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141003.1 Cary Coutant Need to mark ctors and dtors with =default Enhancement Withdrawn Cary Coutant


Section 5.7.7, pg 110

Compare these two ways of declaring a destructor:

struct S {
   ~S() {}
};

struct T {
   ~T() = default;
};

DWARF currently provides no way to distinguish these, but in the
Itanium C++ ABI, there is a difference, and the debugger needs to
know. In the first case, with a user-defined destructor, S is not
a trivial class, and the calling convention for a function that
returns S requires the caller to pass the address of a buffer for
the return value as an implicit first parameter. In the second
case, with a default destructor, T is trivial, and the calling
convention allows it to return the value on the stack.

Similarly, it may be necessary for a consumer to know if a
special member function has been explicitly deleted.

In order to represent deleted and default special member
functions, we should add a DW_AT_provided attribute with the
following values:

    DW_PROV_declared
    DW_PROV_implicit_default
    DW_PROV_explicit_default
    DW_PROV_explicit_delete

The rules in the C++ standard for determining whether a class is
trivial can depend on more details of the class than a consumer
can gather from the DWARF information, and may vary from one
version of the standard to the next. In order to give the
consumer an explicit indication of whether a class is trivial, we
should add a DW_AT_layout attribute with one of the following
values:

    DW_LAYOUT_unspecified
    DW_LAYOUT_non_trivial
    DW_LAYOUT_trivial
    DW_LAYOUT_pod

If the layout is unspecified, the consumer may attempt to
deduce it based on the knowledge it has of the class structure
and of the target ABI.

To allow the producer to represent these attributes with as
little additional space as possible, we should also add a new
FORM code, DW_FORM_const_present, that encodes a constant
value directly in the abbreviation declaration.


Changes to the spec:

Add the following rows to Table 2.2 ("Attribute names"):

    Attribute        Identifies or Specifies
    ---------        -----------------------
    DW_AT_layout     Class layout

    DW_AT_provided   How a special member function is
                     provided in the source code

Add the following non-normative paragraph to Section 5.7
("Structure, Union, Class and Interface Type Entries"):

    C++ has the notion of a "trivial" class, whose objects can be
    bitwise copied, and a "POD" ("Plain Old Data") struct or
    class, representing a structure that can be shared with code
    written in other languages. Trivial or POD classes may have
    different rules for passing objects of that type as
    parameters or return values.

Add the following paragraph to Section 5.7.1 ("Structure, Union
and Class Type Entries"):

    A structure type, union type or class type entry may have a
    DW_AT_layout attribute, whose value indicates whether it is a
    non-trivial, trivial, or POD type. If not present, the layout
    is assumed to be unspecified. <non-normative>If unspecified,
    a consumer may be able to deduce the layout based on its
    knowledge of the type and the ABI.</non-normative>

Add the following paragraph to Section 5.7.7 ("Member Function
Entries"):

    If the member function entry describes a special member
    function then that entry may have a DW_AT_provided attribute
    whose value is one of the following:

    * DW_PROV_declared (assumed if no DW_AT_provided attribute is
    present) identifies a special member function that is
    declared normally in the source code, whether or not there is
    an explicit definition.

    * DW_PROV_implicit_default identifies a special member
    function that is not declared in the class, but was provided
    as a default implementation by the compiler.

    * DW_PROV_explicit_default identifies a special member
    function that is declared in the class with "= default".

    * DW_PROV_explicit_delete identifies a special member
    function that is declared in the class with "= delete".

In Section 7.5.3 ("Abbreviations Tables"), add the following
paragraph immediately following the paragraph beginning "Finally,
the child encoding is followed by a series of attribute
specifications":

    The attribute form DW_FORM_const_present is a special case.
    For attributes with this form, the attribute specification
    contains a third part, which is a signed LEB128 number. The
    value of this number is used as the value of the attribute,
    and no value is stored in the .debug_info section.

In Section 7.5.4 ("Attribute Encodings"), for class "constant",
change "seven forms" to "eight forms", and add to the end of the
paragraph:

    There is also a implicit constant (DW_FORM_const_present),
    whose value is provided as part of the abbreviation
    declaration.

Add the following rows to Table 7.5 ("Attribute encodings"):

    Attribute name          Value   Classes
    --------------          -----   -------
    DW_AT_layout            xxx     constant
    DW_AT_provided          xxx     constant

Add the following row to Table 7.6 ("Attribute form encodings"):

    Form name               Value   Classes
    ---------               -----   -------
    DW_FORM_const_present   xxx     constant

Add the following sections to Chapter 7 ("Data Representation"):

    7.X Class Layout Codes
    
    The encodings of the constants used in the DW_AT_layout
    attribute are given in Table 7.XX.

        Table 7.XX Class layout encodings

        Layout code name         Value
        ----------------         -----
        DW_LAYOUT_unspecified     0x00
        DW_LAYOUT_non_trivial     0x01
        DW_LAYOUT_trivial         0x02
        DW_LAYOUT_pod             0x03

    7.Y Special Member Function Codes

    The encodings of the constants used in the DW_AT_provided
    attribute are given in Table 7.YY.

        Table 7.XX Special member function encodings

        Special member function code name    Value
        ---------------------------------    -----
        DW_PROV_declared                      0x00
        DW_PROV_implicit_default              0x01
        DW_PROV_explicit_default              0x02
        DW_PROV_explicit_delete               0x03

In Appendix A, Table A.1 ("Attributes by tag value"), add
DW_AT_layout to DW_TAG_class_type, DW_TAG_struct_type, and
DW_TAG_union_type. Add DW_AT_provided to DW_TAG_subprogram.

--
Revised - 11/14/2014
Withdrawn -- 12/16/2014


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