I have learned that some Intel/AMD CPUs can do simultanous multiply and add with SSE/AVX:
FLOPS per cycle for sandy-bridge and haswell SSE2/AVX/AVX2.
我想知道如何在代码中最好地做到这一点，我也想知道它在CPU内部是如何完成的.我是说超标量架构.假设我想在SSE中做一个长和，如下所示:
//sum = a1*b1 + a2*b2 + a3*b3 +... where a is a scalar and b is a SIMD vector (e.g. from matrix multiplication)
sum = _mm_set1_ps(0.0f);
a1 = _mm_set1_ps(a[0]);
b1 = _mm_load_ps(&b[0]);
sum = _mm_add_ps(sum, _mm_mul_ps(a1, b1));
a2 = _mm_set1_ps(a[1]);
b2 = _mm_load_ps(&b[4]);
sum = _mm_add_ps(sum, _mm_mul_ps(a2, b2));
a3 = _mm_set1_ps(a[2]);
b3 = _mm_load_ps(&b[8]);
sum = _mm_add_ps(sum, _mm_mul_ps(a3, b3));
...
我的问题是如何将其转换为同时相乘和相加？数据可以依赖吗？我的意思是CPU可以同时做_mm_add_ps(sum, _mm_mul_ps(a1, b1))个吗，或者乘法和加法中使用的寄存器必须是独立的吗？
最后，这如何适用于FMA(与哈斯韦尔一起)？_mm_add_ps(sum, _mm_mul_ps(a1, b1))是否自动转换为单条FMA指令或微操作？