Do you really need more speed? [5]

I’ve heard the argument "you don’t need a compiler, since PHP is rarely the bottleneck" for many years. I think its complete bollox. But I wrote a compiler for PHP, so I would say that.

Unless your PHP server is sitting there idling (which is probably the case for many PHP servers out there), then you could make use of a PHP compiler. For small timers, all components of your application are going to be sitting on the same box, contending for the same resources. Even if you assume the DB is the bottleneck, the resources the interpreter consumes could be more profitably spent on the DB.

The PHP interpreter is also quite memory hungry, as interpreters go. Any PHP value in your program uses 68 bytes of overhead [6]. An array of a million values takes over 68 MB. If HPHP is able to convert your million value array to native C types, it will only take 4MB. I’m sure your caches could make good use of that savings.

However, optimization isn’t only about speed. The main value is that they give you freedom in how you code.

There is a meme in the scripting language communities that {PHP,Ruby,Python,Perl,etc} are "fast-enough". If you need it to go faster, then you should take your hot-loop and rewrite it in C. HPHP will free you from such concerns.

You should consider also that PHP is considered relatively fast. Its not — the interpreter is dog-slow — but programs written in PHP are typically not that slow. This is because most of PHP’s huge standard library is written in C, with a thin layer of PHP over it. Anytime you call a string function, your PHP C string is passed into the C library, the pointers are manipulated and the bits are twiddled, and then it’s handed back to your code. Its a bit like driving in America: it takes a few minutes to get on the freeway, but once you’re on it you’re there in no time.

This is not necessarily a good thing:

1. if you want to write a library, and it needs to be fast, then it needs to be written in C,
2. if there is a PHP function that does almost what you need, and you write your own version instead, it will be slow.

Believe me, if your entire application just ran PHP interpreted code, it would not be fast at all. But people who write PHP functions and libraries don’t want to write C. They like PHP, are productive in it, and any time spent arsing around in C is wasted when there’s a website falling apart and a long list of features due yesterday. HPHP will free you from such concerns.

Compilers also provide other niceties. You don’t have to unroll your own loops, or move constant expressions out of loop headers. I don’t know if HPHP supports these, but I’m sure it could.

Allowing your existing code to go faster is hardly the point though. Really, the point is that you can do more in less time. Suppose you’ve decided that your application needs to response to the user in 500ms. The DB takes 200ms, the request takes 200ms, the framework takes 50ms and your code only has 50ms to run [7]. That’s quite a constraint. This leads to people using PHP as a simple templating layer, instead of as the Turing-complete langauge it is. I expect we’ll hear a lot more about HPHP, simply because of how freeing it is to the user.

So even if you only have a small VPS, instead of massive server farms like Facebook, you’re likely to find a use for HPHP. I’m sure shared hosts will set it all up soon for their users, and everyone will be happy.

Dynamic constructs

A better question is, how widely applicable is a compiler which doesn’t support all of PHP’s dynamic constructs? Funnily enough I did a bit of research on this. We chose the opposite tactic for phc: trying to stay 100% compatible with Zend, all the extensions (even the 3rd-party, unpublished, top-secret, proprietary ones), etc. You would expect we would first research whether this was useful? Well no. We built it, and then I did some analysis to check whether it was useful. You can read it in depth [8]. Basically, I downloaded 700 packages off sourceforge, and wrote some phc plugins to check for evals and dynamic includes (a dynamic include uses a variable as its parameter, instead of a constant or literal).

Result: 40% of PHP packages use dynamic constructs. Now this isn’t quite as scientific as it should be. Lots of those programs were old, and styles have changed. eval is discouraged these days, but it’s probably still used, if only to get around the weaknesses of the PHP parser. In particular, this doesn’t imply that HPHP is somehow fatally flawed for not supporting dynamic constructs. It just means it might not be so useful to you, the common PHP programmer.

An easy way to get round dynamic includes is to just consider the PHP files in the directory structure. There was a good research paper by Wassermann on supporting that, but I find it very hard going, so you probably will too. Still, a naive approach is to just stick a switch statement in, and compile everything that makes sense. This is how you would deal with things like WordPress plugins. It does mean that if you change your plugins, you’re just going to have to recompile. If you’re using a compiler, I doubt you would find that a problem.

What does this mean for phc?

When they annouced HPHP, I would have said it was phc‘s death toll. The original phc authors, Edsko and John, have moved on to other projects, and I’ve run it mostly solo for about two years. But I havent worked on phc in about 6 months, and my hatred of PHP makes it unlikely I will again. My requests for new contributors to step up has fallen on deaf ears, and my summer intern hasn’t decided to take over either.

Since no-one wants to take on the compiler, the new competition from Facebook should probably kill it, right? Maybe not. Over the last week, traffic to the phc website has increased by five times [14]. Facebook has unleashed some sort of latent interest in PHP compilers that I haven’t been able to extract from people. So perhaps this might be the rebirth of phc, not its death.

And phc is better than HPHP in some ways. HPHP is almost certainly faster because they didn’t have to deal with eval, dynamic stuff, and because they don’t use the Zend libraries. But phc was specifically designed to work with the Zend libraries, with eval, with everything. So it’s probably a better fit for most projects than HPHP.

If you want to take over phc, then join the mailing lists, download the source code, read the death notice and contribution page, and email me for commit access.

phc will likely live on anyway. The front-end is pretty slick: Facebook ran it over their million lines of code and only had one or two problems. It gives a lovely AST to allow all sorts of code transformation tools, has a nice plugin interface (for C++ lovers) and an XML interface (for the rest of you), and will spit your code out largely as you put it into it. Its certainly the most mature and well tested part of the whole project.

The optimizer is pretty slick as well, but in a different way. I’m pretty sure its the most advanced static analyser for PHP, and it’s waiting to be put to good use. That said, it’s damn slow, and not mature (read: pretty buggy), and itself doesn’t support eval and dynamic includes (surprise!!). The optimizer is waiting for some love — I could imagine it making a pretty nice "automatically find out what types your function may be passed" kind of linter.

Otherwise, phc will only live on as part of the Roadsend Raven compiler. I understand that they’re going to take the optimizer and the parser from phc, and that will be really interesting.

Finally, what does it mean for me? Well, I’ve left that ship already. I’ve hated PHP for a long time, and have no desire to go back to it. I’m doing a startup now, but when I go back to regular employment I will be looking for another scripting language run-time. There are plenty to choose from, in particlar Unladen swallow and TraceMonkey. Mozilla looks like an amazing place to work, so I think I’ve worked out my backup plan.