Messages in this thread | | | From | Oliver Neukum <> | Subject | Re: [RFC][PATCH] Reduce number of pointer derefs in various files (kernel/exit.c used as example) | Date | Fri, 9 Dec 2005 09:14:21 +0100 |
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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.
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