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Verilog Abstract Syntax Tree (VAST)

XLS outputs Verilog (or SystemVerilog) for synthesis and simulation. As a lowest common denominator, Verilog output enables XLS generated designs to integrate into existing design flows. To make generation of Verilog easier, XLS includes an abstract representation of Verilog called VAST (Verilog Abstract Syntax Tree). VAST is a C++ library which represents Verilog in a recursive tree data structure which is simple to construct and manipulate programmatically. Verilog source code is emitted directly from the VAST data structure.

VAST is intentionally not a complete representation of the Verilog language. VAST is used to emit Verilog for the purposes of code generation within XLS. Given this limited use case, VAST is much smaller and simpler than a complete representation of the entire Verilog language as might be required for a parser, for example.

VAST Overview

Each supported Verilog construct is represented with a C++ class. These classes form a type hierarchy with the class VastNode at the root. Objects are gathered in tree-shaped structures to represent Verilog constructs. Ownership of all VAST objects is maintained by a VerilogFile object which represents a single file of Verilog source code. References between objects are stored as plain pointers.

For example, consider the following Verilog expression:

  foo + 8

In VAST, this is represented with an object of the BinaryInfix class which is derived from the Expression class representing arbitrary Verilog expressions. A BinaryInfix object has three relevant data members:

std::string op_; : The string representation of the operation to perform (e.g., +).

Expression* lhs_; : The left-hand-side of the expression. In this example, this points to a LogicRef object (derived from Expression class) referring to a Verilog reg or wire variable.

Expression* rhs_; : The left-hand-side of the expression. In this example, this points to a Literal object (derived from Expression class) containing the number 8 with unspecified bit width.

The BinaryInfix object representing foo + 8 might be used within other expressions or statements by referring to the object by pointer. For example, the representation of the statement assign bar = foo + 8 would contain an Expression* pointer referring to the foo + 8 object for the right-hand-side of the assignment.

Operator Precedence

To avoid ambiguity, operators in Verilog follow precedence rules. For example, multiplication is higher precedence than addition so the expression 2 + 4 * 10 evaluates to 42 (i.e., 2 + (4 * 10)) not 60 (i.e., (2 + 4) * 10). In VAST, expressions are built as a trees which is evaluated from the leaves to the root. To ensure that the operations are evaluated in the correct order when emitted as Verilog text, VAST automatically adds parentheses where appropriate. For example, the VAST expression consisting of the product (BinaryInfix with operation *) of 10 and the sum of 2 and 4 (BinaryInfix with operation +) will be emitted as 10 * (2 + 4).


VAST has a number of classes which hold a sequence of (pointers to) other VAST objects. At the top-level, this includes the VerilogFile class which can hold a sequence of objects such as include statements and modules. Verilog modules themselves are represented with the Module class containing a sequence of statements, declarations, comments, and other constructs. Other containers include always blocks and functions.

Emitting Verilog text

VAST classes include an Emit method which returns the represented Verilog construct as a string. Typically, Emit is called on the top-level VerilogFile object to create the text of the entire Verilog source file. Underneath the hood, this method calls the Emit method on all contained VAST objects and assembles the returned strings into the Verilog source code.

SystemVerilog support

XLS can emit either Verilog or SystemVerilog so VAST supports both languages. SystemVerilog constructs are included alongside Verilog constructs in VAST. Examples of SystemVerilog features supported by VAST include:

  • always_ff procedure for modeling sequential logic (VAST AlwaysFlop class).
  • Array assignment pattern (VAST ArrayAssignmentPattern class). Example: '{foo, bar, baz}
  • Array declaration using sizes. Example: reg [7:0] foo[42];

Within VAST, there is no distinction between the two languages and it is up to the user of VAST to only use the supported features for the target language (Verilog or SystemVerilog).