[llvm-dev] InstCombine, graph rewriting, Equality saturation (original) (raw)

Daniel Neilson via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 11 08:14:52 PDT 2017

Just thinking out loud…. I’m really not familiar with the vast majority of what instcombine does, so please excuse me if my naiveté is too obvious. I can’t help but notice all of the uses of ‘and’ in Daniel B’s description of what instcombine is right now:

On Sep 8, 2017, at 11:27 PM, Daniel Berlin via llvm-dev <llvm-dev at lists.llvm.org> wrote:

FWIW, Before getting to "how to do it", I think InstCombine needs an actual goal. Right now it's "do a bunch of instruction combination and canonicalization and random kinds of semi-local optimization with some weird analysis and other stuff in the middle. Iterate this as hard as we can" Nobody can really draw any real dividing line for what should be there or not except based on how easy or expensive it is to shove it in. That's a recipe for pass creep.

This makes me wonder… is it sensible to be talking about splitting up instcombine into multiple separate passes? Would such a thing even be possible? For example, split by functionality into separate passes that each do one of:

Or something like that.

As separate passes, each would probably have a natural way to be implemented effectively and those implementations might vary.

-Daniel

Daniel Neilson, Ph.D. Azul Systems

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