Comment by karavelov
5 hours ago
Why not use LLD for linking Rust? You already must have the LLVM for Rust to be buildable, so that do not add any dependency.
5 hours ago
Why not use LLD for linking Rust? You already must have the LLVM for Rust to be buildable, so that do not add any dependency.
We use the llvm linker, lld, to link mold at the earliest point we can, then use mold exclusively after that. We are an LLVM native distro so we build llvm right away as the system compiler used to build the whole tree instead of gcc.
LLD would work fine for the whole tree, and did previously, but is much much slower than mold which is why we switched to mold.
Having to build all dependencies of rust including python and openssl and everything else before being able to use mold erases most of the full-tree build speedups as the path to rust is already about 80% of the full tree build time.
We are a distro that mandates independently verified 100% deterministic builds from source for any given release commit of the tree, so we have to build the whole tree several times for every release.
Can't you just keep using Mold2 for the initial toolchain bootstrap and then build mold3 with Rust as part of the "final" set of toolchains that are to be used to compile everything beyond that? It isn't perfect but it's congruent with how many other components in most open reproducible bootstrapping flows work; ie using GCC 4.4 or whatever just to bootstrap GCC 10. It's annoying but it only needs to be done once and then you just use it to bootstrap newer components forever. Doesn't help with compile times, though.
I would agree if not for the fact rust depends on almost the entire tree, so doing what you propose would increase our full tree verification time from 6ish hours to 12ish hours erasing any wins mold was giving us in the first place.
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Agreed -- not my wheelhouse, but couldn't you just build rust earlier in your process using LLD, then build mold, then build everything else?
Would that meant that building rust is slower, but everything else is the same, and you don't have to maintain a fork of another complex project?
Rust requires python and perl and musl and openssl and 80% of the entire wall time of building the whole tree.
Rust is the single most expensive thing to bootstrap in any given linux distro.
It is the thing you need mold the most for to speed things up.
Hopefully, someone can do some work on decreasing the requirements needed to build rust itself and ease bootstrapping.
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Because mold is faster. Linking is a significant bottleneck when building large projects.
But we're just talking for bootstrapping Rust here.
The complaint is about bootstrapping, where speed is of little relevance.
It is of extreme relevance for us as we have to re-bootstrap every time we change any dependency of rust, and rust depends on almost everything that is expensive to build in a toolchain tree.
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