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SubjectRe: 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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