Portable SIMD project group

[hsivonen]

Proposal

Create a project group for considering what portable SIMD in the standard library should look like.

As a practical matter, I think the multi-ISA-tested running code in the packed_simd crate should be used as the starting point. Compiler back ends in general and LLVM in particular already have a dominant design for the general shape of portable SIMD. It makes sense to expose that kind of general API shape in the standard library in order to avoid exposing lower-level compiler intrinsics. The packed_simd crate already has multi-ISA-tested running code for an API shape that is a good fit for what compiler back ends in general and LLVM in particular expose in this domain. Other kinds of designs can be layered over this by the crate ecosystem.

Notably, packed_simd is already a restart from scratch compared to the simd crate. While the internals differ, the general API shape ended up being similar despite initially trying to simplify things away. (Notably, packed_simd initially tried to not have boolean/mask vectors that simd had but LLVM doesn't have but ended up re-introducing them.) It's not a good use of anyone's time to start from scratch again.

See an FAQ for more.

Mentors or Reviewers

@joshtriplett advised me to file an MCP about this.

Process

The main points of the Major Change Process is 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.

[nikomatsakis]

Closing in favor of rust-lang/lang-team#29