Re: if (unlikely(...)) == unnecessary?

[Davide Libenzi]

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


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/