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From: | Paolo Bonzini |
Subject: | Re: [PATCH] Microoptimization of lib/poll.c |
Date: | Wed, 06 Aug 2008 15:49:07 +0200 |
User-agent: | Thunderbird 2.0.0.16 (Macintosh/20080707) |
3) access to thread-local errno is slow, it's also better to cache it.Pheww! That is micro-optimization indeed.
Well, thread-local errno might involve a function call on some systems. If the function call is not CSE'd by the compiler, it means half a dozen function calls.
Do you have an objection against this further patch? Should save 2 instructions in the case that you were not profiling.
*That* is micro-optimization! :-) It actually makes the code clearer, so go on. But I guess a division is more expensive than those 2 instructions...
(I used Shark to get the profile. rpl_poll was responsible for 6% before my optimizations and 3% afterwards. So not a big change from Amdahl's point of view :-) but a big change for this specific function).
Paolo
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