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

[Nathan Chancellor <nathan@kernel.org>]

On 7/20/2021 1:04 AM, Masahiro Yamada wrote:
> On Fri, Jul 9, 2021 at 8:25 AM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@googlegroups.com> 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>
>> ---
>> 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),)
> 
> 
> Do you need to check $(LLVM) ?
> 
> 
> LLVM=1 is a convenient switch to change all the
> defaults, but yet you can flip each tool individually.
> 
> Instead of LLVM=1, you still should be able to
> get the equivalent setups by:
> 
> 
>    make CC=clang LD=ld.lld AR=llvm-ar OBJCOPY=llvm-objcopy ...
> 
> 
> The --target option is passed to only
> KBUILD_CFLAGS and KBUILD_AFLAGS.
> 
> So, when we talk about --target=,
> we only care about whether $(CC) is Clang.
> Not caring about $(AR), $(LD), or $(OBJCOPY).
> 
> 
> scripts/Makefile.clang is already guarded by:
> 
> ifneq ($(findstring clang,$(CC_VERSION_TEXT)),

$ make ARCH=arm64 CC=clang LLVM_IAS=1

will use the right compiler and assembler but none of the other binary 
tools because '--prefix=' will not be set so CROSS_COMPILE needs to be 
specified still, which defeats the purpose of this whole change. This 
patch is designed to work for the "normal" case of saying "I want to use 
all of the LLVM tools", not "I want to use clang by itself".

>> +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
>> --
>> 2.32.0.93.g670b81a890-goog
>>
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> 
> 
>