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

On 01/20/2021 05:58 AM, Fangrui Song wrote:

On 2021-01-19, Tiezhu Yang wrote:

quoted

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out of date.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 <redacted>
---

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.

quoted

+ $ 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

quoted

+ -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

quoted

+ $ 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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