Why is `std::copy` 5x (!) slower than `memcpy` for reading one int from a char buffer, in my test program?

Question

This is a follow-up to this question where I posted this program:

#include 
#include 
#include 
#include 

        

Answer 1

According to assembler output of G++ 4.8.1, test_memcpy:

movl    (%r15), %r15d

test_std_copy:

movl    $4, %edx
movq    %r15, %rsi
leaq    16(%rsp), %rdi
call    memcpy

As you can see, std::copy successfully recognized that it can copy data with memcpy, but for some reason further inlining did not happen - so that is the reason of performance difference.

By the way, Clang 3.4 produces identical code for both cases:

movl    (%r14,%rbx), %ebp