[llvm] [LoopInterchange] Enable it by default (PR #124911)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> When do we say the penalty on compile-time is acceptable?

ConstraintElim is comparable in complexity: it gets turned on [here](https://llvm-compile-time-tracker.com/compare.php?from=f01a3a893c147c1594b9a3fbd817456b209dabbf&to=577688758ef64fb044215ec3e497ea901bb2db28&stat=instructions:u). Note that there were a lot of binary changes, which justified a geomean regression of 0.3% on ThinLTO. It also does a good job of not blowing up compile-time in degenerate cases (so, never a 0.76%).

That said, I'm very enthusiastic about the prospect of DA operating in LLVM: once the patch lands, I will attempt to add runtime-checks functionality to DA, and gradually move LAA's users to DA (this will be a long-running project). Once there are more users of DA in the tree, and with the potential deprecation of LAA, I expect the costs to eventually be amortized. That said, we don't have very many contributors who have expertise in DA/LCA/LI at this point, and this is a chicken-and-egg problem: I hope that we will further be able to reduce/amortize costs after landing.

https://github.com/llvm/llvm-project/pull/124911