Features

In this chapter, we introduce the features of Veryl along with clear examples.

• Real-time diagnostics
• Auto formatting
• Integrated test
• Dependency management
• Generics
• Clock Domain Annotation
• Trailing comma
• Abstraction of clock and reset
• Documentation comment
• Compound assignment operator in always_ff
• Individual namespace of enum variant
• repeat of concatenation
• if / case expression
• Range-based for / inside / outside
• msb notation
• let statement
• Named block
• Visibility control

Real-time diagnostics

Issues such as undefined, unused, or unassigned variables are notified in real-time while editing in the editor. In the following example, adding the _ prefix to variables flagged as unused explicitly indicates their unused status, suppressing warnings.

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Auto formatting

In addition to the automatic formatting feature integrated with the editor, formatting through the command line and formatting checks in CI are also possible.

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Integrated test

Test code written by SystemVerilog or cocotb can be embeded in Veryl code, it can be executed through veryl test command.

#[test(test1)]
embed (inline) sv{{{
    module test1;
        initial begin
            assert (0) else $error("error");
        end
    endmodule
}}}

Dependency management

Veryl includes a built-in dependency management feature, allowing for easy incorporation of libraries by simply adding the repository path and version of the library on project settings like below.

[dependencies]
"https://github.com/veryl-lang/sample" = "0.1.0"

Generics

Code generation through generics achieves more reusable code than traditional parameter override. Prarmeters in function like the follwoign example, but also module names of instantiation, type names of struct definition, and so on can be parameterized.

function automatic logic [20-1:0] FuncA_20 (
    input logic [20-1:0] a
);
    return a + 1;
endfunction

function automatic logic [10-1:0] FuncA_10 (
    input logic [10-1:0] a
);
    return a + 1;
endfunction

logic [10-1:0] a;
logic [20-1:0] b;
always_comb begin
    a = FuncA_10(1);
    b = FuncA_20(1);
end

function FuncA::<T: const> (
    a: input logic<T>,
) -> logic<T> {
    return a + 1;
}

var a: logic<10>;
var b: logic<10>;
always_comb {
    a = FuncA::<10>(1);
    b = FuncA::<20>(1);
}

Clock Domain Annotation

If there are some clocks in a module, explicit clock domain annotation and unsafe (cdc) block at the clock domain boundaries are required. By the annotation, Veryl compiler detects unexpected clock domain crossing as error, and explicit unsafe (cdc) block eases to review clock domain crossing.

module ModuleA (
    input  i_clk_a,
    input  i_dat_a,
    output o_dat_a,
    input  i_clk_b,
    input  i_dat_b,
    output o_dat_b
);
    // Carefully!!!
    // From i_clk_a to i_clk_b
    assign o_dat_b = i_dat_a;
endmodule

module ModuleA (
    i_clk_a: input  `a clock,
    i_dat_a: input  `a logic,
    i_dat_a: output `a logic,
    i_clk_b: input  `b clock,
    i_dat_b: input  `b logic,
    i_dat_b: output `b logic,
) {
    unsafe (cdc) {
        assign o_dat_b = i_dat_a;
    }
}

Trailing comma

Trailing comma is a syntax where a comma is placed after the last element in a list. It facilitates the addition and removal of elements and reduces unnecessary differences in version control systems.

module ModuleA (
    input  a,
    input  b,
    output o
);
endmodule

module ModuleA (
    a: input logic,
    b: input logic,
    o: input logic,
) {
}

Abstraction of clock and reset

There is no need to specify the polarity and synchronicity of the clock and reset in the syntax; these can be specified during build-time configuration. This allows generating code for both ASICs with negative asynchronous reset and FPGAs with positive synchronous reset from the same Veryl code.

Additionally, explicit clock and reset type enables to check whether clock and reset are correctly connected to registers. If there is a single clock and reset in the module, the connection can be omitted.

module ModuleA (
    input logic i_clk,
    input logic i_rst_n
);

always_ff @ (posedge i_clk or negedge i_rst_n) begin
    if (!i_rst_n) begin
    end else begin
    end
end

endmodule

module ModuleA (
    i_clk: input clock,
    i_rst: input reset,
){
    always_ff {
        if_reset {
        } else {
        }
    }
}

Documentation comment

Writing module descriptions as documentation comments allows for automatic documentation generation. You can use not only plain text but also the following formats:

• Markdown
• Waveform using WaveDrom
• Diagram using Mermaid

// Comment
module ModuleA;
endmodule

/// Documentation comment written by Markdown
///
/// * list
/// * list
/// 
/// ```wavedrom
/// { signal: [{ name: "Alfa", wave: "01.zx=ud.23.456789" }] }
/// ```
module ModuleA {
}

Compound assignment operator in always_ff

There is no dedicated non-blocking assignment operator; within always_ff, non-blocking assignments are inferred, while within always_comb, blocking assignments are inferred. Therefore, various compound assignment operators can be used within always_ff just like within always_comb.

always_ff @ (posedge i_clk) begin
    if (a) begin
        x <= x + 1;
    end
end

always_ff {
    if a {
        x += 1;
    }
}

Individual namespace of enum variant

Variants of an enum are defined within separate namespaces for each enum, thus preventing unintended name collisions.

typedef enum logic[1:0] {
    MemberA,
    MemberB
} EnumA;

EnumA a;
assign a = MemberA;

enum EnumA: logic<2> {
    MemberA,
    MemberB
}

var a: EnumA;
assign a = EnumA::MemberA;

repeat of concatenation

By adopting the explicit repeat syntax as a repetition description in bit concatenation, readability improves over complex combinations of {}.

logic [31:0] a;
assign a = {{2{X[9:0]}}, {12{Y}}};

var a: logic<32>;
assign a = {X[9:0] repeat 2, Y repeat 12};

if / case expression

By adopting if and case expressions instead of the ternary operator, readability improves, especially when comparing a large number of items.

logic a;
assign a = X == 0 ? Y0 :
           X == 1 ? Y1 :
           X == 2 ? Y2 : 
                    Y3;

var a: logic;
assign a = case X {
    0      : Y0,
    1      : Y1,
    2      : Y2,
    default: Y3,
};

Range-based for / inside / outside

With notation representing closed intervals ..= and half-open intervals .., it is possible to uniformly describe ranges using for, inside, and outside (which denotes the inverse of inside).

for (int i = 0; i < 10; i++) begin
    a[i] =   X[i] inside {[1:10]};
    b[i] = !(X[i] inside {[1:10]});
end

for i: u32 in 0..10 {
    a[i] = inside  X[i] {1..=10};
    b[i] = outside X[i] {1..=10};
}

msb notation

The msb notation, indicating the most significant bit, eliminates the need to calculate the most significant bit from parameters, making intentions clearer.

logic a;
logic [WIDTH-1:0] X;
assign a = X[WIDTH-1];

var a: logic;
var X: logic<WIDTH>;
assign a = X[msb];

let statement

There is a dedicated let statement available for binding values simultaneously with variable declaration, which can be used in various contexts that were not supported in SystemVerilog.

logic tmp;
always_ff @ (posedge i_clk) begin
    tmp = b + 1;
    x <= tmp;
end

always_ff {
    let tmp: logic = b + 1;
    x = tmp;
}

Named block

You can define named blocks to limit the scope of variables.

if (1) begin: BlockA
end

:BlockA {
}

Visibility control

Modules without the pub keyword cannot be referenced from outside the project and are not included in automatic documentation generation. This allows distinguishing between what should be exposed externally from the project and internal implementations.

module ModuleA;
endmodule

module ModuleB;
endmodule

pub module ModuleA {
}

module ModuleB {
}