errata/402: 9.5, evaluation of case expression

From: Brad Pierce (Brad.Pierce@synopsys.com)
Date: Sat Jul 26 2003 - 08:08:08 PDT

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    >Number: 402
    >Category: errata
    >Originator: "Brad Pierce" <Brad.Pierce@synopsys.com>
    >Environment:
    >Description:

    Must a case expression be evaluated exactly once and before
    any of the case item expressions? Or it would be legal to
    implement a case by

        if ( case_item_expression_1 == case_expression )
          ...
        elsif ( case_item_expression_2 == case_expression )
          ...

    I thought such an implementation would be incorrect in general,
    because of the possibility of side-effects, but I see nothing
    in the LRM to prohibit it. Here are two examples that highlight
    the difference --

    //------------------------------------------------------------------

    module m_1 (o) ; /* Brad.Pierce@synopsys.com */
    output [5:0] o ;
    reg [5:0] o ;
    integer i ;
    integer j ;

    function [5:0] f ;
    input [5:0] n ;
    begin
      f = i ;
      if ( n == i ) i = i + 1 ;
    end
    endfunction

    task g ;
    input [5:0] n ;
    begin
      $display(n) ;
      j = j + n - 1 ;
    end
    endtask

    always begin
       i = 0 ;
       j = 0 ;
       while ( i <= 10 )
         case ( i )
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
         endcase
       o = j ;
       $display("should be %d? -- is %d", 10*(10+1)/2, o) ;
    end

    endmodule

    //------------------------------------------------------------------

    module m_2 (o) ; /* Brad.Pierce@synopsys.com */
    output [5:0] o ;
    reg [5:0] o ;
    integer i ;
    integer j ;

    function [5:0] f ;
    input [5:0] n ;
    begin
      if ( n == i ) i = i + 1 ;
      f = i ;
    end
    endfunction

    task g ;
    input [5:0] n ;
      j = j + n - 1 ;
    endtask

    always begin
       i = 0 ;
       j = 0 ;
       while ( i <= 10 )
         case ( i )
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
           f(i) : g(i) ;
         endcase
       o = j ;
       $display("should be %d? -- is %d", 0, o) ;
    end

    endmodule

    //------------------------------------------------------------------

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