Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM_IAS=1

[Nathan Chancellor <nathan@kernel.org>]

On 7/8/2021 4:25 PM, 'Nick Desaulniers' via Clang Built Linux wrote:
> We get constant feedback that the command line invocation of make is too
> long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
> target triple, or is an absolute path outside of $PATH, but it's mostly
> redundant for a given SRCARCH. SRCARCH itself is derived from ARCH
> (normalized for a few different targets).
> 
> 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: Nathan Chancellor <nathan@kernel.org>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
I tested arm, arm64, i386, riscv, and x86_64 and verified that the error 
fired for an unsupported arch. Consider this:

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
> 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/.
> 
>   Documentation/kbuild/llvm.rst |  5 +++++
>   scripts/Makefile.clang        | 34 ++++++++++++++++++++++++++++++++--
>   2 files changed, 37 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index b18401d2ba82..80c63dd9a6d1 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -46,6 +46,11 @@ example: ::
>   
>   	clang --target=aarch64-linux-gnu foo.c
>   
> +When both ``LLVM=1`` and ``LLVM_IAS=1`` are used, ``CROSS_COMPILE`` becomes
> +unnecessary and can be inferred from ``ARCH``. Example: ::
> +
> +	ARCH=arm64 make LLVM=1 LLVM_IAS=1
> +
>   LLVM Utilities
>   --------------
>   
> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 297932e973d4..956603f56724 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -1,6 +1,36 @@
> -ifneq ($(CROSS_COMPILE),)
> +# Individual arch/{arch}/Makfiles should use -EL/-EB to set intended endianness
> +# and -m32/-m64 to set word size based on Kconfigs instead of relying on the
> +# target triple.
> +ifeq ($(CROSS_COMPILE),)
> +ifneq ($(LLVM),)
> +ifeq ($(LLVM_IAS),1)
> +ifeq ($(SRCARCH),arm)
> +CLANG_FLAGS	+= --target=arm-linux-gnueabi
> +else ifeq ($(SRCARCH),arm64)
> +CLANG_FLAGS	+= --target=aarch64-linux-gnu
> +else ifeq ($(SRCARCH),hexagon)
> +CLANG_FLAGS	+= --target=hexagon-linux-gnu
> +else ifeq ($(SRCARCH),m68k)
> +CLANG_FLAGS	+= --target=m68k-linux-gnu
> +else ifeq ($(SRCARCH),mips)
> +CLANG_FLAGS	+= --target=mipsel-linux-gnu
> +else ifeq ($(SRCARCH),powerpc)
> +CLANG_FLAGS	+= --target=powerpc64le-linux-gnu
> +else ifeq ($(SRCARCH),riscv)
> +CLANG_FLAGS	+= --target=riscv64-linux-gnu
> +else ifeq ($(SRCARCH),s390)
> +CLANG_FLAGS	+= --target=s390x-linux-gnu
> +else ifeq ($(SRCARCH),x86)
> +CLANG_FLAGS	+= --target=x86_64-linux-gnu
> +else
> +$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)
> +endif # SRCARCH
> +endif # LLVM_IAS
> +endif # LLVM
> +else
>   CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
> -endif
> +endif # CROSS_COMPILE
> +
>   ifeq ($(LLVM_IAS),1)
>   CLANG_FLAGS	+= -integrated-as
>   else
>