Re: [PATCH] rust: x86: remove `-3dnow{,a}` from target features

[Alice Ryhl]

On Tue, Aug 6, 2024 at 4:47 PM Miguel Ojeda <ojeda@xxxxxxxxxx> wrote:
>
> LLVM 19 is dropping support for 3DNow! in commit f0eb5587ceeb ("Remove
> support for 3DNow!, both intrinsics and builtins. (#96246)"):
>
>     Remove support for 3DNow!, both intrinsics and builtins. (#96246)
>
>     This set of instructions was only supported by AMD chips starting in
>     the K6-2 (introduced 1998), and before the "Bulldozer" family
>     (2011). They were never much used, as they were effectively superseded
>     by the more-widely-implemented SSE (first implemented on the AMD side
>     in Athlon XP in 2001).
>
>     This is being done as a predecessor towards general removal of MMX
>     register usage. Since there is almost no usage of the 3DNow!
>     intrinsics, and no modern hardware even implements them, simple
>     removal seems like the best option.
>
> Thus we should avoid passing these to the backend, since otherwise we
> get a diagnostic about it:
>
>     '-3dnow' is not a recognized feature for this target (ignoring feature)
>     '-3dnowa' is not a recognized feature for this target (ignoring feature)
>
> We could try to disable them only up to LLVM 19 (not the C side one,
> but the one used by `rustc`, which may be built with a range of
> LLVMs). However, to avoid more complexity, we can likely just remove
> them altogether. According to Nikita [2]:
>
> > I don't think it's needed because LLVM should not generate 3dnow
> instructions unless specifically asked to, using intrinsics that Rust
> does not provide in the first place.
>
> Thus do so, like Rust did for one of their builtin targets [3].
>
> For those curious: Clang will warn only about trying to enable them
> (`-m3dnow{,a}`), but not about disabling them (`-mno-3dnow{,a}`), so
> there is no change needed there.
>
> Cc: Nikita Popov <github@xxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Link: https://github.com/llvm/llvm-project/commit/f0eb5587ceeb641445b64cb264c822b4751de04a [1]
> Link: https://github.com/rust-lang/rust/pull/127864#issuecomment-2235898760 [2]
> Link: https://github.com/rust-lang/rust/pull/127864 [3]
> Closes: https://github.com/Rust-for-Linux/linux/issues/1094
> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>

Works with Rust Binder on the x86 cuttlefish emulator.

Tested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>