Streaming SIMD Extensions 2 Instructions
How to Write Fast Numerical Code
C++ - Getting started with SSE
Stackoverflow
Introduction
- SSE stands for Streaming SIMD Extensions. It is a set of CPU instructions dedicated to applications like signal processing, scientific computation or 3D graphics.
- SIMD is an acronym itself: Single Instruction, Multiple Data. A CPU instruction is said to be SIMD when the same operation is applied on multiple data at the same time.
- Naming convention:
_mm_<intrin_op>_<suffix>
// a is 16-byte aligned
float a[4] = {1.0, 2.0, 3.0, 4.0};
__m128 t = _mm_load_ps(a); // p: packed, s: single precision
Issues
- Alignment is important (128 bit = 16 byte)
- You need to code explicit loads and stores
- Overhead through shuffles
Example
_mm_store_ps stores results in an array._mm_sqrt_ps computes the square root of 4 float in a single operation.posix_memalign allocates data on the stack;aligned (alignment) allocates aligned data on the heap. (This attribute specifies a minimum alignment for the variable or structure field, measured in bytes.) e.g., float a[] __attribute__ ((aligned (16))) = { 41982., 81.5091, 3.14, 42.666 };
#include <emmintrin.h> // IMPORTANT
#include <math.h>
#include <stdio.h>
#include <chrono>
#include <iostream>
using namespace std;
class Timer {
public:
Timer() { start = Clock::now(); }
void Start() { start = Clock::now(); }
void End() {
auto end = Clock::now();
std::cout << "Elapsed time: "
<< std::chrono::duration_cast<std::chrono::nanoseconds>(end -
start)
.count()
<< " nanoseconds" << std::endl;
}
private:
using Clock = std::chrono::high_resolution_clock;
std::chrono::_V2::system_clock::time_point start;
};
void normal(float* a, int N) {
for (int i = 0; i < N; ++i) {
a[i] = sqrt(a[i]);
}
}
// compute the square root of a very large array on float
void sse(float* a, int N) {
// We assume N % 4 == 0.
int nb_iters = N / 4;
__m128* ptr = (__m128*)a;
for (int i = 0; i < nb_iters; ++i, ++ptr, a += 4) {
_mm_store_ps(a, _mm_sqrt_ps(*ptr));
}
}
// add two arrays of char together
void sse(char* a, const char* b, int N) {
int nb_iters = N / 16;
__m128i* l = (__m128i*)a;
__m128i* r = (__m128i*)b;
for (int i = 0; i < nb_iters; ++i, ++l, ++r)
_mm_store_si128(l, _mm_add_epi8(*l, *r));
}
int main(int argc, char** argv) {
if (argc != 2) {
cout << "Usage: ./sse_test N (N represents the number of floats)" << endl;
return 1;
}
int N = atoi(argv[1]);
float* a;
posix_memalign((void**)&a, 16, N * sizeof(float));
for (int i = 0; i < N; ++i) {
a[i] = 3141592.65358;
}
Timer timer;
{
timer.Start();
normal(a, N);
timer.End();
}
for (int i = 0; i < N; ++i) {
a[i] = 3141592.65358;
}
{
timer.Start();
sse(a, N);
timer.End();
}
}
g++ -o sse_test sse_test.cc -std=c++11 -O3 -msse2
./sse_test 64000000
Elapsed time: 336269639 nanoseconds
Elapsed time: 29902925 nanoseconds
