A rant about PHP compilers in general and HipHop in particular.

we're talking about websites that are in the top 5% of all sites (by traffic) that would even need to think about using something like phpc or hphp. the number of those that would be willing to use a freelancer to handle their scaling issues are probably in the single digits at best.

However, these days new C/C++ apps seem restricted to domains like embedded systems, real-time, and game development, where memory usage (no virtual memory) and performance consistency is very important. So a JIT doesn't make a lot of sense, as it introduces a lot of variability into the application.

In order to do anything resembling JIT, you would have to insert a runtime where one does not already exist.

First up: I'm talking about object-oriented approaches, with polymorphism and dynamic dispatch being the core abstraction tool.

If you try to introduce more static typing into a system with polymorphism, you're trying to push type decorations so that they can follow where the expressions go a little further, so that you can reason more. Parametric polymorphism, or generic types and methods (hence forth called generics to avoid confusion with OO polymorphism), are a key tool. They let you better reason about values of which types cross method boundaries. With generic methods, you can usually infer the type parameters. With generic types, the type arguments can serve as extra documentation. A Java ArrayList<E> seems clearly superior to ArrayList at first glance.

But all sorts of problems start coming up when you mix polymorphism with generics. Type inference isn't so simple anymore, as ad-hoc choices need to be made. Instantiations of generic types have no necessary subtyping relationship. So you have to reintroduce dynamicism in the back door via base classes or interfaces etc. that use a top type (java.lang.Object or whatever) to re-unify types; or you dabble in variance. And you want to do more with your arguments of type parameter types, so you bring in type parameter constraints, or type classes, or other means of generically handling values.

So perhaps you have use-site variance or declaration-site variance combined with type classes combined with polymorphism. Suddenly using or declaring generic types has gotten quite a bit harder. Getting type information to flow into the right places starts requiring confusing tricks, particularly when you want e.g. the methods of your ancestor to return a type related to the concrete instantiation type, for example something like this in C# syntax:

    // so you want a GetThis method that returns a correctly
    // typed thing for the current instance
    class Base<T> where T: Base<T> { public T GetThis() { ??? } }
    class Desc: Base<Desc> {}

One of the paradoxes is that declaratively specifying, at compile-time, all the types that may reach different parts of your program at runtime, is harder than writing the program to build the object graph at runtime in the first place, and treating the occasional runtime type error as a bug that will be eliminated.

Now to the other side of the story, meta-programming. I agree that you can do lots of meta-programming at compile-time, but there are two problems, as I see it. One: compile-time is too early to be making decisions. What do you do when you can't afford to restart the process, but you still need to change the running code and how it works? This isn't a theoretical concern; I've architected such a server system in the past. It could keep on handling requests relating to client sessions associated with a previous versions of the server-side behaviour, but new client sessions would use the latest version of the server-side behaviour. Rolling over the server farm from one version to the next didn't require tricks with load balancers and restarting app servers, etc.; it was all built into the system.

(Actually, the architecture was really interesting, and I should write it up in more detail one day. Another key fallout of this approach was that the session data, which was (usually) stored on disk and was a bit like a Lisp or Smalltalk image only typically 20K in size, had a pointer (a URL) inside it telling the server where to get the behaviour for this session. This meant that problem sessions could be post-mortemed quite effectively: the old session could be resurrected, stepped through, etc.)

And the second problem of compile-time metaprogramming: the formalisms you use to munge with the types, particularly static types which the compiler will later infer further things from, may in practice be quite different to the imperative constructs programmers are used to using to implement behaviour. When they want to add a method to a class, they want to do it using the same tools (or close) to that which they'd use for adding an item to a list or hash table.

Perhaps the meta-programming is implemented as a plugin to the compiler, or a macro system, where the code is modifying the compiler's own definitions of the types etc. But now we've introduced an abstraction level into the chain of source code -> executable text that most programmers are not familiar with, and may be intellectually ill equipped to deal with - not because they're stupid or incompetent, but rather because they simply don't need to know. Consider e.g. hygienic macros and the problems of which symbol refers to which level of abstraction, the compilation level, or the runtime level; the distinction between the level you're at when you're quoting and unquoting, in Scheme terms. Consider the confusion of this poor fellow whom I helped out:

http://stackoverflow.com/questions/326321/how-do-i-create-th...

When you consider the tangled mess of static typing you can get into when you mix polymorphism with generics, and then add in the ability to munge with code and types such that the compiler sees the modified code / types, and hopefully you can see it's easy to add more complexity than the benefit you get out.

And of course, graceful failure is an art in itself. You have to start turning down requests before you actually hit peak capacity, or you risk getting stuck in a thrashing pit.

All interesting philosophical questions. However, since I have editorial control, and determined that he was acting anti-socially, it seems perfectly fine to me to scribble over it (though I could see a problem if I wasn't transparent about it).

The question of "which is better" wasnt really a factor in my decision to build a static compiler instead of a JIT. Building a JIT as a solo effort for a complex language like PHP would probably have prevented me doing enough research to get a PhD. More importantly, I wasn't interested in JIT, I was interested in static analysis and (static) optimization.