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Performance difference between gcc and g++ for C program

I was checking the improvement of performance with using register storage specifier for the loop control variable when I accidentally noticed that program compiled with gcc runs faster than compiled with g++. Can someone explain it to me?

Here is the code:

#include <stdio.h>

const unsigned long scope = 1000000000;

int main()
{
    register unsigned long i;
    for (i=0; i < scope; i++);
    return 0;
}

;

gcc register.c
time ./a.out   
real    0m0.466s
user    0m0.468s
sys     0m0.000s

g++ register.c
time ./a.out 
real    0m0.923s
user    0m0.920s
sys     0m0.000s

Answer 1

I can reproduce the behaviour (gcc-4.6.2), the relevant parts of the produced assembly are

C:

.L3:
    addq    $1, %rbx
.L2:
    movq    scope(%rip), %rax
    cmpq    %rax, %rbx
    jb      .L3

C++:

.L3:
    addq    $1, %rbx
.L2:
    cmpq     $999999999, %rbx
    setbe    %al
    testb    %al, %al
    jne      .L3

so the C compiler produced a better loop test. Don’t ask me why, I have no idea.