Re: [PATCH v6 2/3] Makefile: infer --target from ARCH for CC=clang

[Masahiro Yamada <masahiroy@kernel.org>]

On Tue, Aug 3, 2021 at 3:39 AM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> We get constant feedback that the command line invocation of make is too
> long when compiling with LLVM. CROSS_COMPILE is helpful when a toolchain
> has a prefix of the target triple, or is an absolute path outside of
> $PATH.
>
> Since a Clang binary is generally multi-targeted, we can infer a given
> target from SRCARCH/ARCH.  If CROSS_COMPILE is not set, simply set
> --target= for CLANG_FLAGS, KBUILD_CFLAGS, and KBUILD_AFLAGS based on
> $SRCARCH.
>
> Previously, we'd cross compile via:
> $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1
> Now:
> $ ARCH=arm64 make LLVM=1 LLVM_IAS=1
>
> For native builds (not involving cross compilation) we now explicitly
> specify a target triple rather than rely on the implicit host triple.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1399
> Suggested-by: Arnd Bergmann <arnd@kernel.org>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Acked-by: Arnd Bergmann <arnd@kernel.org>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes v5 -> v6:
> * Use indirection as per Linus. We now have CLANG_TARGET_FLAGS and a
>   few variables suffixed with $SRCARCH.
> * Add Linus' SB tag.
> * Change hexagon target triple from -gnu to -musl.
>
> Changes v3 -> v4:
> * Reword commit message and oneline, as per Nathan.
> * Drop documentation change (it's the next patch, could be squashed in,
>   I guess).
> * Drop LLVM_IAS=1 requirement, as per Masahiro.
> * Carry Arnd's and Nathan's AB/RB/TB tags, confirmed OK on IRC+Discord.
>
> Changes v2 -> v3:
> * Drop check/requirement for LLVM=1, as per Masahiro.
> * Change oneliner from LLVM=1 LLVM_IAS=1 to CC=clang LLVM_IAS=1.
> * Don't carry forward Nathan's RB/TB tags. :( Sorry Nathan, but thank
>   you for testing+reviewing v2.
> * Update wording of docs slightly.
>
> Changes v1 -> v2:
> * Fix typos in commit message as per Geert and Masahiro.
> * Use SRCARCH instead of ARCH, simplifying x86 handling, as per
>   Masahiro. Add his sugguested by tag.
> * change commit oneline from 'drop' to 'infer.'
> * Add detail about explicit host --target and relationship of ARCH to
>   SRCARCH, as per Masahiro.
>
> Changes RFC -> v1:
> * Rebase onto linux-kbuild/for-next
> * Keep full target triples since missing the gnueabi suffix messes up
>   32b ARM. Drop Fangrui's sugguested by tag. Update commit message to
>   drop references to arm64.
> * Flush out TODOS.
> * Add note about -EL/-EB, -m32/-m64.
> * Add note to Documentation/.
>
>  scripts/Makefile.clang | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 297932e973d4..1f4e3eb70f88 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -1,6 +1,27 @@
> -ifneq ($(CROSS_COMPILE),)
> +# Individual arch/{arch}/Makefiles should use -EL/-EB to set intended
> +# endianness and -m32/-m64 to set word size based on Kconfigs instead of
> +# relying on the target triple.
> +CLANG_TARGET_FLAGS_arm         := arm-linux-gnueabi
> +CLANG_TARGET_FLAGS_arm64       := aarch64-linux-gnu
> +CLANG_TARGET_FLAGS_hexagon     := hexagon-linux-musl
> +CLANG_TARGET_FLAGS_m68k                := m68k-linux-gnu
> +CLANG_TARGET_FLAGS_mips                := mipsel-linux-gnu
> +CLANG_TARGET_FLAGS_powerpc     := powerpc64le-linux-gnu
> +CLANG_TARGET_FLAGS_riscv       := riscv64-linux-gnu
> +CLANG_TARGET_FLAGS_s390                := s390x-linux-gnu
> +CLANG_TARGET_FLAGS_x86         := x86_64-linux-gnu
> +CLANG_TARGET_FLAGS             := $(CLANG_TARGET_FLAGS_$(SRCARCH))
> +
> +ifeq ($(CROSS_COMPILE),)
> +ifeq ($(CLANG_TARGET_FLAGS),)
> +$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)


When I was browsing the code, I noticed a nit.

I believe non-build targets such as
'make help', 'make clean' etc. should succeed
without any correct CROSS_COMPILE set
since they do not use the compiler at all.


The remaining architectures may fail to clean.

$ export LLVM=1
$ make ARCH=csky  clean
scripts/Makefile.clang:17: *** Specify CROSS_COMPILE or add
'--target=' option to scripts/Makefile.clang.  Stop.



I decided to not be worried about it because it
is fixable later.
I will pick it up unless a significant issue is found.





> +else
> +CLANG_FLAGS    += --target=$(CLANG_TARGET_FLAGS)
> +endif # CLANG_TARGET_FLAGS
> +else
>  CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
> -endif
> +endif # CROSS_COMPILE
> +
>  ifeq ($(LLVM_IAS),1)
>  CLANG_FLAGS    += -integrated-as
>  else
> --
> 2.32.0.554.ge1b32706d8-goog
>


--
Best Regards


Masahiro Yamada