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

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• File an issue describing the proposal.
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• 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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<description of the concern> 

Concerns can be lifted with:

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See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[Urgau]

@rustbot second

[apiraino]

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