C++ API

LiveHD is built on C++17, LGraph and LNAST are the two key data structures inside LiveHD to support new hardware design.
* LGraph stands for Live Graph. It is graph or netlist data structure at the core of LiveHD. * LNAST stands for Language Neutral AST. It is an Abstract Syntax Tree (AST) designed to be simple but to allow the translation from multiple languages like CHIRRTL, Verilog, and Pyrope.

While LNAST could be seen as a high level API with control flow information, LGraph is a lower level graph API where many LNAST high level constructs are simplified.

There is a division of functionality between LNAST and LGraph:

• LNAST: Language Neutral AST, the high level tree based representation/API

  • Bundles:
    • Flatten fields (only flat attributes passed to LGraph)
    • Find IOs (inputs and outputs). Populate the sub_node accordingly.
    • Detect as array if legal bundle index.
  • Constant propagation (comptime decision)
  • Linear time compiler passes (dead code elimination, constant folding) but not complex (GVN, SAT...)
  • Lgraph creation
    • Inline small LNASTs
    • Partition too large LGraphs
  • Type checking
  • Unroll for and while loops

• LGraph: Live Graph, the low level graph/netlist level based representation/API

  • Attributes
    • Bitwidth
    • Debug flag
  • Complex optimizations
    • cprop (Peephole, constant folding, ...)
    • lecopt, Logic Equivalence based optimizations
    • synthesis