Rule Selection

This section will detail the guidelines followed for creating and organizing rules.

Note

VSG can not fully classify a VHDL file, this will place limitations on the ability to write rules.

Goals

The VHDL Language Reference Manual (LRM) will be the basis used when determining rule creation, naming and the organization of rules.

Guidelines

• [naming_001] Rule naming shall be of the form {identifier}_{number}.

• [naming_002] The identifier shall be the left-hand side of VHDL production.

• [naming_003] The number shall be three digits.

• [naming_004] The rule number shall indicate which phase the rule is executed.

• [naming_005] An identifier and number pair shall never be re-used.

• [scoping_001] Rules shall be scoped to a VHDL production

Example

The following production shows the structure of an architecture.

architecture_body ::=
    *architecture* identifier *of* entity_name *is*
        architecture_declarative_part
    *begin*
        architecture_statement_part
    *end* [ *architecture* ] [ architecture_simple_name ] ;

The VHDL keywords are enclosed with asterisks, architecture, of, is, begin and end. Everything else is another production:

identifier ::=
    basic_identifier | extended_identifier

name ::=
    simple_name
  | operator_symbol
  | character_literal
  | selected_name
  | indexed_name
  | slice_name
  | attribute_name
  | external_name

architecture_declarative_part ::=
    { block_declarative_item }

architecture_statement_part ::=
    { concurrent_statement }

Rules targeting the architecture_body will only include those elements at that level of the production.

For example:

• architecture_body_100 = architecture on it’s own line

• architecture_body_101 = identifier on same line as architecture

• architecture_body_102 = of on same line as identifier

• architecture_body_103 = entity_name on same line as of keyword

• architecture_body_104 = is on same line as entity_name

• architecture_body_105 = no code after is keyword

• architecture_body_106 = begin on it’s own line

• architecture_body_107 = no code after begin keyword

• architecture_body_108 = end on it’s own line

• architecture_body_109 = architecture on same line as end

• architecture_body_110 = architecture_simple_name on same line as end

• architecture_body_111 = semicolon on same line as end

• architecture_body_112 = no code after semicolon

• architecture_body_120 = add/remove optional keyword architecture

• architecture_body_121 = add/remove optional architecture_simple_name

• architecture_body_200 = whitespace between architecture and identifier

• architecture_body_201 = whitespace between identifier and of keyword

• architecture_body_202 = whitespace between of and entity_name

• architecture_body_203 = whitespace between entity_name and is keyword

• architecture_body_204 = whitespace between end and architecture keyword

• architecture_body_205 = whitespace between architecture and architecture_simple_name

• architecture_body_206 = whitespace between architecture_simple_name and semicolon

• architecture_body_400 = indent of architecture keyword

• architecture_body_401 = indent of begin keyword

• architecture_body_402 = indent of end keyword

• architecture_body_600 = case of architecture keyword

• architecture_body_601 = case of identifier

• architecture_body_602 = case of of keyword

• architecture_body_603 = case of entity_name

• architecture_body_604 = case of is keyword

• architecture_body_605 = case of begin keyword

• architecture_body_606 = case of end keyword

• architecture_body_607 = case of ending architecture keyword

• architecture_body_608 = case of architecture_simple_name

• architecture_body_700 = naming restrictions on identifier and architecture_simple_name

One question to answer are blank lines. Are they part of architecture_body or the next production?

Expanding the architecture_declarative_part production shows:

block_declarative_item ::=
    subprogram_declaration
  | subprogram_body
  | subprogram_instantiation_declaration
  | package_declaration
  | package_body
  | package_instantiation_declaration
  | type_declaration
  | subtype_declaration
  | constant_declaration
  | signal_declaration
  | shared_variable_declaration
  | file_declaration
  | alias_declaration
  | component_declaration
  | attribute_declaration
  | attribute_specification
  | configuration_specification
  | disconnection_specification
  | use_clause
  | group_template_declaration
  | group_declaration
  | PSL_Property_Declaration
  | PSL_Sequence_Declaration
  | PSL_Clock_Declaration

Vertical spacing between these elements will be performed by rules in the block_declarative_part group. This will eliminate the issue where multiple blank line rules could collide.

• block_declarative_part_300 = blank line at beginning of block_declarative part

• block_declarative_part_301 = blank line at end of block_declarative part

• block_declarative_part_302 = blank line before block_declarative_item (could be configurable) (could also conflict with block_declarative_item_001)

• etc…

The block_declarative_part can include from 0 to N block_declarative_items. Alignment rules between block_declarative_items, e.g. colon alignment in signal_declaration and constant_declaration, will also be performed at this level.

• block_declarative_part_500 = align identifiers in file, constant and signal declarations

• block_declarative_part_501 = align : in constant and signal declarations

• etc…

Other structural rules could be created:

• block_declarative_part_100 = all constants defined at top of block_declarative_part (maybe this is out of scope of VSG though)

Taking the next step down to the signal_declaration level:

signal_declaration ::=
    signal identifier_list : subtype_indication [ signal_kind ] [ := expression ] ;

The following rules would be generated:

• signal_declaration_100 = signal keyword on it’s own line

• signal_declaration_101 = identifier_list on same line as signal keyword

• signal_declaration_102 = colon on same line as identifier_list

• signal_declaration_103 = subtype_indication on same line as colon

• signal_declaration_104 = signal_kind on same line as subtype_indication

• signal_declaration_105 = := on same line as signal_kind

• signal_declaration_106 = expression on same line as :=

• signal_declaration_107 = semicolon on same line as expression

• signal_declaration_108 = no code after semicolon

• signal_declaration_200 = whitespace between signal keyword and identifier_list

• signal_declaration_200 = whitespace between identifier_list and colon

• signal_declaration_200 = whitespace between colon and subtype_indication

• signal_declaration_200 = whitespace between subtype_indication and signal_kind

• signal_declaration_200 = whitespace between signal_kind and :=

• signal_declaration_200 = whitespace between := and expression

• signal_declaration_400 = indent of signal keyword

• signal_declaration_600 = case of signal keyword

• signal_declaration_601 = case of identifiers in identifier_list

• signal_declaration_700 = naming restrictions on signal identifiers

One could argue rule 601 should be moved to an identifier_list set of rules, but it seems appropriate at this level.

identifier_list ::=
    identifier { , identifier }

I would make the argument that signal_kind case would be done by a signal_kind rule.

signal_kind ::=
    register | bus

One could make the argument that subtype_indication should be handled by it’s own set of rules:

subtype_indication ::=
    [ resolution_indication ] type_mark [ constraint ]

resolution_indication ::=
    resolution_function_name | ( element_resolution )

element_resolution ::= array_element_resolution | record_resolution

type_mark ::=
    type_name | subtype_name

And the same for constraint:

constraint ::=
    range_constraint
  | array_constraint
  | record_constraint

If the subtype_indication and constraint rules were moved out of signal_declaration, and where ever else they are location, that it would reduce the number of rules. VSG currently uses a base rule to handle constraints, which is extended where constraints are used.