[cfe-dev] Meaning of LLVM optimization levels

[Dallman, John]

I'm not a LLVM or Clang developer, but I do spend a lot of time teasing software into
working with the highest possible optimisation levels where it still works correctly.

These guidelines are pretty good, but there are a few details worth considering.

It needs to be possible to debug code at any optimisation level. It's acceptable for
that to be harder at high optimisation levels, but it should be possible. I find myself
doing this when I hit optimizer bugs, and want to make coherent bug reports. The
reports are much better if I can work out what's wrong in the generated code. I
haven't had to report many problems with Clang ... but I haven't turned up the
optimisation all the way either.

Related to optimisation levels, it's quite helpful to have a way of controlling
optimisation on a function-by-function level. This is very useful when you're trying
to work out where in a file with many functions an optimiser problem is happening;
it isn't foolproof, but it helps a lot.

--
John Dallman

From: cfe-dev-bounces at cs.uiuc.edu [mailto:cfe-dev-bounces at cs.uiuc.edu] On Behalf Of Renato Golin
Sent: 06 June 2013 21:41
To: LLVM Dev; Clang Dev
Subject: [cfe-dev] Meaning of LLVM optimization levels

Folks,

I'm trying to rationalize about optimization levels and maybe we should come up with a document like this:

http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Though, I remember a discussion a few months ago, and some people recommended we had names, rather than numbers, to dissociate the idea that 3 is better than 2. Regardless, would be good to have some guidelines on what goes where, so we don't end up in yet another long discussion about where to put the optimization <insert-name-here>.

As far as I can get from our side is:

-O3 : throw everything and hope it sticks
-O2 : optimized build, but should not explode in code size nor consume all resources while compiling
-O1 : optimized debug binaries, don't change the execution order but remove dead code and stuff
-O0 : don't touch it
-Os : optimize, but don't run passes that could blow up code. Try to be a bit more drastic when removing code. When in doubt, prefer small, not fast code.
-Oz : only perform optimizations that reduce code size. Don't even try to run things that could potentially increase code size.

I've been thinking about this, and I think, regarding those criteria, it would make sense to use a try/compare/rollback approach to some passes, at least the most dramatic ones.

For instance, the vectorizer keeps the old loops hanging, and under Os/Oz, it should be possible to rollback the pass if the end result is bigger. Of course, IR size has little to do with final code size, but that's why we have (and rely so much on) heuristics.

AFAIK, for that to work on any pass as they are, we'd have to implement a transactional model on IRBuilder, which is not trivial, but could be done. Does anyone have a strong opinion about this?

cheers,
--renato

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