CIRCT

Circuit IR Compilers and Tools

HW and SV Dialect Rationale

This document describes various design points of the HW and SV dialects, why they are the way they are, and current status. This follows in the spirit of other MLIR Rationale docs .

Introduction 

SystemVerilog is an industry standard language for hardware design and verification, is known by a large number of engineers who write it directly, and is an important interchange format between EDA tools. However, while it is ubiquitous, SystemVerilog is not easy to generate or transform. Furthermore, it is non-trivial for compiler tools to generate high quality human readable SystemVerilog.

The HW and SV dialects attempt to address these problems with three major contributions:

  1. By providing the HW dialect, which contains a common set of abstractions for combinational logic. This dialect is designed to allow easy analysis and transformation, is allows other dialects to “mix in” with it to provide higher level functionality.
  2. By providing the SV dialect, which provides direct access to a wide variety of SystemVerilog constructs, including behavioral constructs, syntactic sugar constructs, and even idioms like ifdef blocks.
  3. By providing a high quality implementation and a number of useful compiler passes for analyzing and transforming these dialects, and a SystemVerilog emitter that generates pretty output.

The combination of these capabilities provides a useful suite of functionality for compiler tools that want to generate high quality SystemVerilog.

The HW Dialect 

The HW dialect is designed as a mid-level compiler IR for combinational logic. It is not designed to model SystemVerilog or any other hardware design language directly. Instead, it is designed to be easy to analyze and transform, and be a flexible and extensible substrate that may be extended with higher level dialects mixed into it.

The HW dialect defines a set of common functionality, such as hw.module for representing hardware modules, and operations like hw.add and hw.mux for logic.

Type System 

TODO: Describe inout types. Analogy to lvalues vs rvalues. Array indices for both forms. Arrays, structs, moving UnpackedArray to SV someday.

InOut types live at the SV dialect level and not the HW dialect level. This allows connects, wires and other syntactic constructs that aren’t necessary for combinational logic, but are nonetheless pretty useful when generating Verilog.

Operations 

TODO: Spotlight on module. Allows arbitrary types for ports.

TODO: Why is add variadic? Why consistent operand types instead of allowing implicit extensions?

No “Replication”, “ZExt”, or “Complement” Operators

We choose to omit several operators that you might expect, in order to make the IR more regular, easy to transform, and have fewer canonical forms.

  • No ~x complement operator: instead use comb.xor(x, -1).

  • No {42{x}} Replication operator (System Verilog 11.4.12.1) to replicate an operand a constant N times. We decided that this was redundant and just sugar for the comb.concat operator, so we just use comb.concat (with the same operand multiple times) instead.

  • No zero extension operator to add high zero bits. This is strictly redundant with concat(zero, value). The hw.sext operator exists because it efficiently models large sign extensions which are common, and would require many operands if modeled as a concat operator (in contrast, a zero extension always requires a single zero value).

The absence of these operations doesn’t affect the expressive ability of the IR, and ExportVerilog will notice these and generate the compact Verilog syntax.

Zero Bit Integers

Combinatorial operations like add and multiply work on values of signless standard integer types, e.g. i42, but they do not allow zero bit inputs. This design point is motivated by a couple of reasons:

  1. The semantics of some operations (e.g. hw.sext) do not have an obvious definition with a zero bit input.

  2. Zero bit operations are useless for operations that are definable, and their presence makes the compiler more complicated.

On the second point, consider an example like hw.mux which could allow zero bit inputs and therefore produce zero bit results. Allowing that as a design point would require us to special case this in our cost models, and we would have that optimizes it away.

By rejecting zero bit operations, we choose to put the complexity into the lowering passes that generate the HW dialect (e.g. LowerToHW from FIRRTL).

Note that this decision only affects the core operations in the HW dialect itself - it is perfectly reasonable to define your operations and mix them into other HW constructs. Also, certain other operations do support zero bit declarations in limited ways:

  • The hw.module operation allows zero bit ports, since it supports an open type system. They are dropped from Verilog emission.
  • Interface signals are allowed to be zero bits wide. They are dropped from Verilog emission.

Endianness: operand ordering and internal representation 

Certain operations require ordering to be defined (i.e. hw.concat, hw.array_concat, and hw.array_create). There are two places where this is relevant: in the MLIR assembly and in the MLIR C++ model.

In MLIR assembly, operands are always listed MSB to LSB (big endian style):

%msb = hw.constant 0xEF : i8
%mid = hw.constant 0x7 : i4
%lsb = hw.constant 0xA018 : i16
%result = hw.concat %msb, %mid, %lsb : (i8, i4, i16) -> i28
// %result is 0xEF7A018

Note: Integers are always written in left-to-right lexical order. Operand ordering for hw.concat was chosen to be consistent with simply abutting them in lexical order.

%1 = hw.constant 0x1 : i4
%2 = hw.constant 0x2 : i4
%3 = hw.constant 0x3 : i4
%arr123 = hw.array_create %1, %2, %3 : i4
// %arr123[0] = 0x3
// %arr123[1] = 0x2
// %arr123[2] = 0x1

%arr456 = ... // {0x4, 0x5, 0x6}
%arr78  = ... // {0x7, 0x8}
%arr = hw.array_concat %arr123, %arr456, %arr78 : !hw.array<3 x i4>, !hw.array<3 x i4>, !hw.array<2 x i4>
// %arr[0] = 0x6
// %arr[1] = 0x5
// %arr[2] = 0x4
// %arr[3] = 0x3
// %arr[4] = 0x2
// %arr[5] = 0x1

Note: This ordering scheme is unintuitive for anyone expecting C array-like ordering. In C, arrays are laid out with index 0 as the least significant value and the first element (lexically) in the array literal. In the CIRCT model (assembly and C++ of the operation creating the array), it is the opposite – the most significant value is on the left (e.g. the first operand is the most significant). The indexing semantics at runtime, however, differ in that the element zero is the least significant (which is lexically on the right).

In the CIRCT C++ model, lists of values are in lexical order. That is, index zero of a list is the leftmost operand in assembly, which is the most significant value.

ConcatOp result = builder.create<ConcatOp>(..., {msb, lsb});
// Is equivalent to the above integer concatenation example.
ArrayConcatOp arr = builder.create<ArrayConcatOp>(..., {arr123, arr456});
// Is equivalent to the above array example.

Array slicing and indexing (array_get) operations both have indexes as operands. These indexes are the runtime index, not the index in the operand list which created the array upon which the op is running.

Bitcasts 

The bitcast operation represents a bitwise reinerpretation (cast) of a value. This always synthesizes away in hardware, though it may or may not be syntactically represented in lowering or export language. Since bitcasting requires information on the bitwise layout of the types on which it operates, we discuss that here. All of the types are packed, meaning there is never padding or alignment.

  • Integer bit vectors: MLIR’s IntegerType with Signless semantics are used to represent bit vectors. They are never padded or aligned.
  • Arrays: The HW dialect defines a custom ArrayType. The in-hardware layout matches C – the high index of array starts at the MSB. Array’s 0th element’s LSB located at array LSB.
  • Structs: The HW dialect defines a custom StructType. The in-hardware layout matchss C – the first listed member’s MSB corresponds to the struct’s MSB. The last member in the list shares its LSB with the struct.
  • Unions: The HW dialect’s UnionType could contain the data of any of the member types so its layout is defined to be equivalent to the union of members type bitcast layout. In cases where the member types have different bit widths, all members start at the 0th bit and are padded up to the width of the widest member. The value with which they are padded is undefined.

Example figure 

15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0 
-------------------------------------------------
| MSB                                       LSB | 16 bit integer vector
-------------------------------------------------
                         | MSB              LSB | 8 bit integer vector
-------------------------------------------------
| MSB      [1]       LSB | MSB     [0]      LSB | 2 element array of 8 bit integer vectors
-------------------------------------------------

      13 12 11 10  9  8  7  6  5  4  3  2  1  0 
                            ---------------------
                            | MSB           LSB | 7 bit integer vector
      -------------------------------------------
      | MSB     [1]     LSB | MSB    [0]    LSB | 2 element array of 7 bit integer vectors
      -------------------------------------------
      | MSB a LSB | MSB b[1] LSB | MSB b[0] LSB | struct
      -------------------------------------------  a: 4 bit integral
                                                   b: 2 element array of 5 bit integer vectors

Cost Model 

As a very general mid-level IR, it is important to define the principles that canonicalizations and other general purpose transformations should optimize for. There are often many different ways to represent a piece of logic in the IR, and things will work better together if we keep the compiler consistent.

First, unlike something like LLVM IR, keep in mind that the HW dialect is a model of hardware – each operation generally corresponds to an instance of hardware, it is not an “instruction” that is executed by an imperative CPU. As such, the primary concerns are area and latency, not “number of operations executed”. As such, here are important concerns that general purpose transformations should consider, ordered from most important to least important.

Simple transformations are always profitable

Many simple transformations are always a good thing, this includes:

  1. Constant folding.
  2. Simple strength reduction (e.g. divide to shift).
  3. Common subexpression elimination.

These generally reduce the size of the IR in memory, can reduce the area of a synthesized design, and often unblock secondary transformations.

Reducing widths of non-trivial operations is always profitable

It is always a good idea to reduce the width of non-trivial operands like add, multiply, shift, divide, and, or (etc) since it produces less hardware and enables other simplifications. This is even true if it grows the IR size by increasing the number of truncations and extensions in the IR.

That said, it is a bad idea to duplicate operations to reduce widths: for example, it is better to have one large multiply with many users than to clone it because one user only needs some of the output bits.

Don’t get overly tricky with divide and remainder

Divide operations (particularly those with non-constant divisors) generate a lot of hardware, and can have long latencies. As such, it is a generally bad idea to do anything to an individual instance of a divide that can increase its latency (e.g. merging a narrow divide with a wider divide and using a subset of the result bits).

Constants and moving bits around is free

Operations for constants, truncation, zero/sign extension, concatenation of signals, and other similar operations are considered free since they generally do not synthesize into hardware. All things being equal it is good to reduce the number of instances of these (to reduce IR size and increase canonical form) but it is ok to introduce more of these to improve on other metrics above.

Ordering Concat and Extract

Theconcat(extract(..)) form is preferred over the extract(concat(..)) form, because

  • extract gets “closer” to underlying add/sub/xor/op operations, giving way optimizations like narrowing.
  • the form gives a more accurate view of the values that are being depended on.
  • redundant extract operations can be removed from the concat args lists, eg: cat(extract(a), b, c, extract(d))

Both forms perform similarly on hardware, since they are simply bit-copies.

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. The driving focus of this dialect is to provide simple and predictable access to SystemVerilog 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, so it does not have its own operations for combinational logic, modules, or other base functionality defined in the HW dialect.

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.

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.

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.

Symbols and Visibility 

Verilog has a broad notion of what can be named outside the context of its declaration. This is compounded by the many tools which have additional source files which refer to verilog names (e.g. tcl files). However, we do not want to require that every wire, register, instance, localparam, port, etc which can be named not be touched by passes. We want only entities marked as public facing to impede transformation.

For this reason, wires, registers, and instances may optionally define a symbol. When the symbol is defined, the entity is considered part of the visible interface and should be preserved in transformation. Entities without a symbol defined are considered private and may be changed by transformation.

Implementation constraints 

Currently, MLIR restricts symbol resolution to looking in and downward through any nested symbol tables when resolving symbols. This assumption has implications for verification, the pass manager, and threading. Until symbol references are more general, SV and HW dialects do not define symbol tables for modules. Therefore, wires, registers, and interfaces exist in the same namespace as modules. It is encouraged that one prefaces the names to avoid conflict with modules. The symbol names on these entities has no bearing on the output verilog, each of these entities has a defined way to assign its name (SSA value name for wires and regs, a non-optional string for instances).

As MLIR symbol support improves, it is desired to move to per-module symbol tables and to unify names with symbol names.

Ports 

Module ports are remotely namable entities in Verilog, but are not easily named with symbols. A suggested workaround is to attach a wire to a port and use its symbol for remote references.

Instance ports have a similar problem.

Future Directions 

There are many possible future directions that we anticipate tackling, when and if the need arises:

First Class Parametric IR

Many in the CIRCT community are interested in adding first-class support for parametric modules – similar but more general than SystemVerilog module parameters. It isn’t clear yet whether this should be part of the HW dialect or something higher level.

Separate from a “good” representation of parametric modules, the SV dialect could grow direct support for representing the SystemVerilog functionality in this space, including even things like “generate” blocks.

EDA Tool-specific Subdialects

The EDA tool ecosystem is filled with a wide range of tools with different capabilities – for example see this table for one compilation of different systems and their capabilities. As such, we expect that the day will come where a frontend wants to generate fancy features for some modern systems, but cannot afford to break compatibility with other ecosystem tools.

Given the design of the HW/SV dialects, there is no need to resort to “lowest common denominator” approach here: we can allow frontends to generate “fancy” features, then use progressive lowering when dealing with tools that can’t handle them. This can also allow IP providers to decide what flavor of features they want to provide to their customers (or provide multiple different choices).

SystemVerilog Parser

As the SV dialect grows out, it becomes natural to think about building a high quality parser that reads SystemVerilog source code and parses it into the SV dialect. Such functionality could be very useful to help build tooling for the SystemVerilog ecosystem.

Such a parser should follow clang-style principles of producing high quality diagnostics, preserving source location information, being built as a library, etc.