Re: [PATCH v2 00/14] Remove clang's -Qunused-arguments from KBUILD_CPPFLAGS

[Masahiro Yamada <masahiroy@kernel.org>]

On Thu, Jan 12, 2023 at 12:05 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Hi all,
>
> Clang can emit a few different warnings when it encounters a flag that it
> recognizes but does not support internally. These warnings are elevated to
> errors within {as,cc}-option via -Werror to catch unsupported flags that should
> not be added to KBUILD_{A,C}FLAGS; see commit c3f0d0bc5b01 ("kbuild, LLVMLinux:
> Add -Werror to cc-option to support clang").
>
> If an unsupported flag is unconditionally to KBUILD_{A,C}FLAGS, all subsequent
> {as,cc}-option will always fail, preventing supported and even potentially
> necessary flags from getting adding to the tool flags.
>
> One would expect these warnings to be visible in the kernel build logs since
> they are added to KBUILD_{A,C}FLAGS but unfortunately, these warnings are
> hidden with clang's -Qunused-arguments flag, which is added to KBUILD_CPPFLAGS
> and used for both compiling and assembling files.
>
> Patches 1-4 address the internal inconsistencies of invoking the assembler
> within kbuild by using KBUILD_AFLAGS consistently and using '-x
> assembler-with-cpp' over '-x assembler'. This matches how assembly files are
> built across the kernel and helps avoid problems in situations where macro
> definitions or warning flags are present in KBUILD_AFLAGS, which cause
> instances of -Wunused-command-line-argument when the preprocessor is not called
> to consume them. There were a couple of places in architecture code where this
> change would break things so those are fixed first.
>
> Patches 5-12 clean up warnings that will show up when -Qunused-argument is
> dropped. I hope none of these are controversial.
>
> Patch 13 turns two warnings into errors so that the presence of unused flags
> cannot be easily ignored.
>
> Patch 14 drops -Qunused-argument. This is done last so that it can be easily
> reverted if need be.
>
> This series has seen my personal test framework, which tests several different
> configurations and architectures, with LLVM tip of tree (16.0.0). I have done
> defconfig, allmodconfig, and allnoconfig builds for arm, arm64, i386, mips,
> powerpc, riscv, s390, and x86_64 with GCC 12.2.0 as well but I am hoping the
> rest of the test infrastructure will catch any lurking problems.
>
> I would like this series to stay together so that there is no opportunity for
> breakage so please consider giving acks so that this can be carried via the
> kbuild tree (and many thanks to the people who have already provided such
> tags).


All applied to linux-kbuild. Thanks.

I left small comments in 07/14.








> ---
> Changes in v2:
> - Pick up tags where provided (thank you everyone!)
> - Patch 6 and 9: Clarify that '-s' is a compiler flag that is only relevant to
>   the linking phase and remove all mention of the assembler's '-s' flag, as the
>   assembler is never directly invoked (Nick, Segher)
> - Patch 7: Move '-z noexecstack' into new ldflags-y variable (Nick)
> - Patch 8: Reword commit message to explain the problem in a clearer manner
>   (Nick)
> - Link to v1: https://lore.kernel.org/r/20221228-drop-qunused-arguments-v1-0-658cbc8fc592@kernel.org
>
> ---
> Nathan Chancellor (12):
>       MIPS: Always use -Wa,-msoft-float and eliminate GAS_HAS_SET_HARDFLOAT
>       MIPS: Prefer cc-option for additions to cflags
>       powerpc: Remove linker flag from KBUILD_AFLAGS
>       powerpc/vdso: Remove unused '-s' flag from ASFLAGS
>       powerpc/vdso: Improve linker flags
>       powerpc/vdso: Remove an unsupported flag from vgettimeofday-32.o with clang
>       s390/vdso: Drop unused '-s' flag from KBUILD_AFLAGS_64
>       s390/vdso: Drop '-shared' from KBUILD_CFLAGS_64
>       s390/purgatory: Remove unused '-MD' and unnecessary '-c' flags
>       drm/amd/display: Do not add '-mhard-float' to dml_ccflags for clang
>       kbuild: Turn a couple more of clang's unused option warnings into errors
>       kbuild: Stop using '-Qunused-arguments' with clang
>
> Nick Desaulniers (2):
>       x86/boot/compressed: prefer cc-option for CFLAGS additions
>       kbuild: Update assembler calls to use proper flags and language target
>
>  Makefile                                    |  1 -
>  arch/mips/Makefile                          | 13 ++-------
>  arch/mips/include/asm/asmmacro-32.h         |  4 +--
>  arch/mips/include/asm/asmmacro.h            | 42 ++++++++++++++---------------
>  arch/mips/include/asm/fpregdef.h            | 14 ----------
>  arch/mips/include/asm/mipsregs.h            | 20 +++-----------
>  arch/mips/kernel/genex.S                    |  2 +-
>  arch/mips/kernel/r2300_fpu.S                |  4 +--
>  arch/mips/kernel/r4k_fpu.S                  | 12 ++++-----
>  arch/mips/kvm/fpu.S                         |  6 ++---
>  arch/mips/loongson2ef/Platform              |  2 +-
>  arch/powerpc/Makefile                       |  2 +-
>  arch/powerpc/kernel/vdso/Makefile           | 25 +++++++++++------
>  arch/s390/kernel/vdso64/Makefile            |  4 +--
>  arch/s390/purgatory/Makefile                |  2 +-
>  arch/x86/boot/compressed/Makefile           |  2 +-
>  drivers/gpu/drm/amd/display/dc/dml/Makefile |  3 ++-
>  scripts/Kconfig.include                     |  2 +-
>  scripts/Makefile.clang                      |  2 ++
>  scripts/Makefile.compiler                   |  8 +++---
>  scripts/as-version.sh                       |  2 +-
>  21 files changed, 74 insertions(+), 98 deletions(-)
> ---
> base-commit: 88603b6dc419445847923fcb7fe5080067a30f98
> change-id: 20221228-drop-qunused-arguments-0c5c7dae54fb
>
> Best regards,
> --
> Nathan Chancellor <nathan@kernel.org>
>


--
Best Regards
Masahiro Yamada