Re: Tracking down gcc-4.0 performance regressions

[Richard Guenther]

On 6/6/05, Georg Bauhaus <[EMAIL PROTECTED]> wrote:
> Daniel Kegel wrote:
> 
> > So, I'm looking around for other reports of performance
> > regressions in gcc-4.0.
> 
> I came across this one:
> 
> int foo(int a, int b)
> {
>           return a + b;
> }
> 
> int bar()
> {
>         int x = 0, y = 10;
>         int c;
> 
>         for (c=0; c < 123123123 && x > -1; ++c, --y)
>                 x = foo(c, y);
>         return x;
> }
> 
> int main()
> {
>         return bar();
> }

Interestingly for mainline with -O3 we transform main into

main ()
{
  int a;
  int x;

<bb 0>:
  a = 0;

<L1>:;
  x = a + (int) (10 - (unsigned int) a);
  a = a + 1;
  if (a <= 123123122 && x > -1) goto <L1>; else goto <L3>;

<L3>:;
  return 10;

}

which the RTL loop optimizer can turn into

.L11:
        incl    %eax
        cmpl    $123123122, %eax
        jle     .L11

4.0 is similar - there's one extra increment in the loop, while 3.4 is
considerably worse:

.L18:
        leal    (%ecx,%ebx), %esi
        movl    %esi, %eax
        incl    %ecx
        decl    %ebx
        notl    %eax
        cmpl    $123123122, %ecx
        setle   %dl
        shrl    $31, %eax
        testl   %edx, %eax
        jne     .L18

So I'd not call it a performance regression ;)

Richard.