Add evex512 target feature for AVX10

[sayantn]

Proposal

Intel has introduced the new AVX10.N-256 instruction set, which enables use of only the 256-bit instructions of the avx512 set. LLVM uses the evex512 feature flag to differentiate between avx512-avx10.N-512 and avx10.N-256. Due to rust-lang/rust#121088, the avx512 target features auto-enable evex512, making it impossible for Rust to use avx10.N-256. A solution will be to

• Add the evex512 target feature to Rustc
• Add the evex512 target feature to all 512-bit intrinsics in stdarch
• Revert Implicitly enable evex512 if avx512 is enabled rust#121088 and update the stdarch submodule

After this change,

• If CPU supports avx512 or avx10.N-512 instructions, it will enable avx512f (and its friends) and evex512
• If CPU supports avx10.N-256 instructions, it will enable avx512f (and its friends) only

(Known) Problems associated with this approach

• As a large part of the Rust ecosystem already uses avx512 (even though it is unstable), this would have a large impact - all of those crates will have to also check for evex512

• This would create a disparity between cpu features and rust target features

• The run-time detection for avx512 in std_detect has been stabilized, and we would need to change the semantics of avx512 feature detection - although this isn't much of a problem as there are no cpus with avx10 yet.

For reference, the Zulip thread is AVX10 target feature (re) organization

Alternatives

• As suggested by @Amanieu, we can add avx256f target-features, which LLVM interprets as only avx512f and the current semantic of avx512f can be preserved (See in Zulip). The possible counter-arguments will be too many target features (AVX512 already has 14, this would mean 14 more)

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Process

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

• 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:

@rustbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rustbot resolve reason-for-concern 

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

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