Change the ABI of powerpc64-unknown-linux-gnu to ELFv2

[Gelbpunkt]

Proposal

I propose changing the ABI of the powerpc64-unknown-linux-gnu target from ELFv1 to ELFv2. This target is currently in tier 2 with host tools, but has no dedicated maintainers, which I believe is largely due to the legacy ABI it uses. I would like to maintain the target, but only if it were to use the ELFv2 ABI.

I have previously proposed a powerpc64-unknown-linux-gnuelfv2 target in rust-lang/rust#144220, but LLVM maintainers were concerned about Rust inventing triples that LLVM does not know about. In the process of that, I did the preliminary work for enabling ELFv2 on a Glibc target in Rust in llvm/llvm-project#149725.

The rationale for the ELFv2 is rather simple: it is the more modern ABI and has less overhead. For those reasons, it is also more widely used and supported. For example, LLD only supports the ELFv2 ABI, LLDB AFAICT only supports ELFv2, Zig has dropped support for ELFv1 and assumes all ppc64 targets to use ELFv2, Go is switching to ELFv2 for ppc64 in 1.27.

Distribution support for ELFv2 is also generally better than ELFv1. Most of the best-supported distributions on ppc64 are musl-based, which is ELFv2 only, but some Glibc-based distributions exist, for example ArchPOWER or Void Linux, which means users of these distributions (which are very actively maintained) cannot make use of the current definition of the powerpc64-unknown-linux-gnu target. Out of other Glibc-based distributions, I'm aware of Gentoo, which currently uses ELFv1 for Glibc but appears to be aware that ELFv2 is a future option for them. NixOS on ppc64 supports both ELFv1 and ELFv2 with Glibc.

I'm interested in maintaining the target, but it only has value to me and much of the ppc64 userbase with the ELFv2 ABI. Since the necessary LLVM changes are present since 22.1.0, we can now finally make the switch. If anyone's interested in keeping ELFv1 on Glibc alive, they can create a separate target, but I believe ELFv2 is the more reasonable default nowadays.

Mentors or Reviewers

none

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member who is knowledgeable in the area can second by writing @rustbot second or kickoff a team FCP with @rfcbot fcp $RESOLUTION.
  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

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cc @rust-lang/compiler

[doko42]

are you sure, that you want to do that for powerpc64-unknown-linux-gnu, and not powerpc64le-unknown-linux-gnu instead?

I am not aware of any distribution using ELFv2 for the big-endian target

[glaubitz]

There are zero users for ELFv2 on big-endian ppc64, so switching the target to ELFv2 would render the target completely useless.

Please don't do this!

Thanks!

CC @thesamesam

[glaubitz]

FWIW, both Debian and Gentoo use ELFv1 and basically the whole ecosystem. I don't know of any relevant usage of ArchPOWER or VoidLinux for PowerPC. Also, the latter project hasn't seen any development for three years.

[jieyouxu]

Please note that the MCP text in the issue as-is currently is outdated: please see discussion in the MCP thread

(In particular, the present direction is not to change the existing powerpc64-unknown-linux-gnu target's ABI)

[glaubitz]

It should also not be used in the future as the motivation for this change makes no sense.

Big-endian ppc64 is a legacy target used on legacy systems such as the Apple PowerMac G5 (PPC970-based) or the Freescale e5500 embedded PowerPC CPU. Both these systems have a baseline around POWER4 and won't run any modern little-endian distributions as these use the POWER8 baseline or higher.

Users with a POWER8 or higher machine will be running the little-endian distributions anyway.

[RalfJung]

As was already said in two comments above, this issue is not meant to be used for technical discussion.
Please take that to Zulip. I will lock the issue for now.