问题
I am using an SSE intrinsic with one of the argument as a memory location (_mm_mul_ps(xmm1,mem)).
I have a doubt which will be faster:
xmm1 = _mm_mul_ps(xmm0,mem) // mem is 16 byte aligned
or:
xmm0 = _mm_load_ps(mem);
xmm1 = _mm_mul_ps(xmm1,xmm0);
Is there a way to specify alignment with _mm_mul_ps() intrinsic ?
回答1:
There are no _mm_mul_ps(reg,mem) form even though mulps reg,mem instruction form exists - https://msdn.microsoft.com/en-us/library/22kbk6t9(v=vs.90).aspx
What you can do is _mm_mul_ps(reg,_mm_load_ps(mem)) and it is going to be exactly the same as writing it in 2 lines.
You can use _mm_load_ps & _mm_loadu_ps to specify if you expect you data to be aligned. BTW, there are no penalty for doing unaligned loads on aligned data starting from Haswell microarch.
On the other side compiler should be smart enough to figure out if it would be better to do the load first and then do the multiply, or do the multiply from the memory.
In some cases it might make sense to do the load a bit in advance to improve software pipelining, but usually this is going to be the next level of optimization.
来源：https://stackoverflow.com/questions/31315892/how-to-specify-alignment-with-mm-mul-ps