Skip to content
A MiniJava compiler written in Rust.
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.

comprakt - A MiniJava compiler

Build Status API documentation for master branch LibFirm API Doxygen Docs

Special features:

  • State-of-the-art error messages and warnings in the style of Elm and Rust
  • A backend using linear-scan register allocation
  • A language extension for attribution of programs, classes and methods
  • A Linter that detects possible logical mistakes / software quality issues
  • A memory-safe mode that aborts the program in a reproducable manner on invalid memory acccesses (NullPointerException, IndexOutOfBounds) (PR awaiting merge)
  • A safe Rust wrapper and abstraction around libfirm.
  • A visual debugger that can be used to inspect the internal compiler state in the browser (and in Visual Studio Code)

Check out the final project presentation (German).

Available Optimizations

  • Inline: method inlining
  • ConstantFolding: constant folding and unreachable code elimination
  • ConstantFoldingWithLoadStore: constant folding, unreachable code elimination and load-store optimization
  • ControlFlow: remove unnecessary (un)conditional jumps
  • NodeLocal: replace some operations with a sequence of shifts
  • CommonSubExpr: block-local common subexpression elimination of commutative operations
  • EarliestPlacement: move all operations to the earliest valid point
  • CostMinimizingPlacement: move all operations to a cost minimizing location (subsumes Loop Invariant Code Motion)
  • CodePlacement: EarliestPlacement + CommonSubExprElim + CostMinimizingPlacement results in GCSE taking all opportunities

Available Optimization Levels

  • -O None: no optimizations
  • -O Moderate (correctness submission): Inline,ConstantFolding,ControlFlow
  • -O Aggressive (BENCH=1 submission): Inline,ConstantFoldingWithLoadStore,ControlFlow,NodeLocal
  • -O Custom:[OptimizationName,]+: a custom list of optimizations, e.g. -O Custom:Inline,ConstantFolding,ControlFlow is equivalent to -O Moderate

(The optimizations in CodePlacement are not recommended given their speed-up vs. compile-time cost trade-off. Just consider them unfinished.)

Checkout & Install Build Dependencies

# Check out this repo and submodules
git clone
git submodule update --init --recursive
# Install Rust using rustup and the toolchain we are using
curl -sSf | sh
rustup toolchain install nightly-2018-12-17

cd comprakt
rustup component add clippy
rustup component add rustfmt

Additionally, make sure the following build dependencies of libFirm are installed (see libfirm-rs-bindings/

  • Python (2.7.x or >=3.3 are supported)
  • Perl
  • an ANSI C99 compiler (gcc, clang, icc are known to work)
  • Git
  • GNU Make

More detailed Rust install instructions can be found at

Debugging and Testing

To run all unit and integration tests, run:

cargo test

To run only regression and integration tests, execute:

cargo test --test integration

To run only a specific kind of integration tests, e.g. binary tests, that run all compiler stages and the generated executable itself, execute:

cargo test --test integration binary::

To run only a specific integration test, e.g. the one that you are currently trying to fix:

cargo test --test integration binary::can_print

If you add a integration test file, through a shortcoming of the default rust test runner, you might have to recompile parts of the crate:

cargo clean -p compiler-cli

If your patch changes a lot of reference files for stderr/stdout/exitcodes, you can automatically regenerate the references by running:

env UPDATE_REFERENCES=true cargo test --test integration

In case for binary tests, run it another time to regenerate the reference files for the generated binaries.

If the compiler crashes during firm graph generation, VCG files are generated in the /compiler-cli/.tmpIR/ folder. (But note that if you are testing the compiler directly by using cargo run instead of cargo test, VCG files are generated in .tmpIR/ in the root of the repository.)


The compiler has an internal timer that can be used to benchmark each compiler phase. You could get an excerpt for a single run using the env flag MEASURE_STDERR=yes or MEASURE_JSON=FILE.json. But please use the benchmark utility instead. Invoking

cargo run --bin benchmark

will benchmark all mjtests in the tests/big folder and print a listing in the following format:

BENCHMARK bench_conways_game_of_life.input

optimization phase                680.66667 +/- 14.38363ms    3 samples      -0.681%     1m 9s ago
  opt #0: Inline                  113.00000 +/-  3.55903ms    3 samples      -3.966%     1m 9s ago
  opt #1: ConstantFolding         449.00000 +/-  9.20145ms    3 samples      +0.074%     1m 9s ago
  opt #2: ControlFlow             117.33333 +/-  1.24722ms    3 samples      -0.283%     1m 9s ago

BENCHMARK bench_math.input

optimization phase               2590.33333 +/- 26.02990ms    3 samples      -3.177%     1m 9s ago
  opt #0: Inline                  438.66667 +/-  6.84755ms    3 samples      -0.529%     1m 9s ago
  opt #1: ConstantFolding        1388.00000 +/-  6.68331ms    3 samples      -6.385%     1m 9s ago
  opt #2: ControlFlow             762.33333 +/- 18.51726ms    3 samples      +1.554%     1m 9s ago

The percentage and relative time, e.g. -0.681% 1m 9s ago, are a comparison to the last invokation of the benchmark utility, e.g. 0.681% faster to the invokation 1 minute and 9 seconds ago.

The benchmark utility accepts some flags (that are more thoroughly explained in its --help dialog):

cargo run --bin benchmark -- --samples NUM_SAMPLES --only REGEX --optimization Aggressive|None|Moderate|Custom:*

--samples specifies the number of invokations per input file. --only is a regex that can be used to filter the input files, e.g. math|conway will only run the benchmarks shown in the example output above. --optimization is identicial to the --optimization flag that the compiler-cli accepts.


We use the tools rustfmt to format our code and clippy as a linter to keep the code clean, idiomatic and correct. To run those tools use

cargo clippy
cargo fmt --all

For code that should not get formatted by rustfmt mark the code with


This should only be used sparingly, e.g. for tabular constant definitions where you want to align the columns and similar cases.

A Clippy lint can be disabled similar to rustc lints:


Fixing a Clippy warning instead of just allowing it, is almost always the better option. Lints should only be disabled if it really does not make sense to fix them or if it is a false positive.


To keep the code quality high and always have a master branch, that passes all test all the time (The Not Rocket Science Rule), we aim to only add code to the project via Pull Requests. Every PR should get reviewed by another person to ensure keeping the code quality high (typos and simple one-line fixes can be accepted by the author). After that the reviewer should use bors to merge the PR. For big and/or major changes it is recommended to have at least two reviewers.

Before creating a PR make sure to run

cargo clippy
cargo fmt --all
cargo test
# or
./build --ci

locally. If one of these commands produce an error the CI will fail.

Every code addition should include a test case for high test coverage.

Merging Pull Requests

To always have a building master branch we want to use bors-ng. Bors is a GitHub bot that prevents merge skew / semantic merge conflicts, so when a developer checks out the master branch, they can expect all of the tests to pass out-of-the-box.

To test a PR comment with bors try.

To approve (and merge) a PR comment with bors r+. If another person reviewed and accepted the PR you can tell this bors, by commenting with (for example) bors r=flip1995.

More commands for the bors-bot can be found in the documentation.

Using the Visual Debugger

The visual debugger is disabled by default. To enable it, enable the debugger_gui feature during compilation. For example, to debug the constant folding optimization run the following commands and then open http://localhost:8000.

# Once per checkout, compile the debugger gui frontend
pushd compiler-lib/debugger-gui
npm install
npm run build
# Every time you want to use the debugger_gui, run the following commands
pushd compiler-cli
cargo run --features "debugger_gui" -- --emit-asm my_file.mj  -O Custom:ConstantFolding.gui

You should see an interface that looks similar to the screenshot below. The debugger will halt at each location annotated with the breakpoint! macro. For a detailed explanation see the module-level documentation of compiler_lib::debugger.

Debugger Screenshot


Licensed under either of

at your option.

You can’t perform that action at this time.