Re: [RFC][PATCH] Reduce number of pointer derefs in various files (kernel/exit.c used as example)
From: Matt Mackall
Date: Fri Dec 09 2005 - 13:27:59 EST
On Fri, Dec 09, 2005 at 09:14:21AM +0100, Oliver Neukum wrote:
> 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@xxxxxxxxx> 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.
Excellent point. It'd require marking functions const, which might not
be a bad idea.
--
Mathematics is the supreme nostalgia of our time.
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