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Subjectgcc -O3 and register usage
Hi all...

I was playing looking at the code gcc gives for some simple operations,
and got this...

Simple C program (do you recognise it ;) ?):

struct list_head
{
struct list_head *next, *prev;
};

static inline int list_empty(struct list_head *head)
{
return head->next == head;
}

int use(struct list_head *l)
{
return list_empty(l);
}

I use gcc 3.3.1.
Compile at -O2:

use:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
popl %ebp
cmpl %eax, (%eax)
sete %al
movzbl %al, %eax
ret

Compile at -O3:

use:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %edx
popl %ebp
cmpl %edx, (%edx)
sete %al
andl $255, %eax
ret

Compile at -O3 and (at least) -march=pentiumpro:

use:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %edx
popl %ebp
cmpl %edx, (%edx)
sete %dl
movzbl %dl, %eax
ret

Go back to -O2, but keep -march=pentiumpro:

use:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
popl %ebp
cmpl %eax, (%eax)
sete %al
movzbl %al, %eax
ret

Does this mean that since PentiumPro gcc has one other register (%dl)
available, and it uses it only at -O3 ?
This can be a _big_ advantage to reduce register spilling (stack
traffic...)

The above effect is due to the -frename-registers activated in -O3.
This option is used in arch/ia64/Makefile, but it is supposed to
benefit more to arches with few registers (I suppose ia64 has a ton more
that ia32...)

Would if be useful ?

TIA

--
J.A. Magallon <jamagallon@able.es> \ Software is like sex:
werewolf.able.es \ It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.22-rc2-jam1m (gcc 3.3.1 (Mandrake Linux 9.2 3.3.1-1mdk))
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