RE: [sv-bc] Mantis 1338 - Clarificaion on String literal behavior in macros

Eric,
 
I believe I agree with you.
 
Note that Mantis 1119 explicitly clarifies that macro argument
substitution does NOT agree within a string literal (using ", not `").
 
Mantis 1507 is open on clarifying the behavior of a backslash followed
by a general character (i.e., one not in the table) within a string
literal. The C precedent is that the backslash has no special meaning,
and that these are two characters. Note that when used as a display
format, the backslash disappears, though, see Mantis 1101.
 
Yulik once commented that the phrase "usual lexical meaning" is not
clear.
 
The LRM also does not specify what happens if `\ appears in the macro
text NOT followed by `". Does it become just backslash, or does it stay
as quote+backslash?
 
The email thread starting at http://www.eda-stds.org/sv-bc/hm/4201.html
clarifies the behavior and the clarifications should enter the LRM text.
 
Regards,
Shalom


________________________________

	From: owner-sv-bc@server.eda.org
[mailto:owner-sv-bc@server.eda.org] On Behalf Of Coffin, Eric
	Sent: Tuesday, November 20, 2007 4:56 PM
	To: Alsop, Thomas R
	Cc: sv-bc@server.eda.org
	Subject: Re: [sv-bc] Mantis 1338 - Clarificaion on String
literal behavior in macros
	
	
	Hi Thomas,
	
	Thanks for writing the 1338 proposal.  I've found this area of
the language a bit problematic myself, and I agree with you in that it
needs clarification.
	
	My take on the three macro-operator character sequences `", ``
and `\`" is that they do not have any special meaning within a string
literal.  If they did have special meaning why would the `"
macro-operator be needed at all?
	
	The 1338 proposal defines two macros.  They are:
	
	`define msg(x,y) `"x: `\`"y`\`"`"
	`define nosubx(x,y) "x: `\`"y`\`"`"
	
	And the examples of their use are:
	
	$display(`msg(left side,right side));
	$display(`nosubx(left side,right side));
	
	I agree with mantis 1338's interpretation of the first macro's
expansion:
	
	$display("left side: \"right side\"");
	
	I disagree with mantis 1338's interpretation of the second
macro.  The second macro should be expanded to:
	"x: `\`"right side`\`"`"  which is an ill-formed argument to the
display systask.  The first double-quote starts a string literal.  The
`\` are just characters within the string literal, and have no special
meaning.  The string literal is terminated with the second double-quote
forming the literal:  "x:`\`".  The next two preprocessor tokens are the
identifiers 'right' and 'side'.  These two idents are followed by the
special macro-operators  `\`" and `".
	
	As an aside, P1800/D4 table 5.9.1 does not explicitly state how
to interpret characters within string literals that are prefixed with
the back slash character but that are not in the table.  Take, for
example, the string literal "\`"-- is it two characters long (a
backslash followed by a tick) or one character (a tick)?  ModelSim 
	
	-Eric
	
	
	
	Alsop, Thomas R wrote: 

		I have updated mantis item 1338 with a proposal on what
I believe is the intended behavior of string literals inside of macros
WRT to argument substitution. 

		

		I am slightly hesitant on adding this wording simply
because some vendor tools do not distinguish on this behavior.  Shalom
is asking for clarification about how we treat macro argument
substitution WRT string literals.  I agree about the ambiguity of the
current wording.  The fact that we have `" in macros that allow us keep
the string character after substitution means that string literals
without the back tick have an alternative meaning.  I would interpret
that meaning as a string literal, not a replacement on the string token.


		

		This does collide with current implementation.  For
example I ran VCS on the following code snippets:

		

		`define msg(x,y) `"x: `\`"y`\`"`"

		`define msg_nobacktick(x,y) "x: \"y\""

		`define msg_nosubsitute(x) "x"

		`define msg_subsitute(x) `"x`"

		

		module foo();

		

		initial begin

		  //$fsdbDumpvars(0,foo,"+all");

		

		  $display(`msg(left side,right side));

		  $display(`msg_nobacktick(left side,right side));

		  $display(`msg_nosubsitute(left side));

		  $display(`msg_subsitute(left side));

		

		end

		endmodule

		

		

		This produced the following results:

		

		left side: "right side"

		left side: "right side"

		left side

		left side

		

		The conclusion being that " is treated exactly the same
way as `".

		

		With the change in the proposal this would have the
following results instead:

		

		left side: "right side"

		x: "y"

		x

		left side

		

		I am happy to shoot down the proposal.  The intent is to
come up with one and discuss it.  I don't see a strong need for this
change.  The alternative proposal is to clarify that " and `" behave the
same.

		

		


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