C++: Mysteriously huge speedup from keeping one operand in a register

Question

I have been trying to get an idea of the impact of having an array in L1 cache versus memory by timing a routine that scales and sums the elements of an array using the foll

Answer 1

I can't actually reproduce this because my compiler (gcc 4.7.2) keeps total in a register.

I suspect the main reason for the slowness doesn't have to do with the L1 cache, but rather is due to the data dependency between the store in

movsd   %xmm0, -72(%rbp)

and the load on the subsequent iteration:

addsd   -72(%rbp), %xmm0