D for Programmers

I wrote a while ago about implementing thread support in the D .net compiler. The idea was to generate code that constructs delegates that are passed to the Start method of the System.Threading.Thread class. I discussed some details of constructing delegates out of nested functions, and I concluded that my next thread-related task was to implement support for the synchronized keyword.

Like the postman who goes for a walk in the park after coming home from work, I relax in the after hours by working on pet projects like C++ debuggers and D compilers. So this Saturday morning I sat down to implement the code generation for synchronized. The D language accepts two statement forms:

synchronized ScopeStatement
synchronized (Expression) ScopeStatement

The former can be easily reduced to the latter by synthesizing a global object and an expression that references it.

Here is a sample of D code that illustrates the use of the synchronized keyword:

import System;

class Semaphore {
  bool go;
}
Semaphore sem = new Semaphore;

void main() {
  void asyncWork() {
    while (true) {           // busy wait
      synchronized(sem) {
          if (sem.go) break;
      }
    }
  }
  Threading.Thread t = new Threading.Thread(&asyncWork);
  t.Start();
  synchronized(sem) {
    sem.go = true;
  }
  t.Join();
}

A synchronized statement can be transformed into the following pseudo-code:

object m = Expression();
try {
  lock(m);
  ScopeStatement();
}
finally {
  unlock(m);
}

Implementing lock / unlock maps naturally to the Enter / Exit methods of the System.Threading.Monitor class. That's really all there is to it, generating the method calls is trivial.

I was a bit disappointed by how easy the task turned out to be, but on the bright side I had plenty of time left to spend with my kid. I took him to Barnes and Noble to check out the Thomas trains and board books and the computer section, where I found the most helpful book title ever: "C# for Programmers". I guess no lawyer accidentally wandering through the computer book section can claim that he bought the book because the title was misleading. I wish that all book titles be so clear: "Finance for Accountants" or "Elmo Board Book for Toddlers".