Capnproto usage in VTR

Capnproto is a data serialization framework designed for portabliity and speed. In VPR, capnproto is used to provide binary formats for internal data structures that can be computed once, and used many times. Specific examples:

• rrgraph
• Router lookahead data
• Place matrix delay estimates

What is capnproto?

capnproto can be broken down into 3 parts:

• A schema language
• A code generator
• A library

The schema language is used to define messages. Each message must have an explcit capnproto schema, which are stored in files suffixed with “.capnp”. The capnproto documentation for how to write these schema files can be found here: https://capnproto.org/language.html

The schema by itself is not especially useful. In order to read and write messages defined by the schema in a target language (e.g. C++), a code generation step is required. Capnproto provides a cmake function for this purpose, capnp_generate_cpp. This generates C++ source and header files. These source and header files combined with the capnproto C++ library, enables C++ code to read and write the messages matching a particular schema. The C++ library API can be found here: https://capnproto.org/cxx.html

Contents of libvtrcapnproto

libvtrcapnproto should contain two elements:

• Utilities for working capnproto messages in VTR
• Generate source and header files of all capnproto messages used in VTR

I/O Utilities

Capnproto does not provide IO support, instead it works from arrays (or file descriptors). To avoid re-writing this code, libvtrcapnproto provides two utilities that should be used whenever reading or writing capnproto message to disk:

• serdes_utils.h provides the writeMessageToFile function - Writes a capnproto message to disk.
• mmap_file.h provides MmapFile object - Maps a capnproto message from the disk as a flat array.

NdMatrix Utilities

A common datatype which appears in many data structures that VPR might want to serialize is the generic type vtr::NdMatrix. ndmatrix_serdes.h provides generic functions ToNdMatrix and FromNdMatrix, which can be used to generically convert between the provideid capnproto message Matrix and vtr::NdMatrix.

Capnproto schemas

libvtrcapnproto should contain all capnproto schema definitions used within VTR. To add a new schema:

1. Add the schema to git in libs/libvtrcapnproto/
2. Add the schema file name to capnp_generate_cpp invocation in libs/libvtrcapnproto/CMakeLists.txt.

The schema will be available in the header file schema filename>.h. The actual header file will appear in the CMake build directory libs/libvtrcapnproto after libvtrcapnproto has been rebuilt.

Writing capnproto binary files to text

The capnp tool (found in the CMake build directiory libs/EXTERNAL/capnproto/c++/src/capnp) can be used to convert from a binary capnp message to a textual form.

Example converting VprOverrideDelayModel from binary to text:

capnp convert binary:text place_delay_model.capnp VprOverrideDelayModel \
  < place_delay.bin > place_delay.txt