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Comment by asveikau | Hacker News Reader

I'm not personally a rust zealot, but I know that rust builds on LLVM and therefore benefits from some of that very same engineering effort that was built to optimize C and C++.

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Clang usually has worse performance than GCC.

Is there anything specific GCC does for performance that Clang/LLVM could adopt? How is this race expected to evolve over time?

it's not so much that gcc does anything specific so much as LLVM is just really really inefficient—they don't track compilation time at all so it's easy for releases to regress, half the stuff in there is academics implementing their PhD thesis aiming for algorithmic accuracy with little regard for efficiency, and LLVM's design itself is somewhat inefficient (multiple IRs, lots and lots of pointers in IR representation, etc)

that said this makes it an excellent testbed but compilation time will keep getting slower every release until they start caring about it

The rust compiler team now tracks LLVM performance, see [0]. They are able to spot regressions as soon as they get merged, and petition to get the change reverted or performance fixed when the impact is large. This has been fairly successful so far, but obviously needs sustained efforts if we are to ever dream of ever getting halfway decent compilation times.

[0]: https://nikic.github.io/2020/05/10/Make-LLVM-fast-again.html

Not answering your question but in my experience reading assembly output of the two compilers the unoptimized output of Clang is atrocious while GCC is closer of what a human could have wrote. Clang seems to have to do better in optimization passes to archive similar results. Usually Clang was a faster compiler in O1 but I don’t think it’s true anymore.

that's just because gcc has certain optimization passes that can't be disabled

(that said gcc -O0 is still absolutely nothing like what a human would write)

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