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.
