They could, and they do.

That said, they don't have infinite time, and if, as in this case, the upstream fix would: (1) be pretty involved and (2) be very likely to get regressed because upstream doesn't have the capability to run tests that would prevent that (e.g. because upstream only runs C++ compilation tests and there is no way to exercise the relevant bugs via C++ code), then investing in fixing upstream may not be the right tradeoff.

In theory, one could first change upstream's test harness to allow Rust code, but that involves upstream tests depending on the Rust compiler frontend, which apart from being a technical problem is probably a political one.

Maybe it would have been possible to do upstream tests via bitcode source instead of Rust or C++; I don't know about LLVM to say offhand. But in either case this is not as easy as just "fix a simple upstream bug"...

Upstream tests are generally done at the LLVM IR level actually. It's mostly just a question of (1) time; (2) worries about ongoing maintenance work upstream; (3) a general feeling that perhaps such optimizations are best done on MIR anyway, because they'll be more effective there than they would be in LLVM.

You're suggesting that rustc should do noalias optimizations on MIR? I'm skeptical of that idea... A lot of duplicate loads that would benefit from being coalesced are only visible after LLVM inlining.

Obviously MIR inlining needs to happen first (and I think it does happen already?) But to me it's clearly the right solution going forward. LLVM's noalias semantics are much weaker than what we can have on MIR, with full knowledge of transitive immutability/mutability properties.

'Classic' LLVM noalias as a function parameter attribute is weak, but the metadata version is much more flexible. I looked into it in the past and IIRC it's not a perfect match for Rust semantics, but close enough that rustc could still use it to emit much more fine-grained aliasing info; it just doesn't. But there was also a plan on LLVM's end to replace it with yet a third approach, as part of the grand noalias overhaul that's also supposed to fix the bug. Not sure if there's been any progress on that.

As far as I can tell, MIR inlining currently happens with -Z mir-opt-level=2 or higher, and that is not implied by -O. But I have no idea what future plans exist in that area.

I admit I have a bias here: it feels to me like everyone (not just Rust) is running away from doing things at LLVM IR level, and the resulting duplication seems inelegant. But on reflection, part of the reason I have that feeling is that I've recently been spending time fixing up Clang's control-flow graph functionality... which itself is 12 years old, so it's not a new trend at all!

Wouldn't MIR be more portable to the future as well? Building on Rust's own equity and all that, because future Rust will probably still use MIR but could replace LLVM(?)

They wouldn't want to do MIR without upstreaming the Rust frontend, which I don't see happening anytime soon.

MIR should stay with Rustc, and that's the point — to work on optimizations that happen in Rustc, and not later when the code is turned over to llvm, or other backend.

That's often done, but Rust is shipped on all major Linux distros using the system LLVM, which is often at least 6 months old, or years old, so it needs the workarounds anyways to be able to work with those. The LLVM fixes take a while to percolate back, and Rust supports up to LLVM versions that are ~2 years old (LTS linux distros). The workarounds can only be removed once versions without the fix are no longer supported.

They actually do sometimes AFAIK