Re: [PATCH bpf-next v2] samples/bpf: Update README.rst and Makefile for manually compiling LLVM and clang

[Tiezhu Yang <yangtiezhu@loongson.cn>]

On 01/20/2021 05:58 AM, Fangrui Song wrote:
> On 2021-01-19, Tiezhu Yang wrote:
>> The current llvm/clang build procedure in samples/bpf/README.rst is
>> out of date. See below that the links are not accessible any more.
>>
>> $ git clone http://llvm.org/git/llvm.git
>> Cloning into 'llvm'...
>> fatal: unable to access 'http://llvm.org/git/llvm.git/': Maximum (20) 
>> redirects followed
>> $ git clone --depth 1 http://llvm.org/git/clang.git
>> Cloning into 'clang'...
>> fatal: unable to access 'http://llvm.org/git/clang.git/': Maximum 
>> (20) redirects followed
>>
>> The llvm community has adopted new ways to build the compiler. There are
>> different ways to build llvm/clang, the Clang Getting Started page 
>> [1] has
>> one way. As Yonghong said, it is better to just copy the build procedure
>> in Documentation/bpf/bpf_devel_QA.rst to keep consistent.
>>
>> I verified the procedure and it is proved to be feasible, so we should
>> update README.rst to reflect the reality. At the same time, update the
>> related comment in Makefile.
>>
>> [1] https://clang.llvm.org/get_started.html
>>
>> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
>> Acked-by: Yonghong Song <yhs@fb.com>
>> ---
>>
>> v2: Update the commit message suggested by Yonghong,
>>    thank you very much.
>>
>> samples/bpf/Makefile   |  2 +-
>> samples/bpf/README.rst | 17 ++++++++++-------
>> 2 files changed, 11 insertions(+), 8 deletions(-)
>>
>> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
>> index 26fc96c..d061446 100644
>> --- a/samples/bpf/Makefile
>> +++ b/samples/bpf/Makefile
>> @@ -208,7 +208,7 @@ TPROGLDLIBS_xdpsock        += -pthread -lcap
>> TPROGLDLIBS_xsk_fwd        += -pthread
>>
>> # Allows pointing LLC/CLANG to a LLVM backend with bpf support, 
>> redefine on cmdline:
>> -#  make M=samples/bpf/ LLC=~/git/llvm/build/bin/llc 
>> CLANG=~/git/llvm/build/bin/clang
>> +# make M=samples/bpf LLC=~/git/llvm-project/llvm/build/bin/llc 
>> CLANG=~/git/llvm-project/llvm/build/bin/clang
>> LLC ?= llc
>> CLANG ?= clang
>> OPT ?= opt
>> diff --git a/samples/bpf/README.rst b/samples/bpf/README.rst
>> index dd34b2d..d1be438 100644
>> --- a/samples/bpf/README.rst
>> +++ b/samples/bpf/README.rst
>> @@ -65,17 +65,20 @@ To generate a smaller llc binary one can use::
>> Quick sniplet for manually compiling LLVM and clang
>> (build dependencies are cmake and gcc-c++)::
>>
>> - $ git clone http://llvm.org/git/llvm.git
>> - $ cd llvm/tools
>> - $ git clone --depth 1 http://llvm.org/git/clang.git
>> - $ cd ..; mkdir build; cd build
>> - $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86"
>> - $ make -j $(getconf _NPROCESSORS_ONLN)
>> + $ git clone https://github.com/llvm/llvm-project.git
>> + $ mkdir -p llvm-project/llvm/build/install
>
> llvm-project/llvm/build/install is not used.

Yes, just mkdir -p llvm-project/llvm/build is OK.

>
>> + $ cd llvm-project/llvm/build
>> + $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
>> +            -DLLVM_ENABLE_PROJECTS="clang"    \
>> +            -DBUILD_SHARED_LIBS=OFF           \
>
> -DBUILD_SHARED_LIBS=OFF is the default. It can be omitted.

I search the related doc [1] [2], remove this option is OK for me.

BUILD_SHARED_LIBS:BOOL

     Flag indicating if each LLVM component (e.g. Support) is built as a 
shared library (ON) or as a static library (OFF). Its default value is OFF.

[1] https://www.llvm.org/docs/CMake.html
[2] https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html

>
>> + -DCMAKE_BUILD_TYPE=Release        \
>> +            -DLLVM_BUILD_RUNTIME=OFF
>
> -DLLVM_BUILD_RUNTIME=OFF can be omitted if none of
> compiler-rt/libc++/libc++abi is built.

I am not very sure about it because the default value of
LLVM_BUILD_RUNTIME is ON? [3]

option(LLVM_BUILD_RUNTIME
"Build the LLVM runtime libraries." ON)

[3] https://github.com/llvm/llvm-project/blob/main/llvm/CMakeLists.txt

If anyone has any more suggestions, please let me know.
I will send v3 after waiting for other feedback.

By the way, Documentation/bpf/bpf_devel_QA.rst maybe need a separate
patch to remove some cmake options?

Thanks,
Tiezhu

>
>> + $ ninja
>>
>> It is also possible to point make to the newly compiled 'llc' or
>> 'clang' command via redefining LLC or CLANG on the make command line::
>>
>> - make M=samples/bpf LLC=~/git/llvm/build/bin/llc 
>> CLANG=~/git/llvm/build/bin/clang
>> + make M=samples/bpf LLC=~/git/llvm-project/llvm/build/bin/llc 
>> CLANG=~/git/llvm-project/llvm/build/bin/clang
>>
>> Cross compiling samples
>> -----------------------
>> -- 
>> 2.1.0
>>
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