RE: [sv-bc] Re: SV31A LRM interpretation for: unique case

Subject: RE: [sv-bc] Re: SV31A LRM interpretation for: unique case
From: Michael McNamara (mac@verisity.com)
Date: Thu Feb 19 2004 - 01:18:17 PST

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In reply to: Stuart Sutherland: "RE: [sv-bc] Re: SV31A LRM interpretation for: unique case"
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While I have not been following the debate to this point, as a former
designer, the value I see in unique casex is that I do not generate a
priority decoder.

As Stuart points out, this example indeed requires a priority decoder
as 1101 would activate two case items.

Recoding the example as to remove this likley unintended overlap gives
us:

unique casex (value)
'b111x, 'b11x1: $display( "case item 1 executed" );
'b0111: $display( "case item 2 executed" );
endcase

Arguably this could be viewed as mapping to:

unique casex (value)
    'b111x: $display( "case item 1 executed" );
    'b11x1: $display( "case item 1 executed" );
    'b0111: $display( "case item 2 executed" );
endcase

and then we are not a unique case, as 'b1111 would indeed trigger the
first two case items. HOWEVER - they do the same thing. BUT they do
generate side effects (there might be two prints to the log file...).

However, if in truth the logic one desired was

if (value[3]&value[2]&(value[1]|value[0]))
$display("case item 1 executed")

if (!value[3]&value[2]&value[1]&value[0])
$display("case item 2 executed")

then we are safe as we have as desired, not generated a priority
decoder; and are assured that only one or the other $display would be
executed.

-- On Feb 18 2004 at 10:33, Stuart Sutherland sent a message:
> To: David.Rich@synopsys.com, dlindner@cadence.com, sv-bc@eda.org, sharp@cadence.com, ricksull@cadence.com
> Subject: "RE: [sv-bc] Re: SV31A LRM interpretation for: unique case"
> Dave,
>
> One of the example previously given had multiple case items that could
> satisfy the same branch.
> ----------
> Example 1: Using "unique casex"
>
> Assume "reg [3:0] value;" is equal to "4'b1111" when the following "unique
> casex" is executed:
>
>
>
> unique casex (value)
>
> 'b111x, 'b11x1: $display( "case item 1 executed" ); //1st case
> item - has multiple true case expressions
>
> 'b1101: $display( "case item 2 executed" ); //2nd
> case item - has no true case expression
>
> endcase
>
> ----------
>
> The given value of the case expression would only satisfy one branch of the
> code. If I understand correctly, your change means that this will still
> generate an error, because two case items are true for the same branch of
> code. Is that your intent?
>
> Note that in that example, if value were 'b1101, then two branches could be
> true, which would--and should--be an error.
>
> Stu
> ~~~~~~~~~~~~~~~~~~~~~~~~~
> Stuart Sutherland
> stuart@sutherland-hdl.com
> 503-692-0898
>
>
>
> _____
>
> From: owner-sv-bc@server.eda.org [mailto:owner-sv-bc@server.eda.org] On
> Behalf Of Dave Rich
> Sent: Wednesday, February 18, 2004 12:20 AM
> To: Dave Lindner
> Cc: sv-bc@server.eda.org; Steven Sharp; Rick Sullivan
> Subject: Re: [sv-bc] Re: SV31A LRM interpretation for: unique case
>
>
> Dave,
>
> Yes I agree with your interpretation.
>
> I suggest the following errata for Section 8.4
>
> "A unique case shall guarantee no overlapping case valuesitems, allowing the
> case items to be evaluated in parallel."
>
> Dave
>
>
> Dave Lindner wrote:
>
>
>
>
> SystemVerilog LRM says:
>
> "A unique case shall guarantee no overlapping case values, allowing the case
> items to be evaluated in parallel."
>
>
>
>
>
> Did you really mean matching multiple case values is an error,
>
> or multiple matching case items is an error, which makes more sense?
>
>
>
>
>
> What happens when:
>
> There is one case item that has multiple true case expressions, and
>
> all other case items have false case expressions.
>
>
>
> We believe that this scenario, demonstrated in Example 1 should not throw a
> "unique" run-time error in the simulator.
>
> Is our interpretation correct?
>
>
>
>
>
> Example 1: Using "unique casex"
>
> Assume "reg [3:0] value;" is equal to "4'b1111" when the following "unique
> casex" is executed:
>
>
>
> unique casex (value)
>
> 'b111x, 'b11x1: $display( "case item 1 executed" ); //1st case
> item - has multiple true case expressions
>
> 'b1101: $display( "case item 2 executed" ); //2nd
> case item - has no true case expression
>
> endcase
>
>
>
> With value set to "4'b1111":
>
> The first case item has multiple true case expressions.
>
> All other case items have false case expressions.
>
> We believe that this is not an error, based upon the following consistency
> argument, demonstrated in Example 2, which uses the "unique if" construct:
>
>
>
>
>
> Example 2: Using "unique if"
>
> Assume "reg [3:0] value, a, b;" are all equal to "4'b1111",
>
> and "reg [3:0] c;" is equal to "4'b1101", when the when the following
> "unique if" is executed:
>
>
>
> unique if ((value == a) || (value==b)) //1st "unique if" condition - is
> true in multiple ways
>
> $display( "if clause 1 executed" );
>
> else if (value == c) //2nd "unique if" condition
> - is false
>
> $display( "if clause 2 executed" );
>
>
>
> With value, a, b, c set to the above values:
>
> The first "unique if" condition is true in multiple ways.
>
> No other "unique if" condition is true.
>
> We know that this should not throw a "unique" run-time error in the
> simulator.
>
> Therefore the "unique casex" analogy should not throw a run time error.
>
>
>
>
>
>
> --
>
> --
>
> David.Rich@Synopsys.com
>
> Technical Marketing Consultant and/or
>
> Principal Product Engineer
>
> http://www.SystemVerilog.org
>
> tele: 650-584-4026
>
> cell: 510-589-2625
>
>
>
> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
> <HTML><HEAD><TITLE></TITLE>
> <META http-equiv=Content-Type content="text/html; charset=us-ascii">
> <META content="MSHTML 6.00.2800.1400" name=GENERATOR></HEAD>
> <BODY text=#000000 bgColor=#ffffff>
> <DIV dir=ltr align=left> color=#0000ff size=2>Dave,</DIV>
> <DIV dir=ltr align=left> color=#0000ff size=2> </DIV>
> <DIV dir=ltr align=left> color=#0000ff size=2>One of the example previously given had multiple case
> items that could satisfy the same branch.</DIV>
> <DIV dir=ltr align=left> color=#0000ff size=2>----------</DIV>
> <DIV>
> <SPAN
> style="FONT-SIZE: 11pt">Example 1:   Using “unique
> casex”
> <SPAN
> style="FONT-SIZE: 11pt">Assume “reg [3:0] value;” is equal to “4’b1111” when the
> following “unique casex” is executed:
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">unique casex (value)
> <SPAN
> style="FONT-SIZE: 11pt">    ’b111x, ‘b11x1:
>    $display( “case item 1 executed” );  
>   //1st case item – has multiple true case
> expressions
> <SPAN
> style="FONT-SIZE: 11pt">   
> ’b1101:          
>      $display( “case item 2 executed”
> );     //2nd case item – has no true case
> expression
> <SPAN
> style="FONT-SIZE: 11pt">endcase</DIV>
> <DIV><SPAN
> class=666431418-18022004>----------</DIV>
> <DIV><SPAN
> class=666431418-18022004> </DIV>
> <DIV><SPAN
> class=666431418-18022004>The given value of the case expression would only
> satisfy one branch of the code.  color=#0000ff size=2><SPAN
> class=666431418-18022004>If I
> understand correctly, your change means that this will still generate an error,
> because two case items are true for the same branch of code.  Is that your
> intent?</DIV>
> <DIV>
> <DIV dir=ltr align=left> </DIV>
> <DIV dir=ltr align=left>Note that in that
> example, if value were 'b1101, then two branches could be true, which would--and
> should--be an error.</DIV>
> <DIV dir=ltr align=left> </DIV>
> <DIV dir=ltr align=left>Stu</DIV>
> <DIV>~~~~~~~~~~~~~~~~~~~~~~~~~ Stuart
> Sutherland stuart@sutherland-hdl.com 503-692-0898 </DIV></DIV>
> <DIV> </DIV> 
> <BLOCKQUOTE dir=ltr
> style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px solid; MARGIN-RIGHT: 0px">
> <DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left>
> <HR tabIndex=-1>
> From: owner-sv-bc@server.eda.org
> [mailto:owner-sv-bc@server.eda.org] On Behalf Of Dave
> Rich Sent: Wednesday, February 18, 2004 12:20 AM To: Dave
> Lindner Cc: sv-bc@server.eda.org; Steven Sharp; Rick
> Sullivan Subject: Re: [sv-bc] Re: SV31A LRM interpretation for:
> unique case </DIV>
> <DIV></DIV>Dave, Yes I agree with your interpretation. I suggest
> the following errata for Section 8.4 size=2>"A unique case shall guarantee no
> overlapping case <STRIKE>values</STRIKE>items,
> allowing the case items to be evaluated in
> parallel." Dave Dave Lindner wrote: 
> <BLOCKQUOTE
> cite=mid9186D1CF264F564C81B42C75972344E6119369@exmbx01chel.cadence.com
> type="cite">
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> <DIV class=Section1>
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">SystemVerilog LRM says:
> <SPAN
> style="FONT-SIZE: 11pt">“A unique case shall guarantee no overlapping case
> values, allowing the case items to be evaluated in
> parallel.”
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">Did you really mean matching multiple case values is
> an error,
> <SPAN
> style="FONT-SIZE: 11pt">or multiple matching case items is an error, which
> makes more sense?
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">What happens when:
> <SPAN
> style="FONT-SIZE: 11pt">   There is one case item that has
> multiple true case expressions, and
> <SPAN
> style="FONT-SIZE: 11pt">   all other case items have false case
> expressions.
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">We believe that this scenario, demonstrated in
> Example 1 should not throw a
> “unique” run-time error in the simulator.
> <SPAN
> style="FONT-SIZE: 11pt">Is our interpretation correct?
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">Example 1:   Using “unique
> casex”
> <SPAN
> style="FONT-SIZE: 11pt">Assume “reg [3:0] value;” is equal to “4’b1111” when
> the following “unique casex” is executed:
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">unique casex (value)
> <SPAN
> style="FONT-SIZE: 11pt">    ’b111x, ‘b11x1:
>    $display( “case item 1 executed” );  
>   //1st case item – has multiple true case
> expressions
> <SPAN
> style="FONT-SIZE: 11pt">   
> ’b1101:          
>      $display( “case item 2 executed”
> );     //2nd case item – has no true case
> expression
> <SPAN
> style="FONT-SIZE: 11pt">endcase
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">With value set to “4’b1111”: 
> <SPAN
> style="FONT-SIZE: 11pt">The first case item has multiple true case
> expressions.
> <SPAN
> style="FONT-SIZE: 11pt">All other case items have false case
> expressions.
> <SPAN
> style="FONT-SIZE: 11pt">We believe that this is <SPAN
> style="FONT-WEIGHT: bold">not an error, based upon the following
> consistency argument, demonstrated in Example 2, which uses the “unique if”
> construct:
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">Example 2:   Using  “unique
> if”
> <SPAN
> style="FONT-SIZE: 11pt">Assume “reg [3:0] value, a, b;” are all equal to
> “4’b1111”,
> <SPAN
> style="FONT-SIZE: 11pt">and “reg [3:0] c;” is equal to “4’b1101”, when the
>  when the following “unique if” is executed:
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">unique if ((value == a) ||
> (value==b))    //1st “unique if” condition – is
> true in multiple ways
> <SPAN
> style="FONT-SIZE: 11pt">    $display( “if clause 1 executed”
> );     
> <SPAN
> style="FONT-SIZE: 11pt">else if (value == c)
>                            
> //2nd “unique if” condition – is false
> <SPAN
> style="FONT-SIZE: 11pt">    $display( “if clause 2 executed”
> );
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 11pt">With value, a, b, c set to the above
> values: 
> <SPAN
> style="FONT-SIZE: 11pt">The first “unique if” condition is true in multiple
> ways.
> <SPAN
> style="FONT-SIZE: 11pt">No other “unique if” condition is
> true.
> <SPAN
> style="FONT-SIZE: 11pt">We know that this should <SPAN
> style="FONT-WEIGHT: bold">not throw a “unique” run-time error in
> the simulator.
> <SPAN
> style="FONT-SIZE: 11pt">Therefore the “unique casex” analogy should <SPAN
> style="FONT-WEIGHT: bold">not throw a run time
> error.
> <SPAN
> style="FONT-SIZE: 11pt"> 
> <SPAN
> style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"> </DIV></BLOCKQUOTE> <PRE class=moz-signature cols="100">--
> --
> <A class=moz-txt-link-abbreviated href="mailto:David.Rich@Synopsys.com">David.Rich@Synopsys.com</A>
> Technical Marketing Consultant and/or
> Principal Product Engineer
> <A class=moz-txt-link-freetext href="http://www.SystemVerilog.org">http://www.SystemVerilog.org>
> tele: 650-584-4026
> cell: 510-589-2625
>
> </PRE></BLOCKQUOTE></BODY></HTML>

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