Re: [PATCH bpf-next v3] samples/bpf: Update build procedure for manually compiling LLVM and Clang

[Nathan Chancellor <natechancellor@gmail.com>]

On Thu, Jan 21, 2021 at 12:08:31AM -0800, Andrii Nakryiko wrote:
> On Wed, Jan 20, 2021 at 9:36 PM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
> >
> > On Thu, Jan 21, 2021 at 01:27:35PM +0800, Tiezhu Yang wrote:
> > > The current LLVM and 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 out of date.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 and Clang, the Clang Getting Started page [1]
> > > has one way. As Yonghong said, it is better to 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.
> > >
> > > Additionally, as Fangrui said, the dir llvm-project/llvm/build/install is
> > > not used, BUILD_SHARED_LIBS=OFF is the default option [2], so also change
> > > Documentation/bpf/bpf_devel_QA.rst together.
> > >
> > > [1] https://clang.llvm.org/get_started.html
> > > [2] https://www.llvm.org/docs/CMake.html
> > >
> > > Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
> > > Acked-by: Yonghong Song <yhs@fb.com>
> >
> > Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> >
> > Small comment below.
> >
> > > ---
> > >
> > > v2: Update the commit message suggested by Yonghong,
> > >     thank you very much.
> > >
> > > v3: Remove the default option BUILD_SHARED_LIBS=OFF
> > >     and just mkdir llvm-project/llvm/build suggested
> > >     by Fangrui.
> > >
> > >  Documentation/bpf/bpf_devel_QA.rst |  3 +--
> > >  samples/bpf/Makefile               |  2 +-
> > >  samples/bpf/README.rst             | 16 +++++++++-------
> > >  3 files changed, 11 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/Documentation/bpf/bpf_devel_QA.rst b/Documentation/bpf/bpf_devel_QA.rst
> > > index 5b613d2..18788bb 100644
> > > --- a/Documentation/bpf/bpf_devel_QA.rst
> > > +++ b/Documentation/bpf/bpf_devel_QA.rst
> > > @@ -506,11 +506,10 @@ that set up, proceed with building the latest LLVM and clang version
> > >  from the git repositories::
> > >
> > >       $ git clone https://github.com/llvm/llvm-project.git
> > > -     $ mkdir -p llvm-project/llvm/build/install
> > > +     $ mkdir -p llvm-project/llvm/build
> > >       $ cd llvm-project/llvm/build
> > >       $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
> > >                  -DLLVM_ENABLE_PROJECTS="clang"    \
> > > -                -DBUILD_SHARED_LIBS=OFF           \
> > >                  -DCMAKE_BUILD_TYPE=Release        \
> > >                  -DLLVM_BUILD_RUNTIME=OFF
> > >       $ ninja
> > > 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..23006cb 100644
> > > --- a/samples/bpf/README.rst
> > > +++ b/samples/bpf/README.rst
> > > @@ -65,17 +65,19 @@ To generate a smaller llc binary one can use::
> > >  Quick sniplet for manually compiling LLVM and clang
> > >  (build dependencies are cmake and gcc-c++)::
> >
> > Technically, ninja is now a build dependency as well, it might be worth
> > mentioning that here (usually the package is ninja or ninja-build).
> 
> it's possible to generate Makefile by passing `-g "Unix Makefiles"`,
> which would avoid dependency on ninja, no?

Yes, although I am fairly certain that building with ninja is quicker so
I would recommend keeping it. One small extra dependency never killed
anyone plus ninja is becoming more common nowadays :)

> > Regardless of whether that is addressed or not (because it is small),
> > feel free to carry forward my tag in any future revisions unless they
> > drastically change.
> >
> > > - $ 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
> > > + $ cd llvm-project/llvm/build
> > > + $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
> > > +            -DLLVM_ENABLE_PROJECTS="clang"    \
> > > +            -DCMAKE_BUILD_TYPE=Release        \
> > > +            -DLLVM_BUILD_RUNTIME=OFF
> > > + $ 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
> > >