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.