LTO builds on recent nightlies fail "to get bitcode from object file"

[phil-opp]

We recently got a bug report for the cargo-xbuild project that builds with LTO fail on recent nightlies: rust-osdev#69. The error message is "error: failed to get bitcode from object file for LTO (Bitcode section not found in object file)". This also seems to affect xargo.

@toku-sa-n created a minimal example to reproduce this issue at https://github.com/toku-sa-n/cargo_bug. Just clone the project and run xargo build --target cargo_settings with a recent nightly to see the error.

We are pretty sure that this is caused by https://github.com/rust-lang/cargo/pull/8192/files (bisect shows rustc commit rust-lang/rust#71925), but don't know what's the best way to fix this. The only thing that worked so far is to set RUSTFLAGS to -Clinker-plugin-lto when compiling the sysroot, but maybe there's a better way.

[RalfJung]

Thanks for the heads-up! I see the cargo-xbuild issue is closed by adding -Clinker-plugin-lto. Do you know if that's also what rustc bootstrap / -Zbuild-std are doing?

[phil-opp]

I couldn't find any related changes for -Zbuild-std, maybe I looked in the wrong places. Looking at rustc bootstrap is a good idea though. It seems like std is build with manually passing -Cembed-bitcode=yes via RUSTFLAGS, which apparently overrides the -Cembed-bitcode=no created by cargo:

https://github.com/rust-lang/rust/blob/a2e0b48e6eef854521d3199ee9e327aab298f071/src/bootstrap/compile.rs#L235-L245

I'll try ot change cargo-xbuild to try that approach instead of passing -Clinker-plugin-lto.

[RalfJung]

I couldn't find any related changes for -Zbuild-std, maybe I looked in the wrong places.

Maybe it doesn't need anything as it builds a special-purpose libstd just for this project -- so either they'll both be LTO or neither of them are.

[phil-opp]

Passing -Cembed-bitcode=yes via RUSTFLAGS seems to work. Both -Cembed-bitcode=no and -Cembed-bitcode=yes are passed now to rustc, but it seems like the latter wins. It seems a bit hacky to me, but I guess if rustc bootstrap does it this way it should be fine.

Interestingly, -Zbuild-std results in rustc commands without any -Cembed-bitcode arguments at all. So your guess might be right.

[johnthagen]

We see build breakages over at the min-sized-rust #16 repository as well. For a very minimal example, see:

• https://github.com/johnthagen/min-sized-rust/tree/1c13bc629f7df3e55b3e3fb712234c033e6dceab/xargo

For those trying to minimize the size of Rust binaries, using Xargo + LTO together is extremely important, so I'm hoping that this is able to be fixed in a way that is transparent to the end user.

[johnthagen]

For those looking to work around this issue, the easiest method I found was to add .cargo/config file in the root of the project with the contents:

[build]
# This is needed to work around: https://github.com/japaric/xargo/issues/292
rustflags = ["-Cembed-bitcode=yes"]

Though this work-around will make the projects not build on stable:

error: unknown codegen option: `embed-bitcode`

[RalfJung]

so I'm hoping that this is able to be fixed in a way that is transparent to the end user.

I don't think I'll have time to work on this (and AFAIK nobody else is on it, either). I'll happily review a PR though. :)

[RalfJung]

Though this work-around will make the projects not build on stable:

I mean xargo can't really be used with stable anyway, building libstd is a nightly-only feature that can only possibly be done on stable by using undocumented internal env vars that should not be used without blessing from the compiler team.

[johnthagen]

I mean xargo can't really be used with stable anyway, building libstd is a nightly-only feature that can only possibly be done on stable by using undocumented internal env vars that should not be used without blessing from the compiler team.

Agreed. It's a super minor point that probably doesn't matter much. Previously some of the examples in min-sized-rust could be built without change on both stable and Xargo with LTO (such as only using no_main or no_std). On stable you wouldn't get as much size reduction as with Xargo, but if you wanted to have the flexibility to choose you could before I added the .cargo/config files to those subprojects.

But yeah, I don't think it's a huge deal as people can read the docs and remove -Cembed-bitcode=yes to build on stable, or use RUSTFLAGS to dynamically pick. I was just trying to keep the examples as simple and flexible for people as possible.