linux-kernel - Re: if (unlikely(...)) == unnecessary?

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Message-ID: <alpine.DEB.1.10.0901281255440.21401@alien.or.mcafeemobile.com>
Date:	Wed, 28 Jan 2009 13:10:43 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Chris Snook <csnook@...hat.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: if (unlikely(...)) == unnecessary?

On Wed, 28 Jan 2009, Chris Snook wrote:

> When you turn on optimizations, gcc will try to avoid branching just to
> execute a few instructions, since the cache miss and page fault penalties
> greatly exceed the cost of a branch mispredict.  The thresholds and heuristics
> vary, but in general, if you stick something like this:
> 
> if (condition) foo++;
> else if (complex condition) {do lots of stuff}
> 
> In the middle of a long function body and compile with optimizations enabled,
> gcc will try to put the foo++ right after the evaluation.  Some ISAs support
> conditional instructions to let the compiler help fill pipeline bubbles, and
> some superscalar processors will speculatively execute it in parallel with
> their evaluation of the second condition, and proceed with whichever execution
> path is chosen when they retire the instruction evaluating the first
> conditional.

OK, been finally able to trigger a different behavior. I thought that 
became a somehow rule, after quite a few trials yesterday all leading to 
the same results.

- Davide

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