[llvm-dev] Why does Machine LICM pass only examine outermost loops before RA?

[Danila Malyutin via llvm-dev]

Hello,

The Machine LICM pass has this comment:
// If this is done before regalloc, only visit outer-most preheader-sporting
// loops.

I was wondering why there is such a restriction? This prevents hoisting from the deeply-nested inner loops into the outer loop body, when the hoisted instruction is loop invariant everywhere except the outermost loop. I've encountered a problem that would be solved by such hoisting, however I couldn't figure out the rationale behind LLVM's MLICM decision. This change appears to be introduced in 2009 (https://github.com/llvm/llvm-project/commit/79618d1de89e76b1a23a02e8146057a6a21260db) but the only reason stated in the commit message seems to be just simplification/performance related.

Has anyone tried enabling MLICM for nested loops? Is it a good idea? What are the possible issues?

Thanks

--
Danila

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