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Message-ID: <20150225092020.GW32724@port70.net>
Date: Wed, 25 Feb 2015 10:20:20 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: x86[_64] memset and rep stos
* ?????? <torshie@...il.com> [2015-02-25 15:54:31 +0800]:
> I'm not an expert on micro optimization, but why not use a dynamic
> routine selection system which would select the optimal routine for a
> given CPU during program initialization. The routine selection
> algorithm could simply be a predefined static table look up.
> IMO, only very small number of functions (like memset, memcpy) would
> benefit from such a system, so no code size overhead to worry about.
my guess is
- for static linking it adds at least an extra indirection
and these functions often used with small input
- code size overhead: now you have to include all possible
versions in libc.so
- for dynamic linking there is a load time dispatch mechanism:
STT_GNU_IFUNC but it's broken due to lack of specs
- maintainance burden, hard to test
- selecting the right algorithm at runtime is not easy
but i guess eventually when more ppl use musl it will make sense
to add more target specific optimizations
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