Bundle Format Spec about hermes

[mhorowitz]

Hi! There aren't docs, but I can point you to some source code to get you started. First, I should point out that the bytecode file format is not intended to be fixed long-term. We update it pretty often, so you should expect to need regular incremental changes.

The overall format of the bytecode file is defined in the serializer/deserializer code. It's driven by a visitor here. This header defines a magic number and the bytecode version, which your tool can use to recognize Hermes bytecode files, and the specific format of each file. The implementations are in BytecodeStream.cpp. The bytecodes themselves are defined in BytecodeList.def.

We also have some tooling to dump bytecode files which might help. If you have a bytecode file, you can run

bin/hermes -dump-bytecode myfile.hbc

to see various data about the file. This was intended for humans, not automated systems, so I wouldn't depend on parsing the output, but it might help you writing your own tool.

The source maps are in the conventional format; we're not doing anything special there.