Re: [PATCH] Handle DW_AT_encoding on DW_TAG_enumeration_type

[Tom Tromey <tromey@adacore.com>]

>>>>> "Andrew" == Andrew Burgess <aburgess@redhat.com> writes:

>> First, this is yet another instance of a general problem with DWARF.
>> See https://sourceware.org/bugzilla/show_bug.cgi?id=32680 for the
>> analysis.

Andrew> Should there be a Bug: .... tag for this bug?

Yeah.  At first I wasn't sure but see below.

Andrew> I tried running your new test against GDB 17 and can confirm that it
Andrew> fails.  But I also tried against GDB 16, and the test passes, and the
Andrew> output looks fine.

Andrew> You commit message seemed (to me) to indicate that you didn't think this
Andrew> should have ever worked, but it looks like something changed 16 to 17
Andrew> that broke this.  It would be nice (maybe?) to understand what changed.

I looked into this and I introduced this difference in

    commit 5363deffcfb1c1825db3a8446d377a0ad56eff70
    Author: Tom Tromey <tromey@adacore.com>
    Date:   Thu Mar 20 09:52:08 2025 -0600

        Use correct sign extension for enumeration types

... without a test, which is disappointing.

Anyway, though, I still think that patch is probably correct, at least
as far as *this* test case is concerned.  Consider one of the
enumeration literals, dumped by eu-readelf to show the form:

 [  13b6]        enumerator           abbrev: 1
                 name                 (strp) "value_129"
                 const_value          (data1) 129

"data1" is inherently ambiguous, from DWARF:

    The data in DW_FORM_data1, DW_FORM_data2, DW_FORM_data4,
    DW_FORM_data8 and DW_FORM_data16 can be anything. Depending on
    context, it may be a signed integer, an unsigned integer, a
    floating-point constant, or anything else. A consumer must use
    context to know how to interpret the bits, which if they are target
    machine data (such as an integer or floating-point constant) will be
    in target machine byte order.

... which is the aforementioned bug.

Now, we could change update_enumeration_type_from_children to also
examine the attribute forms, with the idea that if we see DW_FORM_sdata
then it is definitely signed.  While that wouldn't help this particular
test, it's possible to construct one where it does.  In fact I happened
recently to be investigating this for gnat-llvm, where there's a type:

 [  10cf]      enumeration_type     abbrev: 21
               name                 (strp) "tag_t"
               byte_size            (data1) 1
               decl_file            (data1) p.adb (1)
               decl_line            (data1) 20
               alignment            (udata) 1
...
 [  10de]        enumerator           abbrev: 22
                 name                 (strp) "object"
                 const_value          (sdata) 18446744073709551615 (-1)

Here the enumeration type doesn't specify a sign, but there's a constant
that is clearly signed.  FWIW this occurs in array_of_variant.exp.

I'll address this in a separate patch.

Tom