[Bug symtab/30829] [gdb/symtab] Review write_gdbindex for dwz

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

https://sourceware.org/bugzilla/show_bug.cgi?id=30829

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #0)
> We do this:
> ...
>       const auto insertpair = cu_index_htab.emplace (per_cu, counter);
> ...
> which looks incorrect for per_cu->is_dwz.

This is actually used.

Consider test-case gdb.dwarf2/dwzbuildid.exp.

When running the test-case using target board cc-with-gdb-index, it passes
(well for the one executable that it manages to build), but gdb quietly ignores
the .gdb_index section, because the .gnu_debugaltlink file doesn't have a
.gdb_index section.

The target board uses gdb-add-index, which currently doesn't support this.

So I wrote a new target board cc-with-index-cache, because the index-cache
already does support this.

To my surprise, the test-case still passes.

There seems to be some undocumented behaviour going on (at least, I don't see
any explicit mention of the following here:
https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html#Index-Section-Format
).

Indeed each .gdb_index section has a CU/TU table, and the section for the
.gnu_debugaltlink file contains nothing else.

I think the assumption was that a .gnu_debugaltlink contains only PUs, and that
the toplevel symbol table only contains references to CUs/TUs, which will be
pointing to the toplevel CU/TU table.

However, in this test-case the .gnu_debugaltlink file contains a CU.

And the reference in the toplevel symbol table point towards it, in the sense
that the virtual toplevel CU/TU table (otherwise known as all_units) first
contains the toplevel CU/PUs, then toplevel TUs, then .gnu_debugaltlink CU/PUs,
then .gnu_debugaltlink TUs.

Readelf doesn't understand this, and consequently shows:
...
[512] the_int: T1 [static, variable]
...
But there are no type units, and this could be printed as 'L1', meaning the
first item in the CU list of the .gdb_index section in the .gnu_debugaltlink
file.

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