Re: [RFC][PATCH] Reduce number of pointer derefs in various files (kernel/exit.c used as example)

[Oliver Neukum <oliver@neukum.org>]

Am Freitag, 9. Dezember 2005 02:46 schrieb Matt Mackall:
> On Tue, Dec 06, 2005 at 11:15:28PM +0100, Ingo Molnar wrote:
> > 
> > * Jesper Juhl <jesper.juhl@gmail.com> wrote:
> > 
> > > Ohh, and before I forget, besides the fact that this should speed 
> > > things up a little bit it also has the added benefit of reducing the 
> > > size of the generated code. The original kernel/exit.o file was 19604 
> > > bytes in size, the patched one is 19508 bytes in size.
> > 
> > nice. Just to underline your point, on x86, with gcc 4.0.2, i'm getting 
> > this with your patch:
> > 
> >    text    data     bss     dec     hex filename
> >   11077       0       0   11077    2b45 exit.o.orig
> >   10997       0       0   10997    2af5 exit.o
> > 
> > so 80 bytes shaved off. I think such patches also increase readability.
> 
> Readability improved: good.
> 37 lines of patch for 80-100 bytes saved: not so good.
> 
> So while this is a good style direction, I don't think it's worth the
> churn. And unlike kzalloc and the like, this particular optimization
> is perfectly doable by a compiler. So I'd rather wait for the compiler
> to get smarter than change code for such modest improvements.

How can the compiler do it? If a function call is between two evaluations
of a pointer chain, the compiler would have to make sure no pointer in
the chain is touched. For the case of a computed function call, it is
impossible in principle.

	Regards
		Oliver
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