https://www.doulos.com/knowhow/systemverilog/systemverilog-tutorials/systemverilog-rtl-tutorial/In SystemVerilog, there are specific constructs used to check for violations in if–else statements. These constructs help to identify issues when the conditions don’t match as expected. The key constructs introduced for these checks are:

• unique-if
• unique0-if
• priority-if

Each of these constructs behaves differently and helps in catching violations based on the conditions evaluated in the if–else blocks.

What Does Unique-If Do?

The unique-if construct evaluates conditions in any order and checks for violations based on the following rules:

• Error when no conditions match: If none of the if conditions match and there’s no explicit else clause, an error is reported.
• Error when more than one condition matches: If multiple conditions match, it reports a violation.

Example: Using Unique-If to Check for Violations

Consider the following example where unique-if is used in an if–else construct:

module tb;
    int x = 4;

    initial begin
        // This "unique" construct checks if conditions are met
        // No error is reported because there is an "else" clause
        unique if (x == 3)
            $display("x is %0d", x);
        else if (x == 5)
            $display("x is %0d", x);
        else
            $display("x is neither 3 nor 5");

        // Error is reported here as no conditions match and there is no "else"
        unique if (x == 3)
            $display("x is %0d", x);
        else if (x == 5)
            $display("x is %0d", x);
    end
endmodule

Simulation Log:

ncsim> run
x is neither 3 nor 5
ncsim: *W,NOCOND: Unique if violation: Every if clause was false.
            File: ./testbench.sv, line = 18, pos = 13

What Does Unique0-If Do?

Unlike unique-if, the unique0-if construct does not report a violation if none of the conditions match. It only reports an error if there is more than one match, just like unique-if. However, it ignores the scenario where no conditions match, making it slightly more lenient than unique-if.

Using Unique-If with Multiple Conditions

When multiple conditions match, the unique-if construct reports a violation. Here’s an example where the value of x matches multiple conditions:

module tb;
    int x = 4;

    initial begin
        // When multiple "if" blocks match, an error is reported
        unique if (x == 4)
            $display("1. x is %0d", x);
        else if (x == 4)
            $display("2. x is %0d", x);
        else
            $display("x is not 4");
    end
endmodule

Simulation Log:

ncsim> run
1. x is 4
ncsim: *W,MCONDE: Unique if violation: Multiple true if clauses at {line=8:pos=15 and line=10:pos=13}.
            File: ./testbench.sv, line = 8, pos = 15

What Does Priority-If Do?

The priority-if construct evaluates conditions sequentially. It reports a violation in the following cases:

• No condition is true: If none of the if conditions are true and there’s no else clause, an error is reported.
• No else clause: If no conditions match and there’s no else clause, a violation is reported.

Example: Using Priority-If for Sequential Evaluation

Here’s an example that shows how priority-if works when evaluating conditions sequentially:

module tb;
    int x = 4;

    initial begin
        // This "priority" construct checks conditions in sequence
        // No error is reported because there is an "else" clause
        priority if (x == 3)
            $display("x is %0d", x);
        else if (x == 5)
            $display("x is %0d", x);
        else
            $display("x is neither 3 nor 5");

        // Error is reported here since no conditions match and there's no "else"
        priority if (x == 3)
            $display("x is %0d", x);
        else if (x == 5)
            $display("x is %0d", x);
    end
endmodule

Simulation Log:

ncsim> run
x is neither 3 nor 5
ncsim: *W,NOCOND: Priority if violation: Every if clause was false.
            File: ./testbench.sv, line = 18, pos = 15

Using Priority-If to Exit Early

The priority-if construct stops evaluating further conditions once a match is found. Here is an example showing this behavior:

module tb;
    int x = 4;

    initial begin
        // Exits early after the first matching condition
        priority if (x == 4)
            $display("x is %0d", x);
        else if (x != 5)
            $display("x is %0d", x);
    end
endmodule

Simulation Log:

ncsim> run
x is 4
ncsim: *W,RNQUIE: Simulation is complete.

Key Differences Between Unique-If and Priority-If

To summarize, here’s a comparison table highlighting the main differences between unique-if and priority-if:

Conclusion

In SystemVerilog, both unique-if and priority-if constructs are useful for detecting violations in if–else statements, but they behave differently. The unique-if construct checks for violations when no condition matches or when multiple conditions match, while priority-if evaluates conditions sequentially and reports violations when no condition matches or if there’s no else clause.

By using these constructs correctly, you can ensure that your code is both functional and free of logical errors, improving overall reliability and performance.