Pros of riscv-boom

• Higher performance: Out-of-order superscalar design for better IPC
• More advanced features: Branch prediction, speculative execution, etc.
• Scalable: Configurable pipeline depth and issue width

Cons of riscv-boom

• Higher complexity: More difficult to understand and modify
• Increased area and power consumption
• Potentially longer verification and testing cycles

Code Comparison

ibex (simple in-order core):

always_comb begin
  alu_operator_o = ALU_SLTU;
  alu_op_a_mux_sel_o = OP_A_REG_A;
  alu_op_b_mux_sel_o = OP_B_IMM;
  imm_b_mux_sel_o = IMM_B_I;
  regfile_we = 1'b1;
end

riscv-boom (out-of-order superscalar core):

class Execute(implicit p: Parameters) extends BoomModule()(p) with HasBoomCoreParameters {
  val io = IO(new Bundle {
    val req = Flipped(Vec(memWidth, Valid(new FuncUnitReq)))
    val resp = Vec(memWidth, Valid(new FuncUnitResp))
    val bp = Input(Vec(memWidth, new BranchPrediction))
    val brinfo = Input(new BrResolutionInfo)
  })
  // ... (complex out-of-order execution logic)
}

The code snippets illustrate the difference in complexity between the two designs, with ibex using simpler Verilog constructs and riscv-boom employing more advanced Scala-based hardware description.

Pros of Rocket Chip

• More advanced and feature-rich RISC-V core, supporting RV64G ISA
• Highly configurable and scalable, suitable for a wide range of applications
• Extensive ecosystem and community support through CHIPS Alliance

Cons of Rocket Chip

• Higher complexity and steeper learning curve
• Larger codebase and potentially higher resource usage
• May be overkill for simpler embedded applications

Code Comparison

Ibex (SystemVerilog):

module ibex_core #(
  parameter bit          RV32E           = 0,
  parameter bit          RV32M           = 1,
  parameter int unsigned DmHaltAddr      = 32'h1A110800,
  parameter int unsigned DmExceptionAddr = 32'h1A110808
) (
  // Clock and Reset
  input  logic        clk_i,
  input  logic        rst_ni,

Rocket Chip (Chisel):

class Rocket(implicit p: Parameters) extends CoreModule()(p)
    with HasRocketCoreParameters
    with HasCoreIO {
  val clock_en_reg = Reg(Bool())
  val long_latency_stall = Reg(Bool())
  val pipelined_csr_unit = Module(new CSRFile())
  val decode_units = Seq.fill(decodeWidth) { Module(new DecodeUnit) }

The code snippets highlight the different design approaches and languages used in each project. Ibex uses SystemVerilog, while Rocket Chip is implemented in Chisel, a hardware construction language embedded in Scala.

Pros of VexRiscv

• Written in SpinalHDL, allowing for more flexible and modular design
• Highly configurable with various optional features and extensions
• Better performance in some benchmarks due to optimized design

Cons of VexRiscv

• Less mature and less widely adopted compared to Ibex
• Documentation may be less comprehensive
• Potentially steeper learning curve due to SpinalHDL

Code Comparison

Ibex (SystemVerilog):

module ibex_core #(
  parameter bit          PMPEnable         = 1'b0,
  parameter int unsigned PMPGranularity    = 0,
  parameter int unsigned PMPNumRegions     = 4,
  parameter int unsigned MHPMCounterNum    = 0,
  parameter int unsigned MHPMCounterWidth  = 40,
  parameter bit          RV32E             = 1'b0,
  parameter bit          RV32M             = 1'b1,
  parameter bit          RV32B             = 1'b0,
  parameter bit          BranchPredictor   = 1'b0
) (
  // Clock and Reset
  input  logic        clk_i,
  input  logic        rst_ni,
  // ...
);

VexRiscv (SpinalHDL):

case class VexRiscvConfig(
  withMemoryStage : Boolean = true,
  withWriteBackStage : Boolean = true,
  withRvc : Boolean = true,
  withRvf : Boolean = false,
  withRvd : Boolean = false,
  withRvm : Boolean = true,
  withRva : Boolean = false,
  withRvc : Boolean = true,
  // ...
)

Pros of CVA6

• More advanced 64-bit RISC-V core with out-of-order execution
• Supports RV64GC instruction set, offering broader functionality
• Higher performance suitable for more demanding applications

Cons of CVA6

• Higher complexity and resource usage
• Potentially longer development and verification cycles
• May be overkill for simpler embedded applications

Code Comparison

CVA6 (more complex pipeline):

always_comb begin : commit_instr
  // Complex out-of-order commit logic
  for (int i = 0; i < NR_COMMIT_PORTS; i++) begin
    // ... (additional logic)
  end
end

Ibex (simpler pipeline):

always_comb begin : ex_wb_pipe_reg
  // Straightforward in-order execution
  ex_wb_pipe_o = ex_wb_pipe_i;
  if (ex_valid_i) begin
    // ... (simpler logic)
  end
end

The code snippets illustrate the difference in complexity between CVA6's out-of-order execution and Ibex's simpler in-order pipeline. CVA6 offers higher performance at the cost of increased complexity, while Ibex provides a more straightforward design suitable for less demanding applications.

Pros of Freedom

• More comprehensive SoC design, including peripherals and memory controllers
• Supports multiple RISC-V configurations (E31, S51, U54)
• Includes tools for FPGA development and simulation

Cons of Freedom

• Larger and more complex codebase, potentially harder to understand and modify
• Less focused on a single core implementation
• May require more resources to synthesize and implement

Code Comparison

Ibex (SystemVerilog):

module ibex_core #(
  parameter bit          PMPEnable         = 1'b0,
  parameter int unsigned PMPGranularity    = 0,
  parameter int unsigned PMPNumRegions     = 4,
  parameter int unsigned MHPMCounterNum    = 0,
  parameter int unsigned MHPMCounterWidth  = 40
) (
  // Clock and Reset
  input  logic        clk_i,
  input  logic        rst_ni,

Freedom (Chisel):

class FreedomU500Config extends Config(
  new WithNBigCores(1) ++
  new WithNSmallCores(0) ++
  new WithCoherentBusTopology ++
  new BaseConfig)

class FreedomU500DevKitConfig extends Config(
  new WithJtagDTM ++
  new WithNMemoryChannels(1) ++
  new WithNBanks(8) ++
  new FreedomU500Config)