Forbid disabling SSE on x86 targets that have SSE in their "baseline"

[RalfJung]

Proposal

Passing -Ctarget-feature=-sse on an x86-64 target currently produces an ugly LLVM error. Doing the same on a x86-32 target leads to unsound floating-point behavior.

Therefore, I think we should deprecate and eventually fully forbid toggling the sse/sse2 target features on x86 targets, except for those targets that do not have these features to begin with (e.g. i586-unknown-linux-gnu).

Here, compiler team triage decided "Current Tier 1 x86 targets require SSE-based floats at minimum"; this MCP is basically intended reaffirm that position with a more formal process.

I am implementing some machinery here that could help with that, but properly implementing this will be tricky since one can also use -Ctarget-cpu to disable these target features.

Once this is implemented, we have some options for improving the Rust ABI on these targets as well:

• on x86-32, we could use SSE registers to return float values, instead of PassMode::Indirect
• on all x86 targets, we could pass SIMD vectors of up to 128 bits in registers rather than indirectly

Mentors or Reviewers

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Process

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

• [x] 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.

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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.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

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cc @rust-lang/compiler

[Urgau]

@rustbot second