The LLVM we ship in `rust-dev` doesn't match the LLVM we ship in the `rustc` component

[jyn514]

I tried this code:

; rustc +stage2 --print=target-spec-json --target loongarch64-unknown-linux-gnu -Zunstable-options

I expected to see this happen: rustc prints the target spec successfully, because it knows it exists:

; rustc +stage2 --print=target-list | grep loong
loongarch64-unknown-linux-gnu

Instead, this happened: rustc fails to load the LLVM specification:

; rustc +stage2 --print=target-spec-json --target loongarch64-unknown-linux-gnu -Zunstable-options
error: could not create LLVM TargetMachine for triple: loongarch64-unknown-linux-gnu: No available targets are compatible with triple "loongarch64-unknown-linux-gnu"

Note that this is different from the error we emit for an unrecognized target:

; rustc +stage2 --print=target-spec-json --target unknown -Zunstable-options
error: Error loading target specification: Could not find specification for target "unknown". Run `rustc --print target-list` for a list of built-in targets

Also, this only happens for local builds, because we don't use the LLVM in the rust-dev component in rustup, only when building rustc from source.

... wtf ???

Meta

HEAD is branched from ce1073b

[jyn514]

cc @zhaixiaojuan @heiher, do you know what could be going wrong here?

[heiher]

@jyn514 Thank you for bringing this to my attention. It is possible that the locally builds has enabled llvm.download-ci-llvm, and that prebuilt LLVM does not support LoongArch architecture. It would be great if you could provide specific steps to reproduce. Thank you.

[jyn514]

Yes, that's exactly what's happened, I have download-ci-llvm enabled and the prebuilt LLVM differs.

Here's steps to reproduce:

rm config.toml  # download-ci-llvm is on by default
x build --stage 0 rustc
build/host/stage1/bin/rustc --print=target-spec-json --target loongarch64-unknown-linux-gnu -Zunstable-options

[jyn514]

The RustDev component builds LLVM here:

rust/src/bootstrap/dist.rs

Line 2239 in 276fa29

builder.ensure(crate::llvm::Llvm { target });

but I'm not sure how that differs from the Rustc component?

rust/src/bootstrap/dist.rs

Lines 456 to 461 in 276fa29

	// Copy libLLVM.so to the lib dir as well, if needed. While not
	// technically needed by rustc itself it's needed by lots of other
	// components like the llvm tools and LLD. LLD is included below and
	// tools/LLDB come later, so let's just throw it in the rustc
	// component for now.
	maybe_install_llvm_runtime(builder, host, image);

[heiher]

I think the difference is that the llvm.targets of ci-llvm does not contain LoongArch, but locally it does. Is it time to update ci-llvm?

[nikic]

Maybe need to bump download-ci-llvm-stamp to pick up the LoongArch enablement?

[heiher]

@jyn514 @nikic Thank you for catching that. I was mistaken to think that the cached prebuilt ci-llvm would automatically update over time.

[jyn514]

@heiher no worries :) it updates every six weeks when we make a new release, or if the submodule is updated, but it's cached until then so it's not constantly re-uploaded on every change to bootstrap.

[RalfJung]

Is there any way CI could help prevent such problems? Like, complain if compiler/rustc_llvm gets changed but the stamp file does not, or something like that?

[jyn514]

@RalfJung we can use heuristics, but they're only heuristics and I wouldn't want to fail CI on them. That said, having rustbot leave a comment if you touch rustc_llvm or src/bootstrap/llvm.rs seems reasonable, so the PR author can make a judgement call but also doesn't forget about the stamp.