Hi, (using my subscribed gmail address until my main address, CCed, is approved to also join the list.)
On 2022-05-09 16:48, Ron Brender via Dwarf-Discuss wrote: > It seems to me that the problem here is not so much in the DWARF standard, as > in the DWARF that is produced. > > The DWARF representation generally serves to capture all the semantic > information needed to properly represent > the source program. In the example discussed here it appears that GCC does > that (by including the bit field information) and > CLANG does not (by erroneously omitting the bit field information). > > It is probably the case that in most languages char a; and char a:8; are > semantically equivalent IN ALL RESPECTS. > For such languages,the producer/compiler is probably justified (or at least > can be forgiven) for omitting the bit field information > for char a:8. But evidently for "AMDGP-Lang" the two are not the same--while > the storage layout may be the same, other > subtle but critical aspects of the language and its ABI are quite different > in the two cases. Agreed so far. > > One might have a philosophical debate about whether this difference is > appropriate or not. But as Lancelot says, the ABI is > what it is and any change is unlikely nearterm so such a debate is moot. The > only issue is whether DWARF is able to > convey the needed information. I think DWARF is able, if only the > producer/compiler would do it properly. > Agreed. > So my suggestion is to file a bug report with CLANG, requesting they correct > their DWARF output to reflect all details needed > by your language. An issue here is that DWARF does say this, in (DWARF 5, 5.7.6 Data Member Entries, page 119): "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 on page 55) is the amount of storage needed to hold the value of the data member." Note the part I underlined. In Lancelot's case, the size of the data member IS the same as the size of the type given for the data member. So Clang could well pedantically claim that they _are_ following the spec. Shouldn't the spec be clarified here? This then raises the question of whether a debugger can assume that the presence of a DW_AT_bit_size attribute indicates that the field is a bit field at the C/C++ source level. GDB is assuming that today, as there's really no other way to tell, but I don't think the spec explicitly says so. Pedro Alves _______________________________________________ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
