Sunday, March 3, 2013

LLVM Block Parsing Bug

The other day I ran into what I believed was an LLVM parsing bug and embarrassed myself in the process.

When I get interrupted from coding I'll often create a compiler error in the area of code I'm focussed on. This lets me quickly return to where I was before the interruption. Usually I create the compiler error by simply adding a free floating string above the line I am at. If I'm in the middle of writing a statement though I usually don't need the string since the code wont compile anyway.

In this particular case I got interrupted while writing a simple assignment statement. I got as far as the equal sign and the line looked like this:

        self.dictionary =

When I came back I reflexively compiled and was unsettled when it built successfully. When I ran the code I was more confused to find that the dictionary reference actually pointed to a block object? The next few lines of code looked like this:

        self.dictionary =
        self.operation.completionBlock = ^{
            NSLog(@"my completion block ran.");

It was a long day and my exhausted brain convinced itself that I discovered a really bad but really interesting parsing bug in LLVM.

I was excited at the prospect of this so (obviously) I wrote a blog entry as fast as I could and tweeted about it. For good measure I even CC'ed Mike Ash of Objective-C fame because he clearly needed to know about this. It took roughly 30 seconds for Mike to reply to me and shine a light on my dimness.

It is more obvious what is going on with a slight formatting change. Removing the new line produces this:
         self.dictionary = self.operation.completionBlock = ^{
           NSLog(@"my completion block ran.");

The result of an assignment statement is the value assigned (like x=y=10).

Lessons Learned or Re-learned:
1) Coding on little sleep is dangerous.
2) Mike Ash has better things to do than debug your crappy code.
3) Always remember: No, you did not find a compiler error. If you find yourself saying (or worse typing) those words turn the computer off and take a nap immediately.

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