Re: errata/344: Case Statements with Real Expressions

From: Steven Sharp (sharp@cadence.com)
Date: Thu May 08 2003 - 16:10:06 PDT

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    The following reply was made to PR errata/344; it has been noted by GNATS.

    From: Steven Sharp <sharp@cadence.com>
    To: etf-bugs@boyd.com, steve@icarus.com
    Cc:
    Subject: Re: errata/344: Case Statements with Real Expressions
    Date: Thu, 8 May 2003 19:04:30 -0400 (EDT)

     The standard is not specific on this. However, Verilog-XL allows it,
     and NC-Verilog follows suit.
     
     The behavior seems to follow naturally from normal case statements. If
     any of the case item expressions or case expression are real, then all
     get converted to real. The case expression is compared to each of the
     case item expressions in turn, using "==" since "===" doesn't apply to
     reals.
     
     There might indeed be issues with roundoff errors causing failure to
     match if the expressions involve floating point operations. However,
     there are situations where this is not an issue and where this kind of
     case statement would be useful. For example, if there is a predefined
     set of real values that a variable could be set to (e.g. a choice of
     different input clock frequencies), the case could determine which one
     it had been set to. The input conversion of inexactly represented constants
     such as 1.1 should be the same in all places, so the compare should work.
     
     Since this is allowed by Verilog-XL, and probably most other simulators,
     and there are situations where it could be handy, I don't see any reason
     not to allow it and document it.
     
     I also tested casex and casez in XL and NC, and reals are allowed there
     too. Since any X or Z bits disappear during conversion to real, this
     behaves exactly as if case had been used instead. Since it doesn't add
     any capabilities beyond case, there is less argument in favor of allowing
     reals with casex and casez.
     
     Steven Sharp
     sharp@cadence.com
     

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