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Compiler authors can often make compilation faster, but they have to work at it and think carefully through the problem. It has to be a focus not an afterthough

Compiler authors can often make compilation faster, but they have to work at it and think carefully through the problem. It has to be a focus not an afterthought. Years ago the GNAT Ada compiler authors intentionally avoided implementing precompiled headers. They instead focused on very fast lexical analysis (using a small handwritten lexer optimized for lower case letters because that was the usual case). By careful optimization of a small part of the compiler they made the recompilation of headers faster than loading a precompiled header!! More info: https://dwheeler.com/essays/make-it-simple-dewar.html https://dwheeler.com/essays/make-it-simple-dewar.html Maybe the Rust team needs to add timers to their test suite. This kind of big regression in compilation time should have been caught by the test suite.

estebank 5y ago
We track performance: https://perf.rust-lang.org/ https://perf.rust-lang.org/ I assure you it's not an afterthought. We track performance changes every week, going as far as reverting highly desired features to avoid regressions. The only reason we accept (small) regressions is for correctness fixes. This regression was only partly exercised by our perf test suite. That's how we grow it: see bad behaviour in the wild, add to the suite. The source of rust compilation slowness usually boil down to excercising a part of the language that has O(n²) behaviour, like match arm evaluation, trait bound evaluation, or huge expansion due to monomorphization or macro expansion, and the resulting huge linking times from that, or the bottleneck that proc-macros introduce (they get compiled before they can get executed, only relevant on fresh compiles). The regression was (partly) fixed by adding caching: https://github.com/rust-lang/rust/pull/90423/files https://github.com/rust-lang/rust/pull/90423/files and if I recall correctly was introduced by a feature change that fixed a bunch of incorrectly rejected deeply nested trait bounds.
- dwheeler 5y ago
  Excellent to hear! Don't get me wrong, I like Rust. I'm hoping we can get compilation speeds up over time, though, that is a common complaint.
- misnome 5y ago
  Thanks for reposting this. I recalled reading here a while ago some posts on why and how this was put into place, but I couldn't find the link.
pyjarrett 5y ago
In general, Ada has a pretty good compilation model. It has a formalized version of C++'s physical design with spec/body separation. It's also defined grammatically to required the context clause with dependencies first before everything else in a compilation unit (subunits or library units). `separate` also allows you to separately compile those things which might change a lot.

estebank 5y ago

We track performance: https://perf.rust-lang.org/ https://perf.rust-lang.org/ I assure you it's not an afterthought. We track performance changes every week, going as far as reverting highly desired features to avoid regressions. The only reason we accept (small) regressions is for correctness fixes. This regression was only partly exercised by our perf test suite. That's how we grow it: see bad behaviour in the wild, add to the suite. The source of rust compilation slowness usually boil down to excercising a part of the language that has O(n²) behaviour, like match arm evaluation, trait bound evaluation, or huge expansion due to monomorphization or macro expansion, and the resulting huge linking times from that, or the bottleneck that proc-macros introduce (they get compiled before they can get executed, only relevant on fresh compiles). The regression was (partly) fixed by adding caching: https://github.com/rust-lang/rust/pull/90423/files https://github.com/rust-lang/rust/pull/90423/files and if I recall correctly was introduced by a feature change that fixed a bunch of incorrectly rejected deeply nested trait bounds.

dwheeler 5y ago
Excellent to hear! Don't get me wrong, I like Rust. I'm hoping we can get compilation speeds up over time, though, that is a common complaint.
misnome 5y ago
Thanks for reposting this. I recalled reading here a while ago some posts on why and how this was put into place, but I couldn't find the link.

pyjarrett 5y ago

In general, Ada has a pretty good compilation model. It has a formalized version of C++'s physical design with spec/body separation. It's also defined grammatically to required the context clause with dependencies first before everything else in a compilation unit (subunits or library units). `separate` also allows you to separately compile those things which might change a lot.