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

[Tiezhu Yang]

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out of date.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@xxxxxxxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
---

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++)::
 
- $ 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