Re: [gentoo-dev] -fbranch-probabilities optimisation

[Evan Powers]

On Friday 16 May 2003 08:52 pm, Anupam Kapoor wrote:
> i was under the assumption that most processors already perform branch
> prediction. no ? do you think that -fbranch-probabilities provides a
> more 'comprehensive' view of the executing program ?

Yes, modern processors do perform branch prediction. But you're right, the 
compiler can do a much more 'comprehensive' job.

The reason is that CPU branch prediction algorithms tend to forget any 
information they learn that occurred farther away in time or space than 
certain thresholds. The Xeon, I think, retains information only about the 
last 16 branching instructions and the direction of only the last four 
branches on each of those instructions.

GCC, on the other hand, can retain information about all the branches, and 
complete directional statistics over the entire run, but it can only make 
static predictions. (For example, GCC can't take advantage of knowing that 
the branch will go one way for 200 consecutive times, then the other for 
300--it has to predict the branch will go the second way all the time and 
order accordingly.)

So you get the best performance when you combine the two approaches.

Evan

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