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C++ API

LiveHD is built on C++23. LGraph and LNAST are the two key data structures inside LiveHD to support new hardware design, and both are stored in HHDS (Hierarchical Hardware Data Structures): * LGraph stands for Live Graph. It is the graph or netlist data structure at the core of LiveHD (an hhds::Graph per module, plus LiveHD cell types and attributes layered on top). * LNAST stands for Language Neutral AST. It is an Abstract Syntax Tree (an hhds::Tree per unit) designed to be simple but to allow the translation from multiple languages like Pyrope and SystemVerilog.

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. The LNAST-level passes live in upass/, the LGraph-level passes in pass/:

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

    • Tuples/bundles:
      • Flatten fields (only flat attributes passed to LGraph)
      • Find IOs (inputs and outputs). Populate the io_meta() side-channel accordingly.
      • Detect as array if legal bundle index.
    • SSA construction
    • Constant propagation (comptime decision)
    • Linear time compiler passes (dead code elimination, constant folding) but not complex (GVN, SAT...)
    • Type checking and bitwidth range inference (bw_meta())
    • Unroll for and while loops
    • LGraph creation (upass/tolg)

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

    • Attributes
      • Bitwidth
      • Debug flag
    • Complex optimizations
      • cprop (Peephole, constant folding, ...)
      • bitwidth inference
      • synthesis (through Yosys integration)