Subject: FW: $sv-ec Re: SV 3.1 draft 1 LRM available
From: David W. Smith (david.smith@synopsys.com)
Date: Fri Dec 06 2002 - 13:11:37 PST
Forward of response.
David
-----Original Message-----
From: eolson@cisco.com [mailto:eolson@cisco.com]
Sent: Friday, December 06, 2002 12:43 PM
To: David W. Smith
Subject: Re: $sv-ec Re: SV 3.1 draft 1 LRM available
C++ reference parameters are pass by reference just like pointers.
They differ from pointer parameters in three ways:
1) the data must exist (you can have a NULL pointer, but the data
refered
to by a reference is there)
2) you cannot change which data is referenced (you can the value of a
pointer, but a refrence is a reference). You can duplicate this
with a 'const pointer, non-const data' declaration:
void foo( char * const p);
3) in the definition of the function, the variable is used by its
normal name (much like a pass by value parameter) - like in
David's
example below
Reference parameters are the prefered method of parameter passing in
C++. In the case where the referenced data does not change, the prefered
method is a constant reference parameter:
void foo (const string& p); // p cannot be modified in foo
Most current EE graduate should be familiar with this C++ syntax.
"David W. Smith" wrote:
>
> Kevin,
> Looking through the book "The C++ Programinng Language Special
> Edition" by Bjarne Stroustrup in Section 5.5 there is a discussion on
> references. Let me quote:
>
> A reference is an alternative name for an object. The main use of
> references is for specifying arguments and return values for functions
> in generatl and for overloaded operators in particular. The notation
> X& means a reference to X. For example:
>
> void f()
> {
> int i = 1;
> int &r = i; // r and i now refer to the same int
> int x = r; // x = 1
>
> r = 2; // i = 2
> }
>
> As you can see the use of the variable that is a reference is NOT a
> dereferenced and is NOT a pointer. There is NO similar statement in C
> but there is in C++. A further example on argument passing can be
> found later in the same section. I quote:
>
> A reference can be used to specify a function argument so that the
> function can change the value of an object passed to it. For example:
>
> void increment(int &aa) {a++;}
>
> void f()
> {
> int x = 1;
> increment(x); // x = 2;
> }
>
> My interpretation of the use of var for pass by reference within
> System Verilog is that it is identical in semantic but that we use a
> different syntax for the actual declaration (section 10.5.2 of SV3.1
> draft 1) where the above example would be:
>
> function void increment (var int aa);
> aa++;
> endfunction
>
> function void f ()
> int x = 1;
> increment(x);
> endfunction
>
> Now, the function declaration syntax is clearly not C or C++. This is
> the choice that Verilog made years ago and I do not believe we have
> intended to change it. This actually more obnoxious then having to do
> "var int aa" instead of "int &aa".
>
> So, based on the C++ definition of a reference I do believe we have it
> correct SystemVerilog.
>
> Your issue on [*] versus [] for dynamic arrays does show a difference
> between SV and C/C++. I kinda of like the indication of the * but
> agree that I do not see a requirement for the *.
>
> I will add both of these to the issues list so that they are captured.
> I hope that my first response address your issue though.
>
> Regards
> David
>
> David W. Smith
> Synopsys Scientist
>
> Synopsys, Inc.
> Synopsys Technology Park
> 2025 NW Cornelius Pass Road
> Hillsboro, OR 97124
>
> Voice: 503.547.6467
> Main: 503.547.6000
> FAX: 503.547.6906
> Email: david.smith@synopsys.com
> http://www.synopsys.com
>
> -----Original Message-----
> From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On Behalf Of
> Kevin Cameron
> Sent: Tuesday, December 03, 2002 6:25 PM
> To: sv-ec; sv bc
> Subject: $sv-ec Re: SV 3.1 draft 1 LRM available
>
> I still have a major problem with the lack of syntactic distinction
> between references and actual objects in this extension, and the use
> of "var" (as a keyword). It makes the language a lot harder to parse
> and makes it quite unlike C/C++. [ Can this be added to the issue
> list?]
>
> On page 26 there is an example of an array copy:
>
> int A[100:1]; // fixed-size array of 100 elements
> int B[*]; // empty dynamic array
> int C[*] = new[8]; // dynamic array of size 8
>
> B = A; // ok. B has 100 elements
> B = C; // ok. B has 8 elements
>
> - B & C are effectively references (to dynamically allocated objects),
> similar statements in C:
>
> int A[100],
> *B,
> *C = (int *)malloc(8 * sizeof(int));
>
> B = A;
> B = C;
>
> - have entirely different semantics: SV does a copy of the object, C
> copies the reference (address). [Is there syntax for copying a
> reference in SV?]
>
> Also, the inability to distinguish between pass-by-reference and
> pass-by-value in the caller's context makes it a lot harder to analyze
> modules in isolation when they use non-local functions or tasks.
>
> It would be nice to have the SV syntax/semantics match C and C++
> conventions as much as possible so that it is easy to translate C/C++
> algorithms into SV (and users don't get confused).
>
> In the same vein I don't see why the "*" is necessary in the "[*]" for
> unsized arrays, C just omits the size.
>
> Kev.
> --
> National Semiconductor, Tel: (408) 721 3251
> 2900 Semiconductor Drive, Mail Stop D3-500, Santa Clara, CA 95052-8090
-- Eric Olson eolson@cisco.com 408-526-5530
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