[Dwarf-Discuss] How to map [[no_unique_address]] into DWARF

[David Blaikie]

On Mon, Jun 7, 2021 at 10:58 AM Jan Kratochvil via Dwarf-Discuss
<dwarf-discuss at lists.dwarfstd.org> wrote:
>
> Hi,
>
> clang-12 will create the same DWARF for class B with [[no_unique_address]]
> either present or not. Despite that class C derived from B has different
> layout depending on from which class B it gets derived

Why is this ^ a problem? The layout seems accurate - in that 'a'
shares its address with 'c' - so both members having the same offset
seems like an accurate representation of the layout of the struct?

> :
>
> ------------------------------------------------------------------------------
> struct A {};
> struct By { [[no_unique_address]] A a; };
> struct Cy : By { char c; } cy;
> struct Bn {                       A a; };
> struct Cn : Bn { char c; } cn;
> #include <iostream>
> int main() {
>   std::cout << "sizeof(Cy) = " << sizeof(Cy) << " offsetof(Cy, c) = " << offsetof(Cy, c) << "\n";
>   std::cout << "sizeof(Cn) = " << sizeof(Cn) << " offsetof(Cn, c) = " << offsetof(Cn, c) << "\n";
> }
> // sizeof(Cy) = 1 offsetof(Cy, c) = 0
> // sizeof(Cn) = 2 offsetof(Cn, c) = 1
> ------------------------------------------------------------------------------
>
> gcc-11 creates a different DWARF for the two variants of B but that DWARF does
> not look as compliant to me:
>
> ------------------------------------------------------------------------------
>
> DW_TAG_structure_type
>   DW_AT_name   ("B")
>   DW_AT_byte_size      (1)
>
>   DW_TAG_member
>     DW_AT_name ("a")
>     DW_AT_type (0x0000001e "A")
> By:  DW_AT_data_member_location (-1)
> Bn:  DW_AT_data_member_location (0x00)
>
>   NULL
> ------------------------------------------------------------------------------
>
> From a discussion at https://reviews.llvm.org/D101237 :
>
> (1) Raphael Isemann (teemperor) notes:
>     FWIW, I took a look at the DWARF standard and I think that is actually
>     something we should already emit in the form of a
>     "DW_AT_byte_size 0" attribute at the field? Quote:
>         If the size of a data member is not the same as the size of the type
>         given for the data member, the data member has either
>         a DW_AT_byte_size or a DW_AT_bit_size attribute whose integer constant
>         value (see Section 2.19) is the amount of storage needed to hold the
>         value of the data member.
>
> (2) David Blaikie maybe meant in the beginning new DW_AT_no_unique_address
>     but IIUC that may have been probably later deprecated as not really needed.
>
> (3) One could also omit such member completely but that would break template
>     metaprogramming based on such missing types from DWARF.
>
> I do not see many other options. That (1) would look like:
>
> ------------------------------------------------------------------------------
> DW_TAG_structure_type
>   DW_AT_calling_convention       (DW_CC_pass_by_value)
>   DW_AT_name     ("B")
>   DW_AT_byte_size        (0x01)
>
>   DW_TAG_member
>     DW_AT_name   ("a")
>     DW_AT_type   (0x00000065 "A")
> Bn:
> By: DW_AT_byte_size      (0x00)
>     DW_AT_data_member_location   (0x00)
>
>   NULL
> ------------------------------------------------------------------------------
>
>
> Jan
>
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