Facebook has announced a new tool for PHP development known as the HipHop Virtual Machine (HHVM).
Facebook has introduced a new virtual
machine for PHP called the HipHop Virtual Machine (HHVM) that it is sharing
with the development community.
In 2010, Facebook came out with HipHop
for PHP, a source code transformer, to help support the growing
number of Facebook users. PHP developers found that tool useful, and Facebook
will be making the new technology available as well. HHVM is 60 percent faster
than the current HipHop interpreter with a 90 percent reduction in memory
In a Dec. 9 blog post, Jason Evans, a software
engineer on Facebook's HipHop team, said, "While HipHop has helped us make
significant gains in the performance of our code, its reliance on static
compilation makes optimizing our code time consuming. We were also compelled to
develop a separate PHP interpreter (HPHPI) that requires a lot of effort to
maintain. So, early last year, we put together a small team to experiment with
dynamic translation of PHP code into native machine code. What followed
is a new PHP execution engine based on the HipHop language runtime
that we call the HipHop Virtual Machine (HHVM). We're excited to report that
Facebook is now using HHVM as a faster replacement for the original HipHop
interpreter (HPHPI), with plans to eventually use HHVM for all PHP execution."
PHP is widely used in Web development.
And Facebook uses HPHPI (and now HHVM) for day-to-day software development, but
uses the HipHop compiler (HPHPC) to create optimized binaries that serve
the Facebook Website, Evans said.
"HPHPC is in essence a traditional
static compiler that converts PHP→AST→C++→x64.
We have long been keenly aware of the limitations to static analysis imposed by
such a dynamic language as PHP, not to mention the risks inherent in developing
software with HPHPI and deploying with HPHPC. Our experiences with HPHPC led us
to start experimenting with dynamic translation to native machine code, also
known as just-in-time (JIT) compilation. A dynamic translator can observe data
types as the program executes, and generate type-specialized machine code.
"Unfortunately we didn't have a clean model of PHP language semantics built into
HipHop, as HPHPC and HPHPI are based directly on two distinct abstract syntax
tree (AST) implementations, rather than sharing a unified intermediate
representation. Therefore we developed a high-level stack-based virtual machine
specifically tailored to PHP that executes HipHop bytecode (HHBC). HHVM uses
HPHPC's PHP→AST implementation
and extends the pipeline to PHP→AST→HHBC. We iteratively codeveloped both
an interpreter and a dynamic HHBC→x64
translator that seamlessly interoperate, with the primary goal of cleanly
Evans said most existing translators
use method-at-a-time JIT compilation like Java and C#, though trace-based
translation has also been explored in recent systems such as Tamarin and TraceMonkey.
However, "We decided to try a very simple form of tracing that limits each
trace to a single basic block with known input types," he said. This "tracelet"
approach simplifies the trace cache management problem, because complex
application control flow cannot create a combinatorial explosion of traces.
Evans said HHVM is right now about 1.6
times faster than HPHPI for certain functions, but he said he expects the
performance to increase over the coming months as the dynamic translator
stabilizes and matures.
"In fact, we predict that HHVM will
eventually outperform statically compiled binaries in Facebook's production
environment, in part because we are already sharing enough infrastructure with
the static compiler that we will soon be able to leverage static analysis
results during tracelet creation," he said.
Meanwhile, Evans said the HipHop
Virtual Machine project is about 90 percent done and Facebook will continue to
share HipHop via the public GitHub
site for HipHop.
"We hope that the PHP community will
find HHVM useful as it matures and engage with us to broaden its usefulness
through technical discussions, bug reports and code contributions," Evans said.
"We actively monitor the GitHub
site and mailing
Darryl K. Taft covers the development tools and developer-related issues beat from his office in Baltimore. He has more than 10 years of experience in the business and is always looking for the next scoop. Taft is a member of the Association for Computing Machinery (ACM) and was named 'one of the most active middleware reporters in the world' by The Middleware Co. He also has his own card in the 'Who's Who in Enterprise Java' deck.