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Epsilon

An experimental MLIR compiler that goes from custom dialects to native binaries. Built incrementally, an epsilon at a time.

The Story

This project started as a way to deepen my understanding of MLIR. I learn best by building things, so I set out to construct a small compiler incrementally, one layer at a time.

The initial goal was to understand the core abstractions: dialects (tensor, control flow, buffer), operation semantics (constant folding, type constraints), and optimization passes (DCE, CSE, LICM, tiling, fusion). I wanted to see how MLIR's infrastructure works from the inside - not just reading documentation, but writing TableGen definitions, implementing fold methods, and wiring up pass pipelines.

After the optimization passes were working, I got curious: can I actually run this? So I extended the compiler to bufferize tensor operations into explicit memory management, lower through standard MLIR dialects, emit LLVM IR, and use clang to compile to a native binary. It was satifying seing [[6, 8], [10, 12]] come out of a program that started as epsilon.add on two constant tensors.

At that point, I wanted to close the loop on the end-to-end workflow. Writing MLIR by hand is fine for testing, but real compilers have frontends. So I built an ONNX importer — a Python script that reads machine learning models and emits Epsilon IR. Now you can define a model in Python, export it to ONNX, and compile it to x86 machine code through the full pipeline.

The Build System Battle

In the beginning, I spent days fighting the build system. Linking LLVM/MLIR libraries with a standalone project is not straightforward — CMake configuration, library dependencies, and linking order all have to be exactly right. I tried debugging it with free AI chats and the LLVM documentation and kept hitting walls.

Then I decided to subscribe to Claude Code. The build file problem was solved in minutes. 🤯

The key discovery was a set of LLVM CMake flags that dramatically improved the development experience:

Flag What it does
-DCMAKE_BUILD_TYPE=RelWithDebInfo Optimized build with debug symbols — fast execution, debuggable
-DLLVM_BUILD_LLVM_DYLIB=ON Builds LLVM as a shared library instead of hundreds of static libs
-DLLVM_LINK_LLVM_SHLIB=ON Links tools against the shared lib — cuts link time dramatically
-DLLVM_ENABLE_LLD=ON Uses the LLD linker instead of GNU ld — much faster linking
-DLLVM_USE_SPLIT_DWARF=ON Splits debug info into .dwo files — reduces linker memory pressure
-DLLVM_CCACHE_BUILD=ON Enables ccache for compiler caching — speeds up rebuilds

Without these flags, a full LLVM build takes forever and linking a small tool against the static libraries can eat 16+ GB of RAM. With them, incremental builds are fast and linking is almost instant.

How Claude Code Helped

Thanks to Claude Code, building and learning has been much faster than I could have expected. I didn't need to get bogged down on syntax, CMake incantations, or MLIR boilerplate — I could focus on understanding the concepts while Claude handled the mechanical parts.

I also asked Claude to write the blog posts and code documentation for each feature, so I could read back through the explanations and deepen my understanding of what I'd built.

Architecture

                    ┌─────────────┐
                    │  ONNX Model │
                    │  (.onnx)    │
                    └──────┬──────┘
                           │  epsilon-import-onnx (Python)
                           ▼
                    ┌─────────────┐
                    │   Epsilon   │  epsilon.constant, epsilon.add,
                    │   Tensor    │  epsilon.mul, epsilon.reduce_sum,
                    │   Dialect   │  epsilon.reduce_max, epsilon.fill
                    └──────┬──────┘
                           │  Canonicalization / Constant Folding
                           │  DCE, CSE, LICM, Tiling, Fusion
                           │
                           │  --convert-epsilon-to-epsilon-buf
                           ▼
                    ┌─────────────┐
                    │ epsilon_buf │  epsilon_buf.alloc, epsilon_buf.add,
                    │   Buffer    │  epsilon_buf.mul, epsilon_buf.dealloc,
                    │   Dialect   │  epsilon_buf.constant, epsilon_buf.fill
                    └──────┬──────┘
                           │  Memory Planning, Buffer Reuse
                           │
                           │  --convert-epsilon-buf-to-std
                           ▼
                    ┌─────────────┐
                    │  Standard   │  memref.alloc, scf.for,
                    │    MLIR     │  arith.addi, memref.store
                    └──────┬──────┘
                           │  --convert-scf-to-cf
                           │  --convert-cf-to-llvm
                           │  --convert-arith-to-llvm
                           │  --convert-func-to-llvm
                           │  --finalize-memref-to-llvm
                           ▼
                    ┌─────────────┐
                    │    LLVM     │  llvm.func, llvm.add,
                    │   Dialect   │  llvm.load, llvm.store
                    └──────┬──────┘
                           │  mlir-translate --mlir-to-llvmir
                           ▼
                    ┌─────────────┐
                    │   LLVM IR   │  define i32 @main() { ... }
                    └──────┬──────┘
                           │  llc → clang
                           ▼
                    ┌─────────────┐
                    │   Native    │
                    │   Binary    │
                    │   (x86)     │
                    └─────────────┘

Project Structure

include/Dialect/Tensor/    # Tensor dialect (epsilon.add, epsilon.mul, ...)
include/Dialect/Buf/       # Buffer dialect (epsilon_buf.alloc, epsilon_buf.add, ...)
include/Dialect/CF/        # Control flow dialect (epsilon_cf.br, epsilon_cf.cond_br)
include/Conversion/        # Conversion pass headers
lib/Dialect/               # Dialect implementations
lib/Conversion/            # Conversion passes (tensor → buffer → standard)
test/                      # LIT tests for every dialect, pass, and conversion
tools/epsilon-opt.cpp      # Compiler driver (C++)
tools/epsilon-import-onnx.py  # ONNX frontend (Python)
documentation/blog/        # Blog posts documenting each feature

Build

Prerequisites

  • CMake >= 3.20, Ninja, Clang, LLD
  • ccache (recommended)
  • Python 3 with onnx and numpy (for ONNX import)

Build LLVM/MLIR

cd externals/llvm-project
cmake -G Ninja -B build llvm \
  -DLLVM_ENABLE_PROJECTS=mlir \
  -DLLVM_TARGETS_TO_BUILD="Native" \
  -DCMAKE_BUILD_TYPE=RelWithDebInfo \
  -DLLVM_ENABLE_ASSERTIONS=ON \
  -DLLVM_BUILD_LLVM_DYLIB=ON \
  -DLLVM_LINK_LLVM_SHLIB=ON \
  -DLLVM_ENABLE_LLD=ON \
  -DLLVM_USE_SPLIT_DWARF=ON \
  -DLLVM_CCACHE_BUILD=ON \
  -DCMAKE_C_COMPILER=clang \
  -DCMAKE_CXX_COMPILER=clang++
ninja -C build
cd ../..

Build Epsilon

cmake -G Ninja -B build . \
  -DCMAKE_BUILD_TYPE=RelWithDebInfo \
  -DMLIR_DIR=$(pwd)/externals/llvm-project/build/lib/cmake/mlir \
  -DLLVM_DIR=$(pwd)/externals/llvm-project/build/lib/cmake/llvm \
  -DLLVM_EXTERNAL_LIT=$(pwd)/externals/llvm-project/build/bin/llvm-lit
ninja -C build

Run Tests

# All tests
ninja -C build check-epsilon

# Single test with verbose output
./externals/llvm-project/build/bin/llvm-lit -v build/test/Dialect/Tensor/ops.mlir

Usage

Compile MLIR directly

# Parse and print
./build/bin/epsilon-opt input.mlir

# With constant folding
./build/bin/epsilon-opt input.mlir --canonicalize

# Full pipeline to LLVM dialect
./build/bin/epsilon-opt input.mlir \
  --canonicalize \
  --convert-epsilon-to-epsilon-buf \
  --convert-epsilon-buf-to-std \
  --convert-scf-to-cf \
  --convert-cf-to-llvm \
  --convert-arith-to-llvm \
  --convert-func-to-llvm \
  --finalize-memref-to-llvm \
  --reconcile-unrealized-casts

Import from ONNX

# Set up Python environment (one-time)
python3 -m venv .venv
.venv/bin/pip install onnx numpy

# Generate test models
.venv/bin/python test/ImportONNX/generate_test_models.py

# Import and compile
.venv/bin/python tools/epsilon-import-onnx.py test/ImportONNX/add_constants.onnx \
  | ./build/bin/epsilon-opt --canonicalize

Compile to native binary

# 1. Lower to LLVM dialect
./build/bin/epsilon-opt input.mlir \
  --convert-epsilon-buf-to-std \
  --convert-scf-to-cf --convert-cf-to-llvm \
  --convert-arith-to-llvm --convert-func-to-llvm \
  --finalize-memref-to-llvm --reconcile-unrealized-casts \
  -o lowered.mlir

# 2. Emit LLVM IR
./externals/llvm-project/build/bin/mlir-translate --mlir-to-llvmir lowered.mlir -o output.ll

# 3. Compile and run
./externals/llvm-project/build/bin/llc output.ll -o output.o -filetype=obj
clang output.o -o program
./program

Blog Series

The commit history is clean and incremental — you can follow the commits to see how each feature was built step by step.

Each feature is documented in a blog post that explains the design decisions and implementation details:

  1. Setting Up the Build System with CMake
  2. TableGen: Defining Operations Without the Boilerplate
  3. Building a Tensor Dialect
  4. Control Flow Dialect
  5. Dead Code Elimination
  6. Common Subexpression Elimination
  7. Loop-Invariant Code Motion
  8. Loop Tiling
  9. Loop Fusion
  10. Buffer Dialect
  11. Memory Planning
  12. Buffer Reuse
  13. Lowering to Standard Dialects
  14. Emitting LLVM IR
  15. ONNX Import