使用stl向量优化算术运算

Question

I have some simple structs:

struct ab { double a,b; }
struct abcd { double a,b,c,d; }

struct ch
{
...
  std::vector<abcd> x;
  std::vector<size_t> ir;
...
}

And code:

ch l;
std::vector<ab> x;
double c,f;
...
for(size_t i = ... )
{
    ...
    l.x[i].c = (l.x[i].c / c) + f*x[l.ir[i]].a; // line#1
    ...
}

CodeXl shows that one of the most expensive lines is line#1. And 60% of line#1 take

 mov eax,[edx+eax]

How can I optimize line#1?

Why "mov" operation more expensive than mul and div?

Upd Full decompiling of line#1 from CodeXl:

l.x[i].c = (l.x[i].c / c) + f*x[l.ir[i]].a; => 15.871% of function time
;;
mov ecx,[ebx+4ch]
lea edx,[edi*4+00000000h] => 0.99194%
shl edi,05h
mov eax,[ebx+1ch]
movsd xmm0,[ecx+edi+10h]
divsd xmm0,xmm2 => 1.17793%
mov eax,[edx+eax] => 10.0434%
add eax,eax
movsd xmm1,[esi+eax*8]
mulsd xmm1,xmm4
addsd xmm1,xmm0 => 1.30192%
movsd [ecx+edi+10h],xmm1 => 2.35586%

Upd Microsoft Visual Studio 2013. Release32

Answer 1

mul and div are fast because the arguments are available. mov eax, [eax+edx] requires an argument from memory. Is it in cache or prefetched? I suspect this particular mov is from your x[l.ir[i]] expression, x is sufficiently large to be uncached, and l.ir[i] is sufficiently non-linear to defeat the prefetcher. That means you're waiting for main memory.