alt Hacker NewsThis is a good write up and I agree with pretty much all of it.
Two comments:
- LLVM IR is actually remarkably stable these days. I was able to rebase Fil-C from llvm 17 to 20 in a single day of work. In other projects I’ve maintained a LLVM pass that worked across multiple llvm versions and it was straightforward to do.
- LICM register pressure is a big issue especially when the source isn’t C or C++. I don’t think the problem here is necessarily licm. It might be that regalloc needs to be taught to rematerialize
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"LLVM IR is actually remarkably stable these days."
I'm by no means an LLVM expert but my take away from when I played with it a couple of years ago was that it is more like the union of different languages. Every tool and component in the LLVM universe had its own set of rules and requirements for the LLVM IR that it understands. The IR is more like a common vocabulary than a common language.
My bewilderment about LLVM IR not being stable between versions had given way to understanding that this freedom was necessary.
Do you think I misunderstood?
There is a rematerialize pass, there is no real reason to couple it with register allocation. LLVM regalloc is already somewhat subpar.
What would be neat is to expose all right knobs and levers so that frontend writers can benchmark a number of possibilities and choose the right values.
I can understand this is easier said than done of course.
> It might be that regalloc needs to be taught to rematerialize
It knows how to rematerialize, and has for a long time, but the backend is generally more local/has less visibility than the optimizer. This causes it to struggle to consistently undo bad decisions LICM may have made.