RE: [sv-bc] sign/width casting semantics

Brad,
 
I would agree with 1.
For 2, I think it should have the same signedness as that of the cast
operand.
 
Shalom


________________________________

	From: owner-sv-bc@server.eda.org
[mailto:owner-sv-bc@server.eda.org] On Behalf Of Brad Pierce
	Sent: Wednesday, January 30, 2008 5:30 PM
	To: sv-bc
	Subject: RE: [sv-bc] sign/width casting semantics
	
	
	Shalom,
	 
	I would prefer to remove the current weird text and simply say
that 
	 
	     1) signed'() and unsigned'() are new syntax for $signed()
and $unsigned() 
	 
	     2) the value of a width cast is the same as the result of
an assignment to an unsigned bit or logic variable of that same width,
depending on whether the expression to be cast is purely 2-bit.
	 
	Yes, that breaks backward compatibility with the LRM, but tools
are already in divergence, because the LRM is so confusing.  The more
obvious the semantics, the better.
	 
	-- Brad

________________________________

	From: owner-sv-bc@eda.org [mailto:owner-sv-bc@eda.org] On Behalf
Of Bresticker, Shalom
	Sent: Wednesday, January 30, 2008 5:47 AM
	To: sv-bc
	Subject: [sv-bc] sign/width casting semantics
	
	

	Hi, 

	There is unclearness about the semantics of sign casting and
width casting. 

	For casting to a full type, the semantics are clear: 

	"If the expression is assignment compatible with the casting
type, then the cast shall return the value that a variable of the
casting type would hold after being assigned the expression."

	However, how do sign casting and width casting work? 

	The LRM says, 

	"When changing the size, the signing shall pass through
unchanged and the result type shall be a one-dimensional packed array
with a right bound of zero. When changing the signing, the type of the
expression to be cast shall pass through unchanged, except for the
signing."

	Let's take an example: 

	logic signed [7:0] regS; 
	regS = signed'(4'b1100); 

	My understanding is that "signed'(4'b1100)" turns into 4'sb1100,
which is then sign-extended to the width of regS, giving 8'sb11111100.
This means it works the same as $signed().

	However, some people (e.g., Greg Jaxon) say that the
context-determined width of the entire expression, 8 bits, is propagated
into the cast operand, while the sign cast determines the signedness of
the size-extended expression, giving 8'sb00001100, giving a different
result from $signed().

	Look at 11.7, which contains these examples: 

	logic signed [7:0] regS; 
	regS = $signed (4'b1100); // regS = -4 
	regS = signed'(4'b1100); // regS = -4 

	The $signed() example comes from 1364-2005. The signed'()
example was added by the editor during the LRM merge. He assumed it is
identical to $signed(). Is this correct?

	Another example, from the $clog2() discussion recently: Greg
wrote (slightly edited), 

	     (expr) | signed'(1'b1) 
	... 
	Note that 1'sb1 would be sign-extended to match width (bad!),
whereas 
	signed'(1'b1) will 0-extend the literal before cloaking it with
signedness (so as to not poison expr with unsignedness).

	and I disagree with Greg. 

	The question about width casting is similar. Is the expression
inside the width-cast affected by the signedness of the surrounding
expression before changing its width (e.g., changing it from signed to
unsigned, causing zero-extension instead of sign-extension)?

	The committee needs to decide on the correct semantics and
clarify them in the LRM. Tools are divergent on this already.

	Some relevant links: 

	http://www.eda-stds.org/sv-bc/hm/1346.html
<http://www.eda-stds.org/sv-bc/hm/1346.html>  
	<http://www.eda-stds.org/sv-bc/hm/1714.html>
http://www.eda-stds.org/sv-bc/hm/1714.html 
	<http://www.eda.org/sv-bc/display_issue.cgi?issue_num=38>
http://www.eda.org/sv-bc/display_issue.cgi?issue_num=38 
	<http://www.eda-stds.org/sv-bc/hm/1777.html>
http://www.eda-stds.org/sv-bc/hm/1777.html 
	<http://www.eda-stds.org/sv-bc/hm/1780.html>
http://www.eda-stds.org/sv-bc/hm/1780.html 
	<http://www.eda-stds.org/sv-bc/hm/1790.html>
http://www.eda-stds.org/sv-bc/hm/1790.html 
	<http://www.eda-stds.org/sv-bc/hm/1794.html>
http://www.eda-stds.org/sv-bc/hm/1794.html 
	<http://www.eda-stds.org/sv-bc/hm/1796.html>
http://www.eda-stds.org/sv-bc/hm/1796.html 
	<http://www.eda-stds.org/sv-bc/hm/1800.html>
http://www.eda-stds.org/sv-bc/hm/1800.html 


	Thanks, 
	Shalom 

	Shalom Bresticker 
	Intel Jerusalem LAD DA 
	+972 2 589-6582 
	+972 54 721-1033 

	
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