[PATCH v3 0/2] sparc/build: Rework CFLAGS for clang compatibility

[Koakuma via B4 Relay <devnull+koachan.protonmail.com@kernel.org>]

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@protonmail.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

Best regards,
-- 
Koakuma <koachan@protonmail.com>