systemverilog中的priority if

1 基本概念

在 SystemVerilog 中，priority - if是一种条件判断结构。它和普通的if - else语句类似，但在条件评估和错误检查方面有自己的特点，主要用于按顺序评估多个条件，并且对不符合预期的情况进行报错。报错如下两点

当所有条件都不为真时，

或者如果最后一个 "if" 结构没有 "else" 子句时。

2 语法结构

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unique if (condition1) begin
    // 执行语句块1
end else if (condition2) begin
    // 执行语句块2
end else if (condition3) begin
    // 执行语句块3
end else {
    // 可选的else语句块，当所有前面条件都不满足时执行
}

3 示例

1 No else clause in priority-if（没有else）

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module tb;
	int x = 4;

  	initial begin
      	// This if else if construct is declared to be "unique"
		// Error is not reported here because there is a "else"
      	// clause in the end which will be triggered when none of
      	// the conditions match
    	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");

      	// When none of the conditions become true and there
      	// is no "else" clause, then an error is reported
    	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
           Scope: tb
            Time: 0 FS + 1

ncsim: *W,RNQUIE: Simulation is complete.

2 Multiple matches in unique-if（匹配多个if）

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module tb;
	int x = 4;

  	initial begin

      	// This if else if construct is declared to be "unique"
		// When multiple if blocks match, then error is reported
      	priority 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: priority if violation:  Multiple true if clauses at {line=8:pos=15 and line=10:pos=13}.
            File: ./testbench.sv, line = 8, pos = 15
           Scope: tb
            Time: 0 FS + 1

ncsim: *W,RNQUIE: Simulation is complete.