(r3) MetavalueConditionalConversions < P1076/Ballots

P1076/Ballots Web>Vhdl2019CollectedRequirements>MetavalueConditionalConversions (revision 3) (raw view)~~EditAttach~~
---+ Flag Metavalues detected by ??

%TOC%

---++ Proposal Information
   * Current Owner: JimLewis, ...
   * Contributors: JimLewis, ...
   * Date Proposed: 2014-October-16
   * Date Last Updated: 2014-October-16
   * Priority:
   * Complexity:
   * Focus: All
   * Related Issues: None
   * Competing Issues: None

---++ Issue Summary

When ?? receives a metavalue (U, X, Z, W, -) it quietly returns false

<verbatim>function "??" (l : STD_ULOGIC) return BOOLEAN is
begin
  return l = '1' or l = 'H';
end function "??";</verbatim>

Stack Overflow [[http://stackoverflow.com/questions/25737877/how-to-get-simulation-warning-when-comparing-std-logic-with-x][post]] identifies concerns about correctly handling metavalues.

In fact, if you look at "="[unsigned, unsigned return unsigned], you will note that if one of the inputs has an 'X', an assertion warning is produced.

---++ Proposal

Add an assertion to ?? so that it produces an ERROR

<verbatim>function "??" (l : STD_ULOGIC) return BOOLEAN is
begin
  assert not is_x(l) or NO_WARNING
    report "STD_LOGIC_1164.""??"": metavalue detected, returning FALSE"
    severity warning;
  return l = '1' or l = 'H';
end function "??"; </verbatim>

Often I prefer the readability of if over assert:

<verbatim>function "??" (l : STD_ULOGIC) return BOOLEAN is
begin
  if is_x(l) and not NO_WARNING then
    report "STD_LOGIC_1164.""??"": metavalue detected, returning FALSE"
    severity warning;
  end if ;
  return l = '1' or l = 'H';
end function "??";</verbatim>

Requires edit of LRM 9.2.9. Change:

NOTE 2—The condition operator is defined for type STD_ULOGIC defined in package STD_LOGIC_1164 (see 16.7).<br />Conversion of a value of type STD_ULOGIC converts '1' and 'H' to TRUE and all other values to FALSE.

To:

NOTE 2—The condition operator is defined for type STD_ULOGIC defined in package STD_LOGIC_1164 (see 16.7).<br />Conversion of a value of type STD_ULOGIC converts '1' and 'H' to TRUE and all other values to FALSE. If metavalue is detected, an assert severity WARNING is issued.
---++ Concerns

May slow down evaluation of conditional expressions

---++ Comments

---++ Supporters
Topic revision: r1 - 2020-02-17 - 15:34:56 - TWikiGuest
P1076/Ballots