A Few Synthesizing Superoptimizer Results

http://blog.regehr.org/archives/1252

by jcr • 11 years ago

36 13 11 years ago

blog.regehr.org

I feel like super optimizing llvm code isn't the best idea.

I feel like the way other superoptimizers get good results is by taking advantage of really strange interactions of some of the less commonly understood opcodes. LLVM doesn't have those funky opportunities.

caf3• 11 years ago

It's not either/or, adding superoptimiser-derived optimisations at the IR level doesn't mean you can't also add different ones at the backend.

userbinator• 11 years ago

The problem is that superoptimised IR may obscure more useful optimisations lower down.

andrewchambers3• 11 years ago

My intuition is that stacking general super optimizers will be at best equally or less effective than just running a machine specific one for twice as long.

rurban• 11 years ago

You didn't understand how Souper works. They already found a lot of very good optimizations, which were included into LLVM proper. Check out http://blog.regehr.org/archives/1146 and http://blog.regehr.org/archives/1192

Here he talks about the end-results of Souper and Alive, which should be presented more user-friendly and generalized, with some help from a solver. Encoding every single edge-case into the optimizer is not as good as finding better generalizations, automatically.

andrewchambers3• 11 years ago

I understand that is the approach.

Another idea I want to see tried is a source code annotation which indicates to an llvm plugin to run machine specific super optimization on a specific basic block and cache the result. Might be an interesting way to enhance tight loops.

A third idea I had would be to use super optimization as a way of automatically generating isel selectors with only the lowest level descriptions of what the ir and the opcodes do. That is harvest a program for ir sequences, automatically find good mappings to assembly constructs, automatically emit a backend.

A Few Synthesizing Superoptimizer Results

Related Stories