In a response to a question from ReadWriteWeb, among other things, I wrote:

My main worry here is that people think this is some kind of magic
bullet that will solve their site performance problems. Generating C++
code from PHP code is a nifty trick and people seem to have gotten quite
excited about it. I'd love to see those same people get excited about
basic profiling and identifying the most costly areas of an application.
Speeding up one of the faster parts of your system isn't going to give
you anywhere near as much of a benefit as speeding up, or eliminating,
one of the slower parts of your overall system.

The "nifty trick" part of that seems to have become the story, and them
injecting a "just" in front it of it makes it sound more derogatory. Anyone
who knows me knows that I am a big fan of nifty tricks that solve the problem.
When I first heard about the Facebook effort I was assuming they were writing
a JIT based on LLVM V8 or something along those lines. Writing a good JIT is
hard. Doing static code analysis and generating compilable C++ from it is
indeed a nifty trick. It's not "just" a nifty trick, it is a cool trick that takes
advantage of a number of characteristics of PHP. The main one being that
you can't overload PHP functions. strlen() is always strlen, for example. In
Python, this would be harder because you can overload everything.



I also noted that most sites on the Web have a lot of lower hanging fruit that
would provide a much bigger performance improvement, if fixed, than doubling
the speed of the PHP execution phase. The ReadWriteWeb site, for example,
needs 160 separate HTTP requests and 41 distinct DNS lookups to load the
front page. And once you get beyond the frontend inefficiencies you usually
find Database issues, inefficient system call issues and general architecture
problems that again aren't solved by speeding up PHP execution.



If you have done your homework and find that your web servers are cpu-bound,
you are already using an opcode cache like APC
and your Callgrind callgraph
shows you that the PHP executor is a significant bottleneck, then HipHop PHP is
definitely something you should be looking at.