error: failed to get bitcode from object file for LTO

[toku-sa-n]

Hello. I use cargo xbuild to build my OS. Recently build failed with the following error message.

error: failed to get bitcode from object file for LTO (Bitcode section not found in object file)

My repository is this and cargo_settings.json is specified as the target. You can reproduce this by running make on the master branch. I also tried with phil-opp's .json file, but build failed. The only thing which made it possible to build is to disable lto ( lto = false in Cargo.toml) . cargo clean && cargo xbuild did not solve the problem.
This is my CI history. You can see that the same commit succeeds and fails to build.

[phil-opp]

I never seen this error before, but it sounds more like a rust/cargo problem. Have you tried updating your nightly?

[phil-opp]

If it still occurs on the current nightly, it might be worth opening an issue in the https://github.com/rust-lang/rust repository.

[ascjones]

I'm currently investigating: this looks to be the culprit rust-lang/cargo#8192.

Compiling with rlib enabled in Cargo.toml works, produces the following rustc flags:

-Cembed-bitcode=no

With rlib removed we get:

-C lto

Manually running rustc with -C lto replaced with -C linker-plugin-lto seems to work okay.

I'll look a bit further but that is what I've figured out so far.

[phil-opp]

@ascjones Thanks for investigating! I think you're right that this pull request caused it. We should definitely open a cargo issue with an example that reproduces this eror.

It seems like the rusty-hermit project has the same issue: hermit-os/hermit-rs#15, so it might make sense to cc the project developers in the issue.

[toku-sa-n]

I created a minimal example to reproduce: https://github.com/toku-sa-n/cargo_bug
cargo xbuild should cause the error.

[phil-opp]

Thanks for the example @toku-sa-n. By bisecting rustc I found out that this error is caused by rust-lang/rust#71528, which in combination with rust-lang/cargo#8066 results in a more aggressive removal/ of bitcode for crates that have lto disabled. Since we compile the sysroot as a separate crate, this means that it gets compiled without bitcode because we don't have lto enabled for it.

I think the most simple fix is to always enable lto for sysroot builds. I will create a PR in a moment.

[phil-opp]

I created a fix at #70.

[toku-sa-n]

Still failing with cargo-xbuild 0.5.30 and rustc 1.45.0-nightly (9912925c2 2020-05-10). I tried cargo clean && cargo xbuild on my minimum example, but build failed with the same error. Is it better to send a issue on cargo project?

[ascjones]

Can confirm I also get the same error with the new version 0.5.30

[phil-opp]

Seems like I messed something up… I can also still reproduce the error. I'll take a look at it again right now.

[phil-opp]

Still investigating, but it seems like version 0.5.30 only fixed the breakage between nightlies 2020-04-30 and 2020-05-01. There seems to be another breakage between 2020-05-01 and current nightlies. Starting a new bisect run.

[phil-opp]

Regression is in rust-lang/rust#71925, so it is likely rust-lang/cargo#8192, as @ascjones suspected. I'm still not sure what the best fix is.

I think I will report this to the xargo project too since it also seems to affect them. Maybe they have an idea how to fix this.