We can also go over this at the next meeting. Chuck Swart -------------BEGINNING OF IR---------------- VHDL Issue Number: 2087 Language_Version VHDL-2002 Classification Language Definition Problem Summary Ambiguous rule for type of an alias declaration Relevant_LRM_Sections 4.3.3.1 Object aliases Related_Issues Key_Words_and_Phrases object alias Authors_Name Peter Ashenden Authors_Phone_Number +61 414 709 106 Authors_Fax_Number Authors_Email_Address peter@ashenden.com.au Authors_Affiliation Ashenden Designs Authors_Address1 Authors_Address2 Authors_Address3 Current Status: Analyzed Superseded By: ------------------------ Date Submitted: 9 February 2006 Date Analyzed: 13 March 2006 Author of Analysis: Larry Soule Revision Number: 2 Date Last Revised: 13 March 2006 Description of Problem ---------------------- In 4.3.3.1, page 69, item (b) includes the following: The base type of the name specified in an alias declaration must be the same as the base type of the type mark in the subtype indication (if the subtype indication is present); this type must not be a multidimensional array type. The referent of the word "this" is ambiguous. Does it refer to the base type of the name? If so, then any alias of a multidimensional array object is illegal. Alternatively, does it refer to the base type of the type mark of the subtype indication (if present)? If so, the following would be illegal: type T is array(1 to 3, 1 to 3) of integer; variable V : T; alias A:T is V; whereas the following would be legal: alias A is V; In either case, there appears to be no rationale for proscribing a multidimensional type. Was that intended in general, or only in the case of an alias of a slice? Further, in item (b)(1), it says: If the alias designator denotes a slice of an object, then the subtype of the object is viewed as if it were of the subtype specified by the slice. Presumably this should be "..., then the slice of the object is viewed as if it were of the subtype specified by the slice." In all cases of object aliases, the viewing that goes on is of the object, not of a subtype. Proposed Resolution ------------------- Remove the rule requiring the base type not to be a multidimensional array type. Change the incorrect use of the word "subtype" to "slice". VASG-ISAC Analysis & Rationale ------------------------------ In Section '4.3.3.1 Object aliases', section b) requires that "The base type of the name specified in an alias declaration must be the same as the base type of the type mark in the subtype indication (if the subtype indication is present); this type must not be a multidimensional array type." The restriction against aliasing multidimensional arrays was not intentional and should be removed. The only restriction that needs to be in place is one that prohibits 're-shaping' the array (e.g., from a 2 by 4 to an 8 by 1) as this would depend on the data layout of the implementation. This restriction is already taken care of by b)(2) by "moreover, the subtype denoted by the subtype indication must include a matching element (see 7.2.2) for each element of the object denoted by the name." Also, the typo reported in b)(1) is genuine and should be changed. VASG-ISAC Recommendation for IEEE Std 1076-2002 ----------------------------------------------- Interpret the LRM as if the Recommendation for Future Revisions were in effect. VASG-ISAC Recommendation for Future Revisions --------------------------------------------- As the reported suggests, change the wording of b)(1) to "..., then the slice of the object is viewed as if it were of the subtype specified by the slice." -------------END OF IR----------------Received on Mon Mar 13 17:24:33 2006
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