better load/store scheduling

["Ben Cheng" <bccheng@xxxxxxxxxxxxxxxxx>]

I am trying to tune the performance of hand-unrolled code. I was
wondering what cmd-line options should I specify in order to get h[i+1]
loaded before the store to g[i]:


Code:

void foo(int * __restrict g, int * __restrict h)
{
    int i;
    for (i = 0; i < 4096; i+=2) {
        g[i]   = h[i] + 10;
        g[i+1] = h[i+1] + 10;
    }
}

Command line:

gcc-4.0.2 -O3 loop.c -fargument-noalias-global -fstrict-aliasing -S
loop.s

Assembly code of the loop body:

.L2:
        leal    0(,%ebx,4), %eax
        leal    (%eax,%esi), %ecx
        leal    (%edi,%eax), %eax
        movl    -8(%ecx), %edx                  // = h[i]
        addl    $10, %edx                       // + 10
        movl    %edx, -8(%eax)                  // g[i] = 
        movl    -4(%ecx), %edx                  // = h[i+1]
        addl    $10, %edx                       // + 10
        movl    %edx, -4(%eax)                  // g[i+1] =
        addl    $2, %ebx
        cmpl    $4098, %ebx
        jne     .L2
 

Thanks,
-Ben