CIRCT

Circuit IR Compilers and Tools

Seq(uential) Dialect Rationale

This document describes various design points of the seq dialect, why it is the way it is, and current status. This follows in the spirit of other MLIR Rationale docs.

Introduction 

Digital logic is generally split into two categories: combinational and sequential. CIRCT contains a comb dialect to model the basic combinational operations and the (future) seq dialect which is discussed here. The intention of the seq dialect is to provide a set of stateful constructs which can be used to model sequential logic, independent of the output method (e.g. SystemVerilog).

Definitions 

For the sake of precision, we use the following definitions:

  • Physical devices:
    • Unclocked Latch: A memory element which is only sensitive to the levels of its inputs. Has no clock. Example: SR Latch.
    • Clocked (gated) latch: A latch wherein the inputs are gated by a clock. Transparent the entire time the clock is high. Generally referred to as a “latch”. Examples: “gated SR latch”, “D latch”.
    • Edge-triggered flip-flop: An edge-sensitive memory element. Captures the input value on one or both clock edges. Variants: posedge FF, negedge FF, “edge-sensitive” FF (captures the input value on both edges), resettable FF.
  • Abstract models:
    • Register: A synchronous, resettable memory element. Can be implemented using any of the above “circuit level” elements.

The computational register operation 

The seq.compreg op models an abstract notion of a “register”, independent of its implementation (e.g. latch, D flip-flop). This specific register op is intended to support “computation support” or “reset-agnostic code” and thus it cannot be used to model all the behaviors of a SystemVerilog register. (E.g. FSM and pipeline registers.) It is intended to be ’lowered’ to a specific implementation, which may be an sv.always_ff block, a device primitive instantiation, or even a feedback loop in all comb logic.

Our intention is to allow analysis and optimization of sequential logic without having to reason about implementation-specific behavior. This makes it somewhat distinct from the SystemVerilog register wherein one has to account for details like reset behavior and implementation (latch vs. flip-flop).

CompReg has four operands:

  • input: The value to be captured at ‘clock’. Generally called ’d’. Accepts any type, results in the same type. Does not support any notion of addressing, meaning that this operation sets / reads the entire value.
  • clock: Capture ‘value’ on the positive edge of this signal.
  • reset: Signal to set the state to ‘resetValue’. Optional.
  • resetValue: A value which the state is set to upon reset. Required iff ‘reset’ is present.
  • name: A name for the register, defaults to "". Inferred from the textual SSA value name, or passed explicitly in builder APIs. The name will be passed to the sv.reg during lowering.
  • innerSym: An optional symbol to refer to the register. The symbol will be passed to the sv.reg during lowering if present.
%q = seq.compreg %input, %clk [, %reset, %resetValue ] : $type(input)

Upon initialization, the state is defined to be uninitialized.

Rationale 

Several design decisions were made in defining this op. Mostly, they were made to simplify it while still providing the common case. Providing support for all flavors of registers is an anti-goal of this op. If an omitted feature is needed, it can be added or (if not common or precludes optimization) another op could be added.

  • Logical features:

    • Inclusion of optional ‘reset’ signal: This operand makes lowering to an efficient implementation of reset easier. Omission of it would require some (potentially complex) analysis to find the reset mux if required
    • Inclusion of ‘resetValue’: if we have a ‘reset’ signal, we need to include a value.
    • Omission of ’enable’: enables are easily modeled via a multiplexer on the input with one of the mux inputs as the output of the register. This – we assume – property makes ’enables’ easier to detect than reset in the common case.
  • Timing / clocking:

    • Omission of ’negedge’ event on ‘clock’: this is easily modeled by inverting the clock value.
    • Omission of ‘dual edge’ event on ‘clock’: this is not expected to be terribly common.
    • Omission of edge conditions on ‘reset’: Since this op specifically targets “reset-agnostic code”, the reset style shouldn’t affect logical correctness. It should, therefore, be determined by a lowering pass.

The FIRRTL register operation [Provisional] 

The seq.firreg carries all the information required to represent a FIRRTL register and lower it to SystemVerilog.

FirReg has the following operands:

  • input: Value to set the register to on the positive edge of the clock signal.
  • clk: Clock signal driving the register.
  • name: A name for the register, passed directly to the sv.reg.
  • inner_sym: A optional symbol for the register, passed directly to the sv.reg. Is a symbol is not specified and the register is randomised, one is created during the lowering to SV. Registers without symbols can be removed from the design.
  • reset: Signal to trigger the reset.
  • resetValue: A value which is set upon reset. Must be a constant if the reset is asynchronous.
  • isAsync: Optional boolean flag indicating whether the reset is asynchronous.
%reg = seq.firreg %input clock %clk [ sym @sym ]
    [ reset (sync|async) %reset, %value ] : $type(input)

Examples of registers:

%reg_no_reset = seq.firreg %input clock %clk sym @sym : i32

%reg_sync_reset_rand  = seq.firreg %input clock %clk sym @sym
    reset sync %reset, %value : i64

%reg_async_reset = seq.firreg %input clock %clk sym @sym
    reset async %reset, %value : i1

A register without a reset lowers directly to an always block:

always @(posedge clk or [posedge reset]) begin
  a <= [%input]
end

In the presence of a reset, an if statement and an always block with the proper triggers are emitted:

always @(posedge clk or [posedge reset]) begin
  if ([%reset])
    a <= [%resetValue]
  else
    a <= [%input]
end

Additionally, sv operations are also included to provide the register with a randomized preset value. Since items assigned in an always_ff block cannot be initialised in an initial block, this register lowers to always.

`ifndef SYNTHESIS
  `ifdef RANDOMIZE_REG_INIT
    reg [31:0] _RANDOM;
  `endif
  initial begin
    `INIT_RANDOM_PROLOG_
    `ifdef RANDOMIZE_REG_INIT
      _RANDOM = {`RANDOM};
      a = _RANDOM;
    `endif
  end
`endif

Registers expect the logic assignment to them to be in SSA form. For example, a strict connect to a field of a structure:

%field = firrtl.subfield %a(0)
firrtl.strictconnect %field, %value

Is converted into a hw.struct_inject operation:

%reg = seq.firreg %value clock %clk sym @sym : i32
%value = hw.struct_inject %reg["x"], %value

In order to avoid generating unnecessary assignments, the lowering of the register to sv eliminates the SSA form and emits a single parallel assignment to the field (reg.x = value).

Rationale 

A register specific for FIRRTL is desired as it has a specific lowering while also requiring a preset value and asynchronous resets. The lowering must also be compatible with the reference FIRRTL lowering, which might diverge from the lowering of the computation register.

Future considerations 

  • Enable signal: if this proves difficult to detect (or non-performant if we do not detect and generate the SystemVerilog correctly), we can build it into the compreg op.
  • Reset style and clock style: how should we model posedge vs negedge clocks? Async vs sync resets? There are some reasonable options here: attributes on this op or clock and reset types which are parameterized with that information.
  • Initial value: this register is uninitialized. Using an uninitialized value results in undefined behavior. We will add an initialValue attribute if this proves insufficient.

The High-Level Memory Abstraction 

The seq.hlmem (high-level memory operation) intends to capture the semantics of a memory which eventually map to some form on-chip resources - whether being FPGA or ASIC-based. The abstraction aims to abstract away the physical implementation details of the memory, and instead focus on the external interface and access semantics of the memory. this, in turn, facilitates analysis and transformation (e.g. memory merging, read/write conflicts, etc.) and may serve as a target for other high-level abstractions.

The high-level memory abstraction is split into two parts:

  • Memory allocation is defined by the seq.hlmem operation. This operation defines the internal memory structure. For now, this strictly pertains to the layout of the memory (dimensionality) and element type.
  • Memory access is defined by separate port operations which reference the allocated memory. Port access operations are defined at the same level of abstraction as the core RTL dialects and contain no notion of control flow. As such, for e.g. a write port with a non-zero latency, the encapsulating IR must already have accounted for this latency. The behavior of conflicting writes is defined by the lowering. Generally speaking, it should be considered as undefined.

Example usage:

  %myMemory = seq.hlmem @myMemory %clk : <4xi32>
  %c0_i2 = hw.constant 0 : i2
  %c1_i1 = hw.constant 1 : i1
  %c42_i32 = hw.constant 42 : i32
  %myMemory_rdata = seq.read %myMemory[%c0_i2] rden %c1_i1 { latency = 0} : !seq.hlmem<4xi32>
  seq.write %myMemory[%c0_i2] %c42_i32 wren %c1_i1 { latency = 1 } : !seq.hlmem<4xi32>

Lowering the op is intended to be performed by matching on the seq.hlmem, collecting the port ops which access the memory, and based on this perform a lowering to an appropriate memory structure. This memory could either be behavioral (able to support any combination of memory allocation and port accesses) or specialized (e.g. specifically target FPGA resources, call a memory compiler, … which may only be possible for a subset of allocation and access combinations).

Rationale 

The high-level memory abstraction, as presented here, represents a useful albeit limited abstraction when considering the complexity of instantiating memory resources in both FPGAs and ASICs.

The scope of what the hlmem operations can represent is large. Examples being: multidimensional memories, arbitrary # of read/write ports, and mixed port latencies (all of which could occur together).
In reality, it will only be a limited subset of the possible combinations of these operations which can be lowered reasonably to an FPGA or ASIC implementation.
However, by allowing for such complexity, we ensure that we have a unified IR which can represent such varying levels of complexity, thus ensuring maximum reusability of analysis and transformation passes.

Future considerations 

Port refinements 

The main design decision of seq.hlmem is the choice of abstracting away the structural details of a port into separate ops of which we currently only provide rudimentary read- and write ops. Example future ports could be:

  • Assymetric port widths Specified as a new seq.asym_read port which defines a read data width of some fraction of the native data size.
    %rdata = seq.asym_read %rp[%addr] : !seq.hlmem<4xi32> -> i16
    
    which would then put different typing requirements on the %addr signal. Given the halfing of the word size, the expected address type would then be ceil(log2(4)) << 1 = i3
  • Byte-enable write ports Specified as a new seq.write_be port with an additional byte enable signal.
    %wdata = seq.write_be %wp[%addr] %wdata, %be : i32, i4 -> !seq.hlmem<4xi32>
    
  • Debug ports Could be specified as either an additional read port, or (if further specialization is needed) attached to the memory symbol.
    %mem = seq.debug @myMemory : !seq.hlmem<4xi32>