Re: gcc 3.4 > mainline performance regression

[Steven Bosscher]

On 1/5/07, Andrew Haley <[EMAIL PROTECTED]> 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