Directory Structure

There are over 1000 C++ files in HHVM's codebase, split into a number of different directories. This is a rough overview of what lives where (all paths are under hphp/):

compiler/: The old parser and bytecode emitter. This is deprecated and is currently being removed; the new replacement is in hack/src/hhbc.

doc/: Documentation, of varying age and quality.

hack/src/: The Hack parser, typechecker, bytecode emitter, and various other Hack-related tools.

hhbbc/: HipHop Bytecode-to-Bytecode Compiler, our ahead-of-time static analysis program.

hhvm/: main() for the hhvm binary

neo/: Code to read .hdf files, used for configuration.

parser/: The parser grammar and interface.

pch/: Pre-compiled headers for MSVC (not currently supported).

ppc64-asm/: Code to read and write ppc64 machine code.

runtime/: HHVM's runtime.
runtime/base/: Runtime internals related to Hack types and values (strings, objects, etc...).
runtime/debugger/: Support for debugging Hack programs at runtime.
runtime/ext/: Extensions.
runtime/server/: Built-in servers, both for web requests and other HHVM-specific types of requests.
runtime/test/: C++ unit tests for various runtime components.
runtime/vm/: Most of the internals of the VM. The distinction between runtime/base/ and runtime/vm is a bit fuzzy.
runtime/vm/jit/: HHVM's JIT compiler.
runtime/vm/verifier/: The bytecode verifier.

system/: systemlib, a collection of Hack code that is embedded into hhvm and always available.

test/: Integration tests for the whole compiler pipeline, written in PHP and Hack.

tools/: Utility programs useful in the development and/or use of HHVM.
tools/benchmarks/: Microbenchmarks. If you run these, take them with a pound of salt; we care much more about macrobenchmarks based on real-life workloads.
tools/gdb/: Python scripts to assist in debugging hhvm in gdb.
tools/hfsort: HFSort, the algorithm used to lay out functions in both hhvm and JIT-compiled code.
tools/tc-print/: TCPrint, a tool to analyze the output of the JIT compiler.

util/: Functions and data structures used by HHVM that could be useful to other programs, and have no dependencies on other parts of HHVM.

vixl/: Code to read and write AArch64 machine code.

zend/: Code imported from the standard PHP interpreter.