FLfl0ki·Feb 23, 2024
> In theory, Rust allows even better optimizations than C thanks to stricter immutability and aliasing rules, but in practice this doesn't happen yet. Optimizations beyond what C does are a work-in-progress in LLVM, so Rust still hasn't reached its full potential.
Really glad to see this mentioned, I search for it in every post like this.
LLVM should follow now that the mutable noalias bugs appear to be fixed. Remember when it was a meme that every time it gets enabled in rustc, it has to be disabled again in the next x.y.1 release? It's been left enabled for years.
The biggest challenge here is that most people aren't looking for optimizations beyond what they were already getting with C. Like when Google was switching from GCC to Clang, they'd pick apart the asm and report bugs for Clang and LLVM to converge on GCC. That was a big part
Read the rest
of why the performance gap closed over the years. But that was closing a gap, not speculatively overtaking them.
Then there's a whole other angle; we now know just how much machine code layout can affect real-world performance, in most cases much more than aliasing rules, but we still haven't made it a seamless part of our build tooling. PGO is now fairly easy, but BOLT is still a pain to integrate for no clear reason, and PROPELLER has been left in an embarrassing state where even its own PDF link is broken [1][2].
[1] https://github.com/google/llvm-propeller
[2] To be fair, it may not be Google's highest priority at this time.