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

[Yonghong Song]

On 1/19/21 6:32 PM, Tiezhu Yang wrote:
> 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 out of date.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@xxxxxxxxxxx>
> > > Acked-by: Yonghong Song <yhs@xxxxxx>
> > > ---
> > >
> > > 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?

Please submit updated this patch and Documentation/bpf/bpf_devel_QA.rst
together. This way, it is easy to cross check. Thanks.

> Thanks,
> Tiezhu
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