Re: [PATCH] ifplugd: using the __builtin_expect?

[Maksym Kryzhanovskyy]

> > Maksym Kryzhanovskyy wrote:
> >> I think the main point of __builtin_expect is to make the code faster by 
> >> telling the compiler which branch is more likely, not to make the 
> >> assembly code smaller or easier to read.
> >> Actually I would expect the above sequence to require an extra jump, 
> >> making the code bigger.
> >>     
> > when we use the __builtin_expect in smaller code,
> > the compiler adds a few extra instructions (the result
> > should be slightly larger):
> >
> > if (detect_link_func == 0)
> >
> > 	testl	%eax, %eax
> > 	jne	.L4
> >
> >
> > if (__builtin_expect(detect_link_func == 0, 0))
> >
> > 	testl	%eax, %eax
> > 	sete	%al
> > 	movzbl	%al, %eax
> > 	testl	%eax, %eax
> > 	je	.L4
> >
> >
> > Is it intentionally turned off in busybox?
> >
> >
> > cat >> a.c << EOF
> > int (*detect_link_func)(void) = 0;
> >
> > static int detect_link_0(void)
> > {
> > 	return 0;
> > }
> >
> > static int detect_link(void)
> > {
> > 	// if (detect_link_func == 0)
> > 	if (__builtin_expect(detect_link_func == 0, 0))
> > 		detect_link_func = detect_link_0;
> > 	return detect_link_func();
> > }
> >
> > int main(void)
> > {
> > 	detect_link();
> > 	return 0;
> > }
> > EOF
> > gcc -std=c99 -pedantic -Wall -save-temps -c a.c
> If you want small code, you should turn on optimization.
> Also, it seems that GCC doesn't rearrange the code when optimizing for 
> size (-Os) if that would result in bigger code (and it would, for an 
> "if" without "else").
> 
> Try these versions and see the resulting files:
> $ gcc -Os -S  a.c
> $ gcc -O2 -S  a.c
> 
> Ralf
> 

I think that I've got it sorted now, thanks.

Regards.

Max.