[gentoo-performance] inline considered harmful

[Mario Domenech Goulart]

Hello,

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@×××××××.org
|     * Subject: inline considered harmful.
|     * From: Artur Grabowski <art@××××××××.org>
|     * 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.
| 
| //art
`----

Mario


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