`-Ctarget-feature=+avx` doesn't enable `sse4.2`

[calebzulawski]

See example: https://rust.godbolt.org/z/GxMaecr4T

Adding either sse4.2 or crc32 works, so I believe the issue is due to the tied crc32 feature.

This issue is present on latest master.

[RalfJung]

Seems like LLVM doesn't consider this to be an implication. I don't know enough about target features to be able to say who is right here.

Cc @nikic

[calebzulawski]

X86.td does define avx as implying sse4.2 and -mavx in clang enables sse4.2, I think we're getting it lost somewhere because we define sse4.2 as enabling both sse4.2 and crc32. sse4.2 is the only x86 SIMD feature tied to another feature.

[RalfJung]

Ah, I see. So I guess we must have special logic then that determines when cfg!(target_feature = "sse4.2") holds (checking that both sse4.2 and crc32 are enabled), and maybe that logic is wrong and doesn't properly look at the LLVM-expanded version of the attribute set.

[RalfJung]

-mavx in clang enables sse4.2,

Does it enable crc32 as well?

[calebzulawski]

Yes, it does: https://c.godbolt.org/z/rrd7Pvr4T

[calebzulawski]

I'd be curious to see if this is a problem with other tied features, not just sse4.2

[calebzulawski]

Looking into it a bit more, I think it's incorrect for sse4.2 to enable crc32: https://reviews.llvm.org/D105462

crc32 appears to be a special feature that enables the crc32 instruction when other vector extensions are disabled, so I don't think it's ever necessary when sse4.2 is enabled. I think clang enables the __CRC32__ attribute if one of sse4.2 or crc32 are enabled.