Gentoo Archives: gentoo-dev

From: Evan Powers <powers.161@×××.edu>
To: gentoo-dev@g.o
Subject: Re: [gentoo-dev] -fbranch-probabilities optimisation
Date: Sat, 17 May 2003 23:28:47
In Reply to: Re: [gentoo-dev] -fbranch-probabilities optimisation by Anupam Kapoor
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 -- gentoo-dev@g.o mailing list