[Bug default/30260] New: add support for pointer-to-member types

[Bug default/30260] New: add support for pointer-to-member types

[gprocida at google dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

            Bug ID: 30260
           Summary: add support for pointer-to-member types
           Product: libabigail
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: default
          Assignee: dodji at redhat dot com
          Reporter: gprocida at google dot com
                CC: libabigail at sourceware dot org
  Target Milestone: ---

libabigail currently lacks support for pointer-to-member types.

These are very simple to model but there may be complexities around building
type names and referring to their components (as no standardised naming seems
to exist).

Useful references:

* https://eel.is/c++draft/expr.mptr.oper
* https://eel.is/c++draft/dcl.mptr - "there is no 'reference-to-member' type"
* https://en.cppreference.com/w/cpp/language/pointer#Pointers_to_members

Test case:

struct X {
  void f(int);
  int a;
};
struct Y;

int X::* pmi = &X::a;
void (X::* pmf)(int) = &X::f;
double X::* pmd;
char Y::* pmc;

typedef struct { int t; } T;
auto pmy = &T::t;

namespace {
struct Z { int z; };
}
int Z::* pmz = &Z::z;

union U { int u; };
auto pmu = &U::u;

typedef const U CU;
auto pmcu = &CU::u;

Model:

* no intrinsic name
* containing type (in practice: can be class/struct, union or typedef; after
typedef resolution can be qualified or anonymous)
* (member) type

Naming:

$ c++decl declare x as pointer to member of class NAME function returning int  
int (NAME::*x)()
$ c++decl declare x as array 7 of pointer to member of class NAME int
int NAME::*x[7]

But this doesn't account for the more complex cases for NAME (see the test case
above).

Component naming (for diff reporting):

¯\_(ツ)_/¯

Concrete representation in XML:

¯\_(ツ)_/¯

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[Bug default/30260] add support for pointer-to-member types

[gprocida at google dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

--- Comment #1 from gprocida at google dot com ---
$ c++decl declare x as pointer to member of class NAME array 7 of int
int (NAME::*x)[7]

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[Bug default/30260] add support for pointer-to-member types

[gprocida at google dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

--- Comment #2 from gprocida at google dot com ---
Naming of these types appears to follow all the same rules as naming normal
pointer types, including when parentheses are needed.

pointer to: ... * ...

becomes

pointer to X member: ... X::* ...

Extra complications are:

1. the syntax immediately to the left of the * or X::* may need to be separated
by a space

int*x; // no space needed before *
int X::*x; // space is needed before X
int (X::*x)(); // space would look odd inside the bracket

2. this is normally written as just the tag, without struct / class / union

but you might want to include the keyword for clarity or for simplicity of
coding

int struct X::*x;

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[Bug default/30260] add support for pointer-to-member types

[dodji at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

dodji at redhat dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-10-24
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1

--- Comment #3 from dodji at redhat dot com ---
Indeed, thank you for filing such a detailed enhancement request.  Libabigail
needs to support this.

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[Bug default/30260] add support for pointer-to-member types

[woodard at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

--- Comment #4 from Ben Woodard <woodard at redhat dot com> ---
Please note that pointer to member happens to be one of the challenging areas
in DWARF5, I think it is fine in the underlying type system but if the pointer
to member variable is passed as a parameter to a function and is in a register,
then I believe that the DWARF to represent that is currently undefined.

DWARF6 is supposed to resolve that issue.

https://github.com/ccoutant/dwarf-locations/blob/f74b6d8b95ad5cfea80c4493f11e7dae441f02eb/location-description-on-the-dwarf-stack-concept-proposal.txt#L252

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[Bug default/30260] add support for pointer-to-member types

[dodji at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

--- Comment #5 from dodji at redhat dot com ---
I have started to add patches to support the pointer-to-member feature in the
branch
https://sourceware.org/git/?p=libabigail.git;a=shortlog;h=refs/heads/users/dodji/PR30260.

The code can be tested at this point, FWIW.

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[Bug default/30260] add support for pointer-to-member types

[gprocida at google dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

--- Comment #6 from gprocida at google dot com ---
FWIW, STG diff output looks like this:

function symbol 'int struct S::* s2()' {_Z2s2v} was added

function symbol 'int s10(int struct S::*)' {_Z3s10M1Si} was added

function symbol 'void pmz_fun()' {_Z7pmz_funv} was added

variable symbol 'char struct Y::* pmc' was added

variable symbol 'int union U::* pmcu' was added

variable symbol 'double struct X::* pmd' was added

variable symbol 'void(struct X::* pmf)(struct X*, int)' was added

variable symbol 'int struct X::* pmi' was added

variable symbol 'int union U::* pmu' was added

variable symbol 'int struct { int t; }::* pmy' was added

variable symbol 'int struct S::* s0' was added

variable symbol 'int struct S::** s1' was added

variable symbol 'int struct S::*(* s3)()' was added

variable symbol 'int struct S::* s4[7]' was added

variable symbol 'int* struct S::* s5' was added

variable symbol 'int(* struct S::* s6)()' was added

variable symbol 'int(struct S::* s7)(struct S*)' was added

variable symbol 'int(struct S::* s8)[7]' was added

variable symbol 'const int struct S::* volatile s9' was added

variable symbol changed from 'char struct A::* diff' to 'int struct B::* diff'
  type changed from 'char struct A::*' to 'int struct B::*'
    containing type changed from 'struct A' to 'struct B'
    type changed from 'char' to 'int'

If you have any better ideas than "containing type" and "type", I might steal
them. :-)

"pointed-to-member type"???

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[Bug default/30260] add support for pointer-to-member types

[dodji at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

--- Comment #7 from dodji at redhat dot com ---
Thanks.

The output of abidiff looks like this:

Functions changes summary: 0 Removed, 0 Changed, 0 Added function
Variables changes summary: 0 Removed, 1 Changed, 0 Added variable

1 Changed variable:

  [C] 'int S::* pm' was changed to 'char S::* pm' at
test-ptr-to-mbr1-v1.cc:6:1:
    type of variable changed:
      pointer-to-member type changed from: 'int S::* to: 'char S::*'
      in data member type 'int' of pointed-to-member type 'int S::*':
        type name changed from 'int' to 'char'
        type size changed from 32 to 8 (in bits)
      in containing type 'struct S' of pointed-to-member type 'int S::*' at
test-ptr-to-mbr1-v1.cc:1:1:
        type size changed from 32 to 8 (in bits)
        1 data member change:
          type of 'int m' changed:
            type name changed from 'int' to 'char'
            type size changed from 32 to 8 (in bits)

The output of the relevant tests are available at
https://sourceware.org/git/?p=libabigail.git;a=tree;f=tests/data/test-abidiff-exit/pointer-to-member;h=61ced53e13335af7b31962b8a3736774be5b68db;hb=HEAD.

And one such output is:

https://sourceware.org/git/?p=libabigail.git;a=blob;f=tests/data/test-abidiff-exit/pointer-to-member/test-ptr-to-mbr1-output-1.txt;h=a88d96fbec1d301d52e57f5f6a6e233d97de62b1;hb=HEAD

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[Bug default/30260] add support for pointer-to-member types

[dodji at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30260

dodji at redhat dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #8 from dodji at redhat dot com ---
This feature should now be implemented by commit
https://sourceware.org/git/?p=libabigail.git;a=commit;h=d4ca1088e4cdc4b3e4c3e67284458911c24a7c59
and should be available in libabigail 2.5.

Thank you for filing this enhancement request.

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