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I believe this is mostly due to the switch to LLVM 13[1].

[1]: https://twitter.com/ryan_levick/status/1443202538099073027

I believe the new pass manager isn’t due to be enabled by default until the next version (1.57)

Yeah the significant improvements from 13 will require that last I heard.

That seems to have been a mixed bag. But they also enabled PGO (or was it LTO?) and that was mentioned to be a bigger improvement.

PGO requires a runtime profile, so I doubt they've enabled that by default :-)

Rust has had LTO for quite a while, and it's normally a source of longer compilation times rather than shorter ones (since LTO in LLVM-world involves mashing all of the bitcode together and (re-)running a lot of expensive analyses to further optimize across translation unit boundaries.

OTOH they've been making continuous improvements to the incremental compilation mode since 1.51/2, so that's probably among the sources of improvements here.

They're referring to the use of PGO when building the compiler itself: https://github.com/rust-lang/rust/pull/88069

TIL. That's fantastic!

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