Re: gcc 3.4 > mainline performance regression

["Steven Bosscher" <stevenb dot gcc at gmail dot com>]

On 1/5/07, Andrew Haley <aph@redhat.com> wrote:
> This is from the gcc-help mailing list.  It's mentioned there for ARM,
> but it's just as bad for x86-64.
>
> It appears that memory references to arrays aren't being hoisted out
> of loops: in this test case, gcc 3.4 doesn't touch memory at all in
> the loop, but 4.3pre (and 4.2, etc) does.
>
> Here's the test case:
>
> void foo(int *a)
> {       int i;
>         for (i = 0; i < 1000000; i++)
>    a[0] += a[1];
> }
>
> gcc 3.4.5 -O2:
>
> .L5:
>         leal    (%rcx,%rsi), %edx
>         decl    %eax
>         movl    %edx, %ecx
>         jns     .L5
>
> gcc 4.3pre -O2:
>
> .L2:
>         addl    4(%rdi), %eax
>         addl    $1, %edx
>         cmpl    $1000000, %edx
>         movl    %eax, (%rdi)
>         jne     .L2
>
> Thoughts?

What does the code look like if you compile with -O2  -fgcse-sm?

Gr.
Steven