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

[Benno Lossin]

On 06.08.24 16:45, Miguel Ojeda 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 They were never much used, as they were effectively superseded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>
> ---
>  scripts/generate_rust_target.rs | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Works on my machine:

Tested-by: Benno Lossin <benno.lossin@xxxxxxxxx>

---
Cheers,
Benno

> diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
> index 87f34925eb7b..404edf7587e0 100644
> --- a/scripts/generate_rust_target.rs
> +++ b/scripts/generate_rust_target.rs
> @@ -162,7 +162,7 @@ fn main() {
>              "data-layout",
>              "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
>          );
> -        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
> +        let mut features = "-mmx,+soft-float".to_string();
>          if cfg.has("MITIGATION_RETPOLINE") {
>              features += ",+retpoline-external-thunk";
>          }
> @@ -179,7 +179,7 @@ fn main() {
>              "data-layout",
>              "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128",
>          );
> -        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
> +        let mut features = "-mmx,+soft-float".to_string();
>          if cfg.has("MITIGATION_RETPOLINE") {
>              features += ",+retpoline-external-thunk";
>          }
> 
> base-commit: de9c2c66ad8e787abec7c9d7eff4f8c3cdd28aed
> --
> 2.46.0
>