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LLVM IR Trace Profiler (LLVM-Tracer) 2.0

LLVM-Tracer is an LLVM instrumentation pass to print out a dynamic LLVM IR trace, including dynamic register values and memory addresses.

If you use LLVM-Tracer in your research, please cite:

ISA-Independent Workload Characterization and its Implications for Specialized Architectures, Yakun Sophia Shao and David Brooks, International Symposium on Performance Analysis of Systems and Software (ISPASS), April 2013

Requirements:

We highly recommend that users use the provided Docker image, available for download here. This will solve basically all environment issues. If you cannot use Docker, then read on.

  1. LLVM 6.0 and Clang 6.0 64-bit. Users cannot download a pre-built package because the Release build type strips value names from LLVM IR, and working around this is difficult. Instead, users must built the toolchain from source.
  2. GCC 5.4 or later.
  3. CMake 2.8.12 or newer.

Changelog:

Feburary 2020: v2 new features

Breaking changes from v1.2 to v2.0:

  • LLVM-Tracer now uses LLVM 6.0. Support for all previous versions of LLVM have been removed. Minor versions 6.0.0 and 6.0.1 have been tested to work.
  • The Release build type of LLVM 6.0 and Clang 6.0 will strip value names from the generated LLVM IR, which breaks the tracer. To avoid this problem, it is highly recommended that you build the Debug build type by adding the cmake flag -DCMAKE_BUILD_TYPE=Debug.

C++ support

LLVM-Tracer now supports C++ for uninstrumented code! Users can write C++ to build applications and only have LLVM-Tracer instrument and generate dynamic traces for the parts that are written in pure C with C-style linkage. limited to writing pure C programs. Now, you can make use of all the features For example: if you write a C++ class that calls a function foo which is in an extern C context, you can trace foo(), but you cannot trace a function like the class's constructor.

See playground/test.cc for a live example.

Multithreading support

LLVM-Tracer can now trace multithreaded applications. Each thread should call the llvmtracer_set_trace_name API first to assign a unique name for the dynamic trace file that will be produced.

See playground/multithreading.cc for an example.

November 2016: v1.2 changelog

Breaking changes from v1.1 to v1.2:

  • CMake is no longer optional. LLVM-Tracer only supports CMake for building. This is because of a new Clangtool component (under ast-pass/) which requires CMake.
  • LLVM-Tracer will install all built files to the subdirectories bin/ and lib/ instead of full-trace/ and profile-func/. The BUILD_ON_SOURCE option has been removed.

Build:

CMake is a configure tool which allows you to do out-of-source build. LLVM-Tracer requires CMake newer than 2.8.12. By default, CMake searches for LLVM 6.0.

  1. Set LLVM_HOME to where you installed LLVM

    export LLVM_HOME=/path/to/your/llvm/installation
    
  2. Clone LLVM-Tracer.

    git clone https://github.com/ysshao/LLVM-Tracer
    cd LLVM-Tracer/
    
  3. Configure with CMake and build LLVM-Tracer source code.

    If you have LLVM installed:

    mkdir build/
    cd build
    cmake ..
    make
    make install
    

    If you want CMake to install LLVM for you (CAUTION: takes an hour!):

    mkdir build/
    cd build
    cmake .. -DLLVM_ROOT=/where/to/install/LLVM -DAUTOINSTALL=TRUE -DCMAKE_BUILD_TYPE=Debug
    make
    make install
    
  4. (Optional) Try running triad example, which is built by CMake.

    cd /path/to/build/LLVM-Tracer/
    ctest -V
    
  5. Available CMake settings

    -DLLVM_ROOT=/where/your/llvm/install   (default : $LLVM_HOME)
      You may denote the path of LLVM to find/install by this option. if
      this option is not defined, environment variable LLVM_HOME will be
      used.
    
    -DAUTOINSTALL=TRUE,FALSE    (default : FALSE)
      By this option, CMake scripts will automatically download, build and
      install LLVM for you if finds no LLVM installation. Using this
      function requires tar-1.22 or newer to extract xz format.
    
      The default installation path is under /your/build_dir/lib/llvm-x.x.
      You can manually define the installation path by
      -DLLVM_ROOT=/where/to/install.
    
    -DCMAKE_BUILD_TYPE=None,Debug,Release    (default : None)
      It is recommended to use the Debug build type for LLVM 6.0.
    

Run:

After you build LLVM-Tracer, you can use example triad programs in the example directory to test LLVM-Tracer.

Example program: triad

  1. Go to /your/path/to/LLVM-Tracer/example/triad

  2. Run LLVM-Tracer to generate a dynamic LLVM IR trace a. LLVM-Tracer tracks regions of interest inside a program. In the triad example, we want to analyze the triad kernel instead of the setup and initialization work done in main. To tell LLVM-Tracer the functions we are interested in, set the enviroment variable WORKLOAD to be the top-level function name:

    export WORKLOAD=triad
    

    If you have multiple functions, separate them with commas.

    export WORKLOAD=md,md_kernel
    

    LLVM-Tracer will trace them differently based on the -trace-all-callees flag, which can be specified to the opt command (see step d).

    • If this flag is specified, then any function called by any function in the WORKLOAD variable will be traced. This is a simple way to trace multiple "top-level" functions at once.
    • If this flag is not specified, then only functions in the WORKLOAD variable will be traced.

    b. Generate the source code labelmap.

    export TRACER_HOME=/your/path/to/LLVM-Tracer
    ${TRACER_HOME}/bin/get-labeled-stmts triad.c -- -I${LLVM_HOME}/lib/clang/6.0/include
    

    c. Generate LLVM IR:

    clang -g -O1 -S -fno-slp-vectorize -fno-vectorize -fno-unroll-loops -fno-inline -emit-llvm -o triad.llvm triad.c
    

    d. Run LLVM-Tracer pass. Before you run, make sure you already built LLVM-Tracer and have set the environment variable TRACER_HOME to where you put LLVM-Tracer code.

    opt -S -load=${TRACER_HOME}/full-trace/full_trace.so -fulltrace -labelmapwriter [-trace-all-callees] triad.llvm -o triad-opt.llvm
    llvm-link -o full.llvm triad-opt.llvm ${TRACER_HOME}/profile-func/trace_logger.llvm
    

    The -trace-all-callees flag is optional and defaults to false.

    e. Generate machine code:

    llc -filetype=asm -o full.s full.llvm
    gcc -fno-inline -o triad-instrumented full.s
    

    f. Run binary. It will generate a file called dynamic_trace under current directory.

    ./triad-instrumented
    

    g. There is a script provided which performs all of these operations.

    python llvm_compile.py $TRACER_HOME/example/triad triad
    

triad is part of the SHOC benchmark suite. We provide a version of SHOC that is ready to be used with LLVM-Tracer. Please go to Aladdin and look under the SHOC directory.


Sophia Shao, Sam Xi, and Emma Wang