Whole Program LLVM
This project, WLLVM, provides tools for building whole-program (or
whole-library) LLVM bitcode files from an unmodified C or C++
source package. It currently runs on *nix platforms such as Linux,
FreeBSD, and Mac OS X.
WLLVM provides python-based compiler wrappers that work in two steps. The wrappers first invoke the compiler as normal. Then, for each object file, they call a bitcode compiler to produce LLVM bitcode. The wrappers also store the location of the generated bitcode file in a dedicated section of the object file. When object files are linked together, the contents of the dedicated sections are concatenated (so we don't lose the locations of any of the constituent bitcode files). After the build completes, one can use a WLLVM utility to read the contents of the dedicated section and link all of the bitcode into a single whole-program bitcode file. This utility works for both executable and native libraries.
This two-phase build process is necessary to be a drop-in replacement for gcc or g++ in any build system. Using the LTO framework in gcc and the gold linker plugin works in many cases, but fails in the presence of static libraries in builds. WLLVM's approach has the distinct advantage of generating working binaries, in case some part of a build process requires that.
WLLVM works with either clang or the gcc dragonegg plugin. If you are not interested in dragonegg support, and speed is an issue for you, you may want to try out gllvm.
As of August 2016 WLLVM is now a pip package. You can just do:
pip install wllvm
or
sudo pip install wllvm
depending on your machine's permissions.
If you want to develop or use the development version:
git clone https://github.com/travitch/whole-program-llvm
cd whole-program-llvm
Now you need to install WLLVM. You can either install globally on your system in develop mode:
sudo pip install -e .
or install WLLVM into a virtual python environment in develop mode to avoid installing globally:
virtualenv venv
source venv/bin/activate
pip install -e .
WLLVM includes four python executables: wllvm for compiling C code
and wllvm++ for compiling C++, an auxiliary tool extract-bc for
extracting the bitcode from a build product (object file, executable, library
or archive), and a sanity checker, wllvm-sanity-checker for detecting
configuration oversights.
Three environment variables must be set to use these wrappers:
LLVM_COMPILERshould be set to eitherdragoneggorclang.LLVM_GCC_PREFIXshould be set to the prefix for the version of gcc that should be used with dragonegg. This can be empty if there is no prefix. This variable is not used if$LLVM_COMPILER == clang.LLVM_DRAGONEGG_PLUGINshould be the full path to the dragonegg plugin. This variable is not used if$LLVM_COMPILER == clang.
Once the environment is set up, just use wllvm and wllvm++ as your C
and C++ compilers, respectively.
In addition to the above environment variables the following can be optionally used:
-
LLVM_CC_NAMEcan be set if your clang compiler is not calledclangbut something likeclang-3.7. SimilarlyLLVM_CXX_NAMEcan be used to describe what the C++ compiler is called. Note that in these sorts of cases, the environment variableLLVM_COMPILERshould still be set toclangnotclang-3.7etc. We also pay attention to the environment variablesLLVM_LINK_NAMEandLLVM_AR_NAMEin an analagous way, since they too get adorned with suffixes in various Linux distributions. -
LLVM_COMPILER_PATHcan be set to the absolute path to the folder that contains the compiler and other LLVM tools such asllvm-linkto be used. This prevents searching for the compiler in your PATH environment variable. This can be useful if you have different versions of clang on your system and you want to easily switch compilers without tinkering with your PATH variable. ExampleLLVM_COMPILER_PATH=/home/user/llvm_and_clang/Debug+Asserts/bin. -
WLLVM_CONFIGURE_ONLYcan be set to anything. If it is set,wllvmandwllvm++behave like a normal C or C++ compiler. They do not produce bitcode. SettingWLLVM_CONFIGURE_ONLYmay prevent configuration errors caused by the unexpected production of hidden bitcode files. It is sometimes required when configuring a build.
export LLVM_COMPILER=clang
tar xf pkg-config-0.26.tar.gz
cd pkg-config-0.26
CC=wllvm ./configure
make
This should produce the executable pkg-config. To extract the bitcode:
extract-bc pkg-config
which will produce the bitcode module pkg-config.bc.
A gentler set of instructions on building apache in a vagrant Ubuntu 14.04 can be found here, and for Ubuntu 16.04 here.
export LLVM_COMPILER=dragonegg
export LLVM_GCC_PREFIX=llvm-
export LLVM_DRAGONEGG_PLUGIN=/unsup/llvm-2.9/lib/dragonegg.so
tar xf pkg-config-0.26.tar.gz
cd pkg-config-0.26
CC=wllvm ./configure
make
Again, this should produce the executable pkg-config. To extract the bitcode:
extract-bc pkg-config
which will produce the bitcode module pkg-config.bc.
export LLVM_COMPILER=clang
tar -xvf bullet-2.81-rev2613.tgz
mkdir bullet-bin
cd bullet-bin
CC=wllvm CXX=wllvm++ cmake ../bullet-2.81-rev2613/
make
# Produces src/LinearMath/libLinearMath.bca
extract-bc src/LinearMath/libLinearMath.a
Note that by default extracting bitcode from an archive produces an archive of bitcode. You can also extract the bitcode directly into a module.
extract-bc -b src/LinearMath/libLinearMath.a
produces src/LinearMath/libLinearMath.a.bc.
To see how to build freeBSD 10.0 from scratch check out this guide.
Sometimes it is necessary to disable the production of bitcode.
Typically this is during configuration, where the production
of unexpected files can confuse the configure script. For this
we have a flag WLLVM_CONFIGURE_ONLY which can be used as
follows:
WLLVM_CONFIGURE_ONLY=1 CC=wllvm ./configure
CC=wllvm make
export LLVM_COMPILER=clang
tar xvfz jansson-2.7.tar.gz
cd jansson-2.7
CC=wllvm ./configure
make
mkdir bitcode
cp src/.libs/libjansson.a bitcode
cd bitcode
extract-bc libjansson.a
llvm-ar x libjansson.bca
ls -la
Sometimes it can be useful to preserve the bitcode files produced in a
build, either to prevent deletion or to retrieve it later. If the
environment variable WLLVM_BC_STORE is set to the absolute path of
an existing directory,
then WLLVM will copy the produced bitcode file into that directory.
The name of the copied bitcode file is the hash of the path to the
original bitcode file. For convenience, when using both the manifest
feature of extract-bc and the store, the manifest will contain both
the original path, and the store path.
To support cross-compilation WLLVM supports the -target triple used by clang.
More information can be found
here.
Additionally, WLLVM leverages objcopy for some of its heavy lifting. When
cross-compiling you must ensure to use the appropriate objcopy for the target
architecture. The BINUTILS_TARGET_PREFIX environment variable can be used to
set the objcopy of choice, for example, arm-linux-gnueabihf.
In some situations it is desirable to pass certain flags to clang in the step that
produces the bitcode. This can be fulfilled by setting the
LLVM_BITCODE_GENERATION_FLAGS environment variable to the desired
flags, for example "-flto -fwhole-program-vtables".
The WLLVM tools can show various levels of output to aid with debugging.
To show this output set the WLLVM_OUTPUT_LEVEL environment
variable to one of the following levels:
ERRORWARNINGINFODEBUG
For example:
export WLLVM_OUTPUT_LEVEL=DEBUG
Output will be directed to the standard error stream, unless you specify the
path of a logfile via the WLLVM_OUTPUT_FILE environment variable.
For example:
export WLLVM_OUTPUT_FILE=/tmp/wllvm.log
Too many environment variables? Try doing a sanity check:
wllvm-sanity-checker
it might point out what is wrong.
WLLVM is released under the MIT license. See the file LICENSE for details.
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