lldb-eval - blazing fast debug expression evaluation

What

lldb-eval is an LLDB-based library for evaluating debug expressions in the context of the debugged process. All modern debuggers support evaluating expressions to inspect the target process state: print out variables, access member fields, etc. lldb-eval is basically a REPL-like library, that allows to inspect the process state using the familiar C++ syntax. The primary use-case is IDE integration (for example, Stadia for Visual Studio).

Why

LLDB has a very powerful built-in expression evaluator (available via expr command). It can handle almost any valid C++ as well as perform function calls. But the downside of this power is poor performance, especially for large programs with lots of debug information. This is not as critical for interactive use, but doesn't work well for implementing IDE integrations. For example, Stadia debugger for Visual Studio evaluates dozens and hundreds of expressions for every "step", so it has to be fast.

lldb-eval makes a trade-off between performance and completeness, focusing on performance. It features a custom expression parser and relies purely on the debug information, aiming at sub-millisecond evaluation speed.

Build & Test

Dependencies

Linux

Install the dependencies:

sudo apt install lld-11 clang-11 lldb-11 llvm-11-dev libclang-11-dev liblldb-11-dev libc++-11-dev libc++abi-11-dev

Or build them from source, see the instructions in the LLDB documentation.

cmake \
    -GNinja \
    -DCMAKE_INSTALL_PREFIX="~/src/llvm-project/build_optdebug/install" \
    -DCMAKE_BUILD_TYPE=RelWithDebInfo \
    -DLLVM_ENABLE_PROJECTS="lldb;clang;lld" \
    -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \
    -DLLVM_TARGETS_TO_BUILD="X86" \
    ../llvm

Windows

On Windows we need to build LLDB (and other parts) from source. The steps are basically the same as for Linux. You will need: CMake, Ninja and Visual Studio (tested with Visual Studio 2019 16.6.5).

Hint: You can install the dependencies via Chocolatey.

Run the x64 Native Tools Command Prompt for VS 2019:

git clone https://github.com/llvm/llvm-project.git
cd llvm-project
mkdir build_x64_optdebug
cd build_x64_optdebug

cmake ^
    -DCMAKE_INSTALL_PREFIX='C:\src\llvm-project\build_optdebug\install' ^
    -DCMAKE_BUILD_TYPE=RelWithDebInfo ^
    -DLLVM_ENABLE_PROJECTS='lldb;clang;lld' ^
    -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \
    -DLLVM_TARGETS_TO_BUILD="X86" \
    -DLLDB_ENABLE_PYTHON=0 ^
    -GNinja ^
    ../llvm

ninja install

Build

lldb-eval uses Bazel, you can find the installation instructions on its website.

You need to set the LLVM_INSTALL_PATH environmental variable with a location to your LLVM installation:

# (Linux) If you installed the packages via "apt install".
export LLVM_INSTALL_PATH=/usr/lib/llvm-10

# If you built from source using the instruction above.
export LLVM_INSTALL_PATH=~/src/llvm-project/build_optdebug/install

# (Windows) If you built from source using the instructions above.
$env:LLVM_INSTALL_PATH = C:\src\llvm-project\build_optdebug\install

Now you can build and test lldb-eval:

# Build and run all tests
bazel test ...:all

# Evaluate a sample expression
bazel run tools:exec -- "(1 + 2) * 42 / 4"

Depending on your distribution of LLVM, you may also need to provide --@llvm_project//:llvm_build={static,dynamic} flag. For example, if your liblldb.so is linked dynamically (this is the case when installing via apt), then you need to use llvm_build=dynamic. The build script tries to choose the correct default value automatically, but it can be wrong in some situations (please, report and contribute 🙂).

Hint: You can add this option to your user.bazelrc !

Local per-repo Bazel config

You can create user.bazelrc in the repository root and put there your local configuration. Check Bazel docs for the format. For example:

# Building on Linux (usually don't need this, Bazel detects automatically)
build --config=linux
# Using statically linked liblldb.so
build --@llvm_project//:llvm_build=static

Publications

• Blazing fast expression evaluation for C++ in LLDB
• Building a faster expression evaluator for LLDB at LLVM Developers' Meeting 2021
• lldb-eval fuzzer: Finding bugs in an LLDB-based expression evaluator at LLVM Developers' Meeting 2021

Disclaimer

This is not an officially supported Google product.