Notes

Compiler basics (3): LLVM

March 10, 2019

Previously in compiler basics:
Compiler basics (1): lisp to assembly
Compiler basics (2): user-defined functions and variables

In this post we'll extend the compiler to emit LLVM IR as an option instead of x86 assembly.

All source code is available on Github.

LLVM IR is a portable, human-readable, typed, assembly-like syntax that LLVM can apply optimizations on before generating assembly for the target architecture. Many language implementors choose to compile to LLVM IR specifically to avoid needing to implement sophisticated optimizations.

But the biggest reason I'm adding an LLVM backend is so that I can punt on implementing register allocation. This is the technique that allows you to generically use as many registers as possible before storing local variables on the stack. While register allocation algorithms are not that difficult, I got bored/lazy trying to implement this for ulisp. And LLVM IR provides "infinite" locals that get mapped as needed to registers and the stack -- implementing register allocation.

LLVM IR basics

In LLVM IR, all local variables must be prefixed with %. All global variables (including function names) must be prefixed with @. LLVM IR must be in single-static assignment (SSA) form, which means that no variable is assigned twice. Additionally, literals cannot be assigned to variables directly. So we'll work around that by adding 0 to the literal. Furthermore, we'll take advantage of the add, sub, and mul operations built into LLVM IR.

; x = 4
%x = add i32 4, 0

The type that the operation is operating on must be specified after the operation name. In this case we are specifying that add is operating on and returning 32-bit integers.

While this might seem very inefficient, we'll see in the end that LLVM easily optimizes this away.

Function definition

Functions are defined at the top-level and are much simpler than x86 assembly since the details of calling conventions are handled by LLVM.

; (def plus (a b) (+ a b))
define i32 @plus (i32 a, i32 b) {
  %res = add i32 a, b
  ret i32 %res
}

In ulisp, all functions will return a result (and the only supported type for now are 32-bit integers). So we annotate the definition with this return type (i32 in define i32). Finally, we return inside the function with the ret instruction that must also specify the type (again i32).

Generating LLVM IR

We are going to generate LLVM IR as text. But any large project will benefit from generating LLVM IR via API.

Supporting multiple backends

The goal is to be able to switch at compile-time between generating x86 assembly or generating LLVM IR. So we'll need to reorganize ulisp a little bit.

We'll edit src/ulisp.js to accept a second argument to specify the backend (and from now on we'll default to LLVM).

const cp = require('child_process');
const fs = require('fs');

const { parse } = require('./parser');
const backends = require('./backend');

function main(args) {
  const input = fs.readFileSync(args[2]).toString();

  let backend;
  switch (args[3]) {
    case 'llvm':
    case undefined:
      backend = backends.llvm;
      break;
    case 'x86':
      backend = backends.x86;
      break;
    default:
      console.log('Unsupported backend ' + args[3]);
  }

  const [ast] = parse(input);
  const program = backend.compile(ast);

  try {
    fs.mkdirSync('build');
  } catch (e) {}
  backend.build('build', program);
}

main(process.argv);

The LLVM backend

We'll add src/backend/llvm.js and And expose compile and build functions.

compile(ast)

This will work the same as it did for the x86 backend, creating a new Compiler helper object, creating a scope manager (which we'll get into in more detail shortly), and generating code from the AST wrapped in a begin.

module.exports.compile = function(ast) {
  const c = new Compiler();
  const scope = new Scope();
  c.compileBegin(ast, scope.symbol(), scope);
  return c.getOutput();
};

build(buildDir, output)

The job of build will be to clean up the build directory, write any output as needed to the directory, and compile the written output. Since we're dealing with LLVM IR, we first call llc on the IR file to get an assembly file. Then we can call gcc on the assembly to get a binary output.

const cp = require('child_process');
const fs = require('fs');

...

module.exports.build = function(buildDir, program) {
  const prog = 'prog';
  fs.writeFileSync(buildDir + `/${prog}.ll`, program);
  cp.execSync(`llc -o ${buildDir}/${prog}.s ${buildDir}/${prog}.ll`);
  cp.execSync(`gcc -o ${buildDir}/${prog} ${buildDir}/${prog}.s`);
};

Taking advantage of locals

Before we get too far into the specifics of LLVM IR code generation, let's build out the infrastructure to take advantage of "infinite" locals. In particular, we want a local-manager (Scope) with four functions:

It is important to track and map user variables into safe names so we don't accidentally conflict between variable names used by the user and names used by the compiler itself.

register(local)

When we register, we'll want to replace any unsafe characters that Lisp allows but LLVM likely won't. For now, we'll just replace any dashes in the name (since dashes are fine in variables in Lisp) with underscores. Then we'll add a number to the end of the local name until we have a safe name that doesn't exist already. Finally we return that safe name after storing the mapping.

class Scope {
  constructor() {
    this.locals = {};
  }

  register(local) {
    let copy = local.replace('-', '_');
    let n = 1;
    while (this.locals[copy]) {
      copy = local + n++;
    }

    this.locals[local] = copy;
    return copy;
  }
}

symbol()

This is a simple function that will return one new unused safe name that we can store things in.

class Scope {
  ...

  symbol() {
    const nth = Object.keys(this.locals).length + 1;
    return this.register('sym' + nth);
  }

  ...
}

We start off by making up a name based on the prefix sym and a suffix of the current key length and pass that into the register function to make sure we get a safe name.

get(local)

This function is a very simple lookup to return the safe name for a user variable. It is up to the caller of this function to handle if the user variable does not exist in scope (and perhaps throw a compiler error back to the programmer).

class Scope {
  ...

  get(local) {
    return this.locals[local];
  }

  ...
}

copy()

Finally, we want to expose a copy function so we can duplicate the local storage before entering a new scope. (A variable inside a function should not exist in scope outside the function.)

class Scope {
  ...

  copy() {
    const c = new Scope();
    c.locals = { ...this.locals };
    return c;
  }

  ...
}

Back to codegen!

As mentioned in module.exports.compile, we're going to use a Compiler that exposes a number of compiler helpers:

emit(depth, code)

Like we had in the x86 backend, this will indent the code two spaces depth times and write it to the buffer we track generated code.

class Compiler {
  constructor() {
    this.outBuffer = [];
  }

  emit(depth, code) {
    const indent = new Array(depth + 1).join('  ');
    this.outBuffer.push(indent + code);
  }
}

compileBegin(ast, destination, scope)

Our first compiler function actually does no code generation itself. We'll call compileExpression on each item within the begin block. And we'll pass the destination to the last expression in the list so that the value of a begin block is set to the value of its last expression. All other expressions will receive a temporary variable to store results.

class Compiler {
  ...

  compileBegin(body, destination, scope) {
    body.forEach((expression, i) =>
      this.compileExpression(
        expression,
        i === body.length - 1 ? destination : scope.symbol(),
        scope,
      ),
    );
  }

  ...
}

Example:

(begin 1 2) ; returns 2

compileExpression(ast, destination, scope)

This is the most generic compile function. If the ast is a list (representing a function call), it will pass compilation off to compileCall. Otherwise the only non-function call parts of the language are variable references and numeric literals.

class Compiler {
  ...

  compileExpression(exp, destination, scope) {
    // Is a nested function call, compile it
    if (Array.isArray(exp)) {
      this.compileCall(exp[0], exp.slice(1), destination, scope);
      return;
    }

    // If numeric literal, store to destination register by adding 0.
    if (Number.isInteger(exp)) {
      this.emit(1, `%${destination} = add i32 ${exp}, 0`);
      return;
    }

    // If is local, store to destination register similarly.
    const res = scope.get(exp);
    if (res) {
      this.emit(1, `%${destination} = add i32 %${res}, 0`);
    } else {
      throw new Error(
        'Attempt to reference undefined variable or unsupported literal: ' +
          exp,
      );
  }

  ...
}

Example:

1
...
a
...
(+ 1 a)

compileCall(functionName, arguments, destination, scope)

Most function calls will automatically compile arguments before calling the function. However, certain control-flow primitives don't do this (e.g. def, if, etc.). Macros in Lisp allow you to add new control-flow primitives (even if you don't use it to modify control-flow). But we will ignore user-defined primitives for now.

We'll keep a list of control-flow primitives and pass off compilation to them if the function name matches a primitive. Otherwise, we'll look up the function name in scope (to find its safe name), compile the arguments, and call the function with the results of the arguments.

class Compiler {
  constructor() {
    this.outBuffer = [];
    this.primitiveFunctions = {
      def: this.compileDefine.bind(this),
      begin: this.compileBegin.bind(this),
    };
  }

  ...

  compileCall(fun, args, destination, scope) {
    if (this.primitiveFunctions[fun]) {
      this.primitiveFunctions[fun](args, destination, scope);
      return;
    }

    const validFunction = scope.get(fun);
    if (validFunction) {
      const safeArgs = args
        .map((a) => {
          const res = scope.symbol();
          this.compileExpression(a, res, scope);
          return 'i32 %' + res;
        })
        .join(', ');
      this.emit(1, `%${destination} = call i32 @${validFunction}(${safeArgs})`);
    } else {
      throw new Error('Attempt to call undefined function: ' + fun);
    }
  }

  ...
}

Yay LLVM for simplifying calls!

Example:

(foo 1)
...
(+ 1 2)

compileDefine([functionName, parameters, ...body], destination, scope)

This is the last undefined compile function we've used. The call signature may look funny but we write less code if we keep the primitive signatures the same. In any case, JavaScript's destructuring makes it pretty enough.

Aside from code generation, we also need to add the function itself to scope so we can look it up later in use. Additionally we need to create a copy of the current scope for the body of the function. And we'll add the parameter names themselves to the child scope.

class Compiler {
  ...

  compileDefine([name, params, ...body], destination, scope) {
    // Add this function to outer scope
    const safeName = scope.register(name);

    // Copy outer scope so parameter mappings aren't exposed in outer scope.
    const childScope = scope.copy();

    const safeParams = params.map((param) =>
      // Store parameter mapped to associated local
      childScope.register(param),
    );

    this.emit(
      0,
      `define i32 @${safeName}(${safeParams
        .map((p) => `i32 %${p}`)
        .join(', ')}) {`,
    );

    // Pass childScope in for reference when body is compiled.
    const ret = childScope.symbol();
    this.compileExpression(body[0], ret, childScope);

    this.emit(1, `ret i32 %${ret}`);
    this.emit(0, '}\n');
  }

  ...
}

Example:

(def plus (a b) (+ a b))

compileOp(op)

The last function mentioned above will help us expose some useful primitives. This function will take a string builtin operation and return a function that can be used to generate code when the operation is called.

class Compiler {
  ...

  compileOp(op) {
    return ([a, b], destination, scope) => {
      const arg1 = scope.symbol();
      const arg2 = scope.symbol();
      this.compileExpression(a, arg1, scope);
      this.compileExpression(b, arg2, scope);
      this.emit(1, `%${destination} = ${op} i32 %${arg1}, %${arg2}`);
    };
  }

  ...
}

This allows us to add some builtin ops as primitives (even though they aren't control-flow modifying).

class Compiler {
  constructor() {
    this.outBuffer = [];
    this.primitiveFunctions = {
      def: this.compileDefine.bind(this),
      begin: this.compileBegin.bind(this),
      '+': this.compileOp('add'),
      '-': this.compileOp('sub'),
      '*': this.compileOp('mul'),
    };
  }

  ...
}

Example:

(+ 1 2)

Hello world!

Putting it all together, we'll compile this Lisp program:

(def plus-two (a b)
     (+ a (+ b 2)))

(def main ()
     (plus-two 3 (plus-two 1 1)))

To get 9.

$ node src/ulisp.js tests/function_definition.lisp
$ ./build/prog
$ echo $?
9

Generated code

The generated LLVM can be found in ./build/prog.ll:

define i32 @plus_two(i32 %a, i32 %b) {
  %sym7 = add i32 %a, 0
  %sym9 = add i32 %b, 0
  %sym10 = add i32 2, 0
  %sym8 = add i32 %sym9, %sym10
  %sym6 = add i32 %sym7, %sym8
  ret i32 %sym6
}

define i32 @main() {
  %sym6 = add i32 3, 0
  %sym8 = add i32 1, 0
  %sym9 = add i32 1, 0
  %sym7 = call i32 @plus_two(i32 %sym8, i32 %sym9)
  %sym5 = call i32 @plus_two(i32 %sym6, i32 %sym7)
  ret i32 %sym5
}

You can see all these unnecessary add, ... 0 instructions. But let's look at the x86 assembly that LLVM generates in build/prog.s:

...

_plus_two:                              ## @plus_two
        .cfi_startproc
## %bb.0:
                                        ## kill: def $esi killed $esi def $rsi
                                        ## kill: def $edi killed $edi def $rdi
        leal    2(%rdi,%rsi), %eax
        retq
        .cfi_endproc
                                        ## -- End function
...

And we see that LLVM easily optimized the inefficiencies away. :)

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