Smart Contract Compiler

How Can I Add New Features or Fix Bugs in the Smart Contract Compiler?

Basics of the Smart Contract Compiler

The NEO•ONE Smart Contract Compiler is by far the largest NEO•ONE package. The compiler takes in (almost) regular TypeScript code and compiles it to NeoVM bytecode, which can then be deployed to the Neo blockchain and run on the NeoVM. NEO•ONE uses the TypeScript compiler API to parse the TypeScript code into a tree of "nodes" with information about each node. Our compiler then "visits" each node and begins to output specific bytecode for the VM instructions that are needed in order to execute the logic that is specified by the TypeScript code. The bytecode outputted by the compiler corresponds to human-readable opcodes that each correspond to an action that the NeoVM will perform. These actions are the manipulation of data by the NeoVM that will ultimately translate to changes to the state of the Neo blockchain.

Where to Look in the Code

Now that you have a very basic understanding of how the NEO•ONE compiler works, you can start digging into the neo-one-smart-contract-compiler package. Most likely you’ll be looking in neo-one-smart-contract-compiler/compiler/<subfolder> (where <subfolder> is one of constants, declaration, expression, helper, scope, statement) for the specific syntax that is broken or where you want to add a feature. For example, if you want to change how we compile the == token, you would look for EqualsEqualsEqualsHelper.ts in neo-one-smart-contract-compiler/compiler/helper/relational/EqualsEqualsHelper.ts. In there you’ll see how this helper will "emit" different opcodes, syscalls, and other helpers to manipulate the Evaluation Stack. The comment line above each emit shows a representation of the Evaluation Stack after that bytecode is evaluated.

Write Unit Tests for What You’re Working On

Once you have an understanding of what helpers or syntax compilers you need to change in order to make your compiler change, the best way to begin is to write a unit test that you will run to test your change. You’ll see that nearly every helper and syntax compiler has a corresponding set of unit tests in neo-one-smart-contract-compiler/src/__tests__. For example, the IfStatementCompiler.ts has a set of unit tests in IfStatementCompiler.test.ts. In there you’ll see that we typically use the built in assertEqual method to test if values are what we expect them to be. You’ll also see that we have helpers, like the helpers.executeString() helper, that make it easy to compile a string and test for certain behavior.

Here is an example unit test you would write to test your changes:

import { helpers } from '../../../__data__';

describe('MyNewCompiler', () => {
  test.only('simple test', async () => {
    await helpers.executeString(`
      if (!true) {
        throw 'Failure';

      const x = '10';
      assertEqual(x, '10');

Start Hacking

Once you’ve created a unit test that either recreates the bug you’re trying to fix, or tests for the expected behavior of your new feature, you can start to make changes to the compiler’s source code and run your unit test. If you want to add logging (ie. console.log) to the source code to get more information you can, just make sure to change the console settings in neo-one-build-tests/environments/test/jestSetup.js. From there you should make sure to only run one unit test at a time so that you’re only getting logs from the compilation of that one unit test. This will make it easier to learn what the compiler is doing when compiling a specific string. To run a specific unit test, rather than all unit tests, run rush test -t <path/to/testfile>.

And that’s it! Once you have this workflow setup you can hack away at the compiler code and run the unit test to test your changes, get logs, etc.

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