List Archive: gentoo-performance
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With gcc, is there a way to enable all -O3 options but function
inlines? Would -fno-inline work or something like that?
Mario Domenech Goulart wrote:
>There's an interesting discussion in the OpenBSD mailing
>list about the use of inline.
>Here's the beginning of the thread about this topic:
>,----[ http://www.sigmasoft.com/cgi-bin/wilma_hiliter/openbsd-tech/200407/msg00175.html ]
>| inline considered harmful.
>| * To: tech@...
>| * Subject: inline considered harmful.
>| * From: Artur Grabowski <art@...>
>| * Date: 21 Jul 2004 03:54:46 +0200
>| * User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.2
>| Today we did a bunch of removal of inline functions in the kernel.
>| It all started to make floppies fit, but now it's a quest.
>| If you think that I'm crazy doing this because it might hurt your
>| precious performance, go back to your vax and leave the performance
>| tuning to people who have a cache.
>| Every single inline we removed today (and there are more in the
>| pipeline and even more waiting to be fixed) shrunk the code and MADE
>| IT FASTER. Yes, modern cpus have something called "cache". The cache
>| prefers the code to be smaller, rather than free from function calls.
>| Yes, some cpus have expensive function call overhead. Don't use them.
>| i386 has quite expensive function calls, on the other hand it doesn't
>| have any relevant amount of registers either. So a function call
>| instead of the same function inlined can potentially make the job
>| easier for the register allocator in the compiler which could eat the
>| overhead. At the same time the instruction cache can run the same code
>| in the same place, instead of loading it from main memory 4711 times.
>| And guess what? The stack on i386 is in the cache too, so the function
>| call overhead isn't that bad anyway.
>| I'm tired of seeing code where everything is made inline just because
>| someone acted on a meme that hasn't been true for over a decade. Bloat,
>| bloat and more bloat. Since people can't use inline correctly (it does
>| have valid and correct uses), from now on inline in the OpenBSD kernel
>| is considered to be a bug until proven otherwise. So. Next time I see
>| code that adds to the bloat with inlines, I expect performance figures
>| and kernel size comparisons that show that the inline actually
>| contributes anything. Otherwise the code does not go in.
>| There's still a lot of work to be done in the kernel (yes, macros can
>| be evil too, just see nfs), so send diffs. And there's a whole
>| unexplored field in userland too.
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