[PATCH] ifplugd: using the __builtin_expect?

[Denys Vlasenko]

On Thursday 08 July 2010 15:32, Maksym Kryzhanovskyy wrote:
> > Denys Vlasenko wrote:
> > > > On Wednesday 07 July 2010 12:58, Maksym Kryzhanovskyy wrote:
> > > >   
> > >> >> Hi Denis,
> > >> >>
> > >> >> can we use the __builtin_expect function in busybox?
> > >> >>     
> > > > -       if (!G.detect_link_func) {
> > > > +       if (__builtin_expect(G.detect_link_func == NULL, 0)) {
> > > >
> > > > I don't see any changes in size, only obfuscated code.
> > > >   
> > 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.
> > 
> > Ralf
> > 
> 
> 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


Haha, wonderful. It does tha same thing, but is bigger AND slower...

> Is it intentionally turned off in busybox?

I guess -Os disables this optimization.

-- 
vda