CIRCT

Circuit IR Compilers and Tools

SV Dialect Rationale

This document describes various design points of the sv dialect, a common dialect that is typically used in conjunction with the hw and comb dialects. Please see the [RationaleHW.md](HW Dialect Rationale) for high level insight on how these work together. This follows in the spirit of other MLIR Rationale docs .

Introduction to the sv dialect 

The sv dialect is one of the dialects that can be mixed into the HW dialect, providing access to a range of syntactic and behavioral constructs in SystemVerilog. The driving focus of this dialect is to provide simple and predictable access to these features: it is not focused primarily on being easy to analyze and transform.

The sv dialect is designed to build on top of the hw dialect and is often used in conjunction with the comb or other dialects, so it does not have its own operations for combinational logic, modules, or other common functionality.

sv Type System 

Like the HW dialect, the SV dialect is designed to tolerate unknown types where possible, allowing other dialects to mix in with it. In addition to these external types, and the types used by the HW dialect, the SV dialect defines types for SystemVerilog interfaces.

TODO: Describe interface types, modports, etc.

Overview of sv dialect operations 

Because the SV dialect aims to align with the textual nature of SystemVerilog, many of the constructs in the SV dialect have an “AST” style of representation. The major classes of operations you’ll find are:

  1. Statements like sv.if, sv.ifdef, sv.always and sv.initial that expose primary task-like operations and the behavioral model.
  2. Procedural assignment operators, including the sv.bpassign and sv.passign operators that expose the blocking (x = y) and non-blocking (x <= y) procedural operators.
  3. Directives like sv.finish and sv.alias and behavioral functions like sv.fwrite.
  4. Access to verification constructs with sv.assert, sv.assume, and sv.cover.
  5. Escape hatches that allow direct integration of textual expressions (sv.verbatim.expr) and full statements (sv.verbatim).

These operations are designed to directly model the syntax of the SystemVerilog language and to be easily printable by the ExportVerilog pass. While there are still many things in SystemVerilog that we cannot currently express in the SV dialect, this design makes it easy to incrementally build out new capabilities over time.

Statements 

TODO.

Declarations 

TODO: Describe sv.wire, sv.reg,

Expressions 

TODO: Describe sv.read_inout and sv.array_index_inout.

Indexed Part Select 

Unlike Bit-selects which extract a particular bit from integer types, part-select can extract several contiguous bits in a vector net, vector reg, integer variable, or time variables.

SystemVerilog supports two types of part-selects, a constant part-select and an indexed part-select. SV dialect has two ops named sv.part_select and sv.part_select_inout, that is lowered to the indexed part-select operation. The sv.part_select is defined on Integer type input and sv.part_select_inout is defined on inout type.

Part select consits of 3 arguments, the input value, a width and a base and an optional boolean attribute decrement. The width shall be a compile-time constant expression. The base can be a runtime integer expression.

The operation selects bits starting at the base and ascending or descending the bit range. The number of bits selected is equal to the width expression. The bit addressing is always ascending starting from the base, unless the decrement attribute is specified.

Part-selects that address a range of bits that are completely out of the address bounds of the net, reg, integer, or time, or when the part-select is x or z, shall yield the value x when read, and shall have no effect on the data stored when written.

Part-selects that are partially out of range shall when read return x for the bits that are out of range, and when written shall only affect the bits that are in range.

In this example, bits starting from %c2 and of width 1 are addressed. Hence %0 is of width 1.

%0 = sv.part_select_inout %combWire[%c2 : 1] : !hw.inout<i10>, i3, !hw.inout<i1>

Verbatim op 

The verbatim operation produces a typed value expressed by a string of SystemVerilog. This can be used to access macros and other values that are only sensible as Verilog text. There are three kinds of verbatim operations:

  1. VerbatimOp(sv.verbatim, the statement form
  2. VerbatimExprOp(sv.verbatim.expr), the expression form.
  3. VerbatimExprSEOp(sv.verbatim.expr.se), the effectful expression form.

For the verbatim expression form, the text string is assumed to have the highest precedence - include parentheses in the text if it isn’t a single token. sv.verbatim.expr is assumed to not have side effects (is NoSideEffect in MLIR terminology), whereas sv.verbatim.expr.se may have side effects.

Verbatim allows operand substitutions with ‘{{0}}’ syntax. For macro substitution, optional operands and symbols can be added after the string. Verbatim ops may also include an array of symbol references. The indexing begins at 0, and if the index is greater than the number of operands, then it is used to index into the symbols array. It is invalid to have macro indices greater than the total number of operands and symbols. Example,

sv.verbatim "MACRO({{0}}, {{1}} reg={{4}}, {{3}})" 
            (%add, %xor) : i8, i8
            {symRefs = [@reg1, @Module1, @instance1]}

Cost Model 

The SV dialect is primarily designed for human consumption, not machines. As such, transformations should aim to reduce redundancy, eliminate useless constructs (e.g. eliminate empty ifdef and if blocks), etc.