Ohm

A "white-box" profiler built on top of the LLVM infrastructure. Unlike sampling profilres (e.g., perf), Ohm modifies the program's Intermediate Representation (IR) during compilation to inject precise measurement probes. It turns opaque binaries into transparent systems that report their own execution metrics.

Architecture

The project follows a dual-component architecture design to separate instrumentation logic from measurement logic.

The Injector (LLVM Pass)

• Role: The "Surgeon".
• Time: Compile-time.
• Function: Analysis the LLVM IR, identifies function entry and exit points (Basic Blocks), and injects calls to the runtime library. It does not measure anything itself; it merely prepares the code.

The Runtime Library

• Role: The "Lab".
• Time: Runtime.
• Function: A lightweight static library linked to the target binary. It implements the probes, captures CPU cycle counts, and aggregates statistics.

Prerequisites

• LLVM & Clang
• CMake (3.13+)
• C++ Compiler supporting C++17/20

Build Instructions

1. Clone & enter repo

git clone https://github.com/monvit/ohm.git monvit-ohm && cd monvit-ohm

2. Prepare build directory

mkdir build && cd build

3. Configure with Cmake

cmake ..

4. Build the Pass and Runtime

cmake --build .

Usage

To instrument a target program (e.g., examples/main.cc):

1. Compile to Bitcode

clang -emit-llvm -c main.cpp -o main.bc

2. Load the Pass and Instrument

opt -load-pass-plugin=./injector/libOhmPass.so -passes="ohm" main.bc -o main_inst.bc

3. Link with Runtime and Compile to Executable:

clang main_inst.bc -L./runtime -lmonvit_rt -o my_program

4. Run

./my_program