linux-kernel - Re: [PATCH v3 0/2] sparc/build: Rework CFLAGS for clang compatibility

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Message-ID: <cfc033a8-a7de-469b-8afa-0069593c5c6f@gaisler.com>
Date: Sat, 16 Nov 2024 09:31:16 +0100
From: Andreas Larsson <andreas@...sler.com>
To: koachan@...tonmail.com, "David S. Miller" <davem@...emloft.net>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 glaubitz@...sik.fu-berlin.de, Masahiro Yamada <masahiroy@...nel.org>,
 Nicolas Schier <nicolas@...sle.eu>, Jonathan Corbet <corbet@....net>
Cc: sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 0/2] sparc/build: Rework CFLAGS for clang compatibility

On 2024-10-29 15:49, Koakuma via B4 Relay wrote:
> Hello~
> 
> This changes the CFLAGS for building the SPARC kernel so that it can be
> built with clang, as a follow up from the discussion in this thread:
> 
> https://lore.kernel.org/lkml/JAYB7uS-EdLABTR4iWZdtFOVa5MvlKosIrD_cKTzgeozCOGRM7lhxeLigFB1g3exX445I_W5VKB-tAzl2_G1zCVJRQjp67ODfsSqiZWOZ9o=@protonmail.com/T/#u
> 
> The changes are removal of various `-fcall-used-*` flags, and adding
> clang target flags for SPARC:
> 
> - `-fcall-used-*` flags is gated behind cc-option as it is
>   not supported in clang. It should be safe; clang won't use the registers
>   specified as temporaries, but it is a safe change wrt. the ABI.
>   Assembly code can still use those registers as needed.
>   A cursory look at the assembly generated by GCC 13.2 shows that
>   the compiler was able to reallocate uses of those registers into
>   other temporary registers without adding extra spills, so there
>   should be no change in performance.
> 
> - More trivial is to add CLANG_TARGET_FLAGS for SPARC target.
> 
> Building with these changes still result in a working kernel,
> at least for Sun T5120, Oracle T4-1, and qemu virtual machines.
> 
> On the LLVM side, the effort for building Linux/SPARC is tracked here:
> https://github.com/llvm/llvm-project/issues/40792
> 
> Signed-off-by: Koakuma <koachan@...tonmail.com>
> ---
> Changes in v3:
> - Use cc-option to allow GCC to still use -fcall-used-* flags.
> - Add documentation on building on SPARC, along with required LLVM version.
> - Link to v2: https://lore.kernel.org/r/20240717-sparc-cflags-v2-0-259407e6eb5f@protonmail.com
> 
> Changes in v2:
> - Remove the -mv8plus change; it will be handled on clang side:
>   https://github.com/llvm/llvm-project/pull/98713
> - Add CLANG_TARGET_FLAGS as suggested in v1 review.
> - Link to v1: https://lore.kernel.org/r/20240620-sparc-cflags-v1-1-bba7d0ff7d42@protonmail.com
> 
> ---
> Koakuma (2):
>       sparc/build: Put usage of -fcall-used* flags behind cc-option
>       sparc/build: Add SPARC target flags for compiling with clang
> 
>  Documentation/kbuild/llvm.rst | 3 +++
>  arch/sparc/Makefile           | 4 ++--
>  arch/sparc/vdso/Makefile      | 2 +-
>  scripts/Makefile.clang        | 1 +
>  4 files changed, 7 insertions(+), 3 deletions(-)
> ---
> base-commit: c2ee9f594da826bea183ed14f2cc029c719bf4da
> change-id: 20240620-sparc-cflags-e7f2dbbd4b9d
Reviewed-by: Andreas Larsson <andreas@...sler.com>

I see no pressing need for a v4. Picking the series up to my for-next.

Thanks,
Andreas