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 -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.Received on Mon Sep 7 00:56:59 2009
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