Bootstrap is slow · Issue #126423 · rust-lang/rust (original) (raw)

A fully cached ./x run --stage 1 miri takes 2.35s. This is really slow, and added to most bootstrap invocations.

Bootstrap is doing a lot of work, and it's nontrivial to optimize all of that, but it leads to these slow times.
There's not gonna be a silver bullet, but I believe this is something we should at least consider and attempt to improve if possible.

One particularly slow aspect is updating the submodules. Another thing I found in my profile is that it spends 10% of the time parsing C++. I do not think every bootstrap invocation should involve parsing C++ code. There's also lots of overhead from cargo, maybe there are unnecessary cargo invocations?

I've attached the flamegraph from perf record --call-graph=dwarf ./x run --stage 1 miri.