RE: [sv-ec] rand in struct with union members

1. I think it could have been done, as Steven wrote, 

"It would have been more general if it had said that a struct member could be made rand, as long as that member was itself of a type that could be made rand. "

18.4 says, "The solver can randomize singular variables of any integral type," and packed unions are singular (6.4) and integral (6.11.1).

I think the possible distinction between packed and unpacked unions was simply not considered when the restriction in 18.4 was written.


2. However, in your example, you wrote:
> 	typedef union packed {reg [3:0] r; enum 
> bit[3:0]{raz,dwa,trzy} e;}
> TT;
> 	typedef struct {TT a; rand byte b;} TT1;
> 	typedef struct packed{TT a; byte b;} TT2;
> 
> 	typedef struct {
> 		TT u1;		//cannot be random
> 		rand int i;//ok 
> 		rand TT1 s1;//ok
> 		TT2 s2;//this also cannot be random as if so 
> then it will
> randomize s2.a which is a union
> 	} T;

I think u1 can be random, as I wrote above. At least, I think it is a legitimate interpretation of the LRM.
I think s2 can also be random as it is a packed structure, not an unpacked structure.

Shalom


> -----Original Message-----
> From: owner-sv-ec@server.eda.org 
> [mailto:owner-sv-ec@server.eda.org] On Behalf Of Daniel Mlynek
> Sent: Monday, September 07, 2009 10:50 AM
> To: 'Steven Sharp'; sv-ec@server.eda.org
> Subject: RE: [sv-ec] rand in struct with union members
> 
> 1. Thank you for yours exaplanation.
> I still have doubts why not to allow packed union to be rand member of
> unpacked struct and treat them as variable vector with size 
> and sign given
> by its declaration (ignore fields specification). Another 
> problem is that
> union are allowed as arguments for std::randomize function so this is
> inconsistence.
> 
> 2. Let me present  an example for current LRM rules - are my 
> comments right?
> 
> module top;
> 	typedef union packed {reg [3:0] r; enum 
> bit[3:0]{raz,dwa,trzy} e;}
> TT;
> 	typedef struct {TT a; rand byte b;} TT1;
> 	typedef struct packed{TT a; byte b;} TT2;
> 
> 	typedef struct {
> 		TT u1;		//cannot be random
> 		rand int i;//ok 
> 		rand TT1 s1;//ok
> 		TT2 s2;//this also cannot be random as if so 
> then it will
> randomize s2.a which is a union
> 	} T;
> 	T s1;
> 
> 	
> endmodule 
> 
> -----Original Message-----
> From: Steven Sharp [mailto:sharp@cadence.com] 
> Sent: 4 września 2009 23:21
> To: sv-ec@eda.org; daniel.mlynek@aldec.com
> Subject: Re: [sv-ec] rand in struct with union members
> 
> 
> >From: "Daniel Mlynek" <daniel.mlynek@aldec.com>
> 
> >LRM:"Members of unpacked structures containing a union as well as 
> >members of packed structures shall not be allowed to have a random
> modifier."
> >
> >Why does the LRM forbids to have "rand" on  structs  
> containing an union?
> >What was the motivation for this restriction?
> 
> I don't remember anything about this restriction, and may not 
> have been
> involved.  However, I can make deductions about what was intended.
> 
> The wording of this restriction in 8.4 closely matches the 
> restriction in
> 7.2.2:
> 
> "Members of unpacked structures containing a union as well as 
> members of
> packed structures shall not be assigned individual default 
> member values."
> 
> I assume that the wording in 8.4 was borrowed from 7.2.2.
> 
> The issue here is presumably that it is unclear which member 
> of the union is
> supposed to be assigned the value.  And for a member of a 
> packed struct, it
> may be unclear how to handle the conflict between the overall 
> initialization
> of the struct as an integral value, and the initialization of 
> the member.  A
> packed struct is effectively a union between the separate 
> members and the
> vector value that the entire struct represents.
> 
> The text in 7.2.2 refers to unions that are members of 
> unpacked structs
> because it is only members of unpacked structs that can have 
> default values
> like this.  So there is no need to say it about unions in general.
> 
> Similarly, the text about rand in 18.4 comes immediately 
> after a reference
> to rand qualifiers on members of unpacked structs.
> 
> Allowing a rand qualifier on a union would be a problem, 
> since it would be
> unclear which member of the union should be assigned the 
> random value.  So
> why does this only mention unions that are members of 
> unpacked structs?  It
> appears that this is because other unions have already been 
> excluded.  8.4
> lists everything you can qualify with rand, and it does not 
> list unions.
> But when it says it is allowed for a member of an unpacked 
> struct, that
> seems to allow a loophole, since that member could be a 
> union.  So this text
> is saying that a union still isn't allowed.
> 
> It would have been more general if it had said that a struct 
> member could be
> made rand, as long as that member was itself of a type that 
> could be made
> rand.
> 
> 
> >Does above forbids the code:
> >
> >typedef union packed { reg a; bit b;} TU;
> >
> >typededef struct {
> >
> >    rand int a;//illegal as this struct has  union inside
> 
> This is fine, as the member does not contain a union.
> 
> >    rand TU v; //illegal
> 
> This is illegal, as the member contains a union.  It isn't a 
> major issue in
> this simple case, but one can imagine cases where it would 
> be.  For example,
> what if it were a union between an enum and an int?  Which 
> member of the
> union should be randomized?  If it is the enum, then I assume it is
> restricted to the defined literals of the enum.  If it is the 
> int, then it
> could be any value in its range.  These give different 
> results.  It could
> have been defined to randomize the first (or last) member of 
> the union, but
> instead it was made illegal.
> 
> 
> Steven Sharp
> sharp@cadence.com
> 
> 
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