerDataTime[workerID]);
__m128i mask = _mm_cmpeq_epi64(v,_mm_setzero_si128());
mask = _mm_xor_si128(mask, _mm_set1_epi32(~0));
v= _mm_add_epi64(v,mask);
_mm_storeu_si128((__m128i*)&WorkerDataTime[workerID], v);
}
for (;WorkerID<WorkersON;++WorkerID){
if(WorkerDataTime[WorkerID] > 0) WorkerDataTime[WorkerID]-=1;
}
編輯:根據Stephen Canon的評論,我了解到有一種更有效的方法可以使用SSE4.2的pcmpgtq
指令比較一般的64位無符號整數:
__m128i a,b;
__m128i sign64 = _mm_set1_epi64x(0x8000000000000000L);
__m128i aflip = _mm_xor_si128(a, sign64);
__m128i bflip = _mm_xor_si128(b, sign64);
__m128i cmp = _mm_cmpgt_epi64(aflip,bflip);
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