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

[Yonghong Song <yhs@fb.com>]

On 4/11/21 3:22 AM, Sedat Dilek wrote:
> On Sat, Apr 10, 2021 at 6:49 PM 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
>> +else
>>   CC := $(CROSS_COMPILE)gcc
>> +endif
>>
> 
> Why not use include "include ../../../scripts/Makefile.include" here
> and include CC and GNU or LLVM (bin)utils from there?

There is a comment above my change,

 >>   # This mimics the top-level Makefile. We do it explicitly here so 
that this
 >>   # Makefile can operate with or without the kbuild infrastructure.

It is intentionally not depending on kbuild 
(../../../scripts/Makefile.include).

> 
> Should the CC line have a $(CROSS_COMPILE) for people doing cross-compilation?
> 
> CC := $(CROSS_COMPILE)clang

The top linux/Makefile has

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

There is no CROSS_COMPILE prefix for llvm.
Also see here:
   https://clang.llvm.org/docs/CrossCompilation.html
for clang, cross compilation is mostly related to
tweaking compiler options than building a different
compiler.

Hence, I didn't add $(CROSS_COMPILER) prefix.

> 
> - Sedat -
> 
> 
>>   ifeq (0,$(MAKELEVEL))
>>       ifeq ($(OUTPUT),)
>> --
>> 2.30.2
>>