Add pie relocation-model

[hlopko]

Proposal

We currently compile crates (--crate-type=rlib) in relocation-model=pic as PIC. When rustc knows the code will not be linked into a shared library (when it builds a binary crate) it uses PIE.

This is a reasonable default, PIC objects are relocatable and can be used for both executables and shared libraries. PIE objects are also relocatable, but their code can be faster than with PIC. The downside is that they cannot be used for shared libraries.

We would like introduce a way to build crates as PIE in performance-critical scenarios where we need relocatable code and we know we won't need to produce shared libraries.

We propose to add pie as another relocation-model value. Prototype PR: rust-lang/rust#88820.

Alternatives considered

LLVM doesn't have pie as a relocation-model value, instead it has a concept of PIC level and PIE level. If we wanted to mirror the LLVM design, we could add a new rustc option for controlling PIE when relocation-model=pic is used.

We think using pie relocation-model value is more understandable both from the user perspective and from the implementation side.

Mentors or Reviewers

@petrochenkov
@tmandry

Process

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

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

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.

[rustbot]

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.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[petrochenkov]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted