[Bug debug/115272] New: [debug] complex type is hard to related back to base type

["vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115272

            Bug ID: 115272
           Summary: [debug] complex type is hard to related back to base
                    type
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider test.c:
...
$ cat test.c
__complex__ float cf;
__complex__ double cd;
__complex__ long double cld;
...
compiled using an arm-linux-gnueabihf target compiler:
...
$ gcc test.c -g -c
...

The three types are represented like this:
...
 <1><2e>: Abbrev Number: 2 (DW_TAG_base_type)
    <2f>   DW_AT_byte_size   : 8
    <30>   DW_AT_encoding    : 3        (complex float)
    <30>   DW_AT_name        : complex float
 <1><44>: Abbrev Number: 2 (DW_TAG_base_type)
    <45>   DW_AT_byte_size   : 16
    <46>   DW_AT_encoding    : 3        (complex float)
    <46>   DW_AT_name        : complex double
 <1><5b>: Abbrev Number: 2 (DW_TAG_base_type)
    <5c>   DW_AT_byte_size   : 16
    <5d>   DW_AT_encoding    : 3        (complex float)
    <5d>   DW_AT_name        : (indirect string, offset: 0): complex long
double
...

Note that the sizes of complex double and complex long double are the same.

When printing the type of the real component in gdb, we get:
...
$ gdb -q -batch test.o \
    -ex 'p $_creal(cf)' -ex 'ptype $' \
    -ex 'p $_creal(cd)' -ex 'ptype $' \
    -ex 'p $_creal(cld)' -ex 'ptype $'
$1 = 0
type = float
$2 = 0
type = double
$3 = 0
type = double
...

We'd prefer to get "long double" for the real component of complex long double.

I submitted a fix for gdb that achieves this here (
https://sourceware.org/pipermail/gdb-patches/2024-May/209415.html ).  The fix
involves grepping for "long" in the name of the complex type, which is hacky
and fragile, but it's the best we can do with the current state of debug info.

If gcc would emit a DW_AT_type pointing to the component type, this could be
solved cleanly and unambiguously. [ Note: this was also discussed as a solution
to PR93988, but a different solution was chosen there (
https://dwarfstd.org/issues/210218.2.html ). ]

As things stand, a DW_TAG_base_type is not allowed to have a DW_AT_type, but we
could add this as an extension.