Re: [PATCH bpf-next v3 1/5] selftests: set CC to clang in lib.mk if LLVM is set

[Yonghong Song <yhs@fb.com>]

On 4/13/21 3:13 PM, Nick Desaulniers wrote:
> On Tue, Apr 13, 2021 at 3:05 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>>
>> On Tue, Apr 13, 2021 at 8:34 AM Yonghong Song <yhs@fb.com> wrote:
>>>
>>> selftests/bpf/Makefile includes lib.mk. With the following command
>>>    make -j60 LLVM=1 LLVM_IAS=1  <=== compile kernel
>>>    make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1 V=1
>>> some files are still compiled with gcc. This patch
>>> fixed lib.mk issue which sets CC to gcc in all cases.
>>>
>>> Cc: Sedat Dilek <sedat.dilek@gmail.com>
>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>> ---
>>>   tools/testing/selftests/lib.mk | 4 ++++
>>>   1 file changed, 4 insertions(+)
>>>
>>> diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
>>> index a5ce26d548e4..9a41d8bb9ff1 100644
>>> --- a/tools/testing/selftests/lib.mk
>>> +++ b/tools/testing/selftests/lib.mk
>>> @@ -1,6 +1,10 @@
>>>   # This mimics the top-level Makefile. We do it explicitly here so that this
>>>   # Makefile can operate with or without the kbuild infrastructure.
>>> +ifneq ($(LLVM),)
>>> +CC := clang
>>
>> Does this mean that cross-compilation with Clang doesn't work at all
>> or is achieved in some other way?
> 
> Right, this probably doesn't support cross compilation w/ Clang.
> Rather than invoke `$(CROSS_COMPILE) clang`, you'd do `clang
> --target=$(CROSS_COMPILE)`.  Even then, cross linking executables is
> hairy.  But at least this should enable native compilation, which is a
> start.

See https://clang.llvm.org/docs/CrossCompilation.html.
As Nick said, clang prefers --target=$(CROSS_COMPILE) to
indicate cross compilation. User can pass additional
flags (CFLAGS) for cross compilation for the time being.
This is the same as main kernel Makefile.

ifneq ($(LLVM),)
CC              = clang
LD              = ld.lld
AR              = llvm-ar
NM              = llvm-nm
OBJCOPY         = llvm-objcopy
OBJDUMP         = llvm-objdump
READELF         = llvm-readelf
STRIP           = llvm-strip
else
CC              = $(CROSS_COMPILE)gcc
LD              = $(CROSS_COMPILE)ld
AR              = $(CROSS_COMPILE)ar
NM              = $(CROSS_COMPILE)nm
OBJCOPY         = $(CROSS_COMPILE)objcopy
OBJDUMP         = $(CROSS_COMPILE)objdump
READELF         = $(CROSS_COMPILE)readelf
STRIP           = $(CROSS_COMPILE)strip
endif

>>
>>
>>> +else
>>>   CC := $(CROSS_COMPILE)gcc
>>> +endif
>>>
>>>   ifeq (0,$(MAKELEVEL))
>>>       ifeq ($(OUTPUT),)
>>> --
>>> 2.30.2
>>>
> 
> 
>