[theresistor]

> 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.

[pizlonator]

> It knows how to rematerialize

That's very cool, I didn't realize that.

> but the backend is generally more local/has less visibility than the optimizer

I don't really buy that. It's operating on SSA, so it has exactly the same view as LICM in practice (to my knowledge LICM doesn't cross function boundary).

LICM can't possibly know the cost of hoisting. Regalloc does have decent visibility into cost. Hence why this feels like a regalloc remat problem to me

[CalChris]

> to my knowledge LICM doesn't cross function boundary

LICM is called with runOnLoop() but is called after function inlining. Inlining enlarges functions, possibly revealing more invariants.