[llvm-dev] Potential DWARF debug info bug: DW_TAG_label DIE has a DW_AT_prototyped attribute

via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 18 14:35:07 PST 2019

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Adrian Prantl via llvm-dev
> Sent: Friday, January 18, 2019 5:20 PM
> To: Snider, Todd
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] Potential DWARF debug info bug: DW_TAG_label DIE
> has a DW_AT_prototyped attribute
> > On Jan 18, 2019, at 1:06 PM, Snider, Todd via llvm-dev <llvm-
> dev at lists.llvm.org> wrote:
> >
> >
> > In llvm/lib/MC/MCDwarf.cpp’s EmitGenDwarfAbbrev(), the DWARF
> abbreviation declaration for a DW_TAG_label includes a DW_AT_prototyped
> attribute, but the DWARF 4 specification indicates that the
> DW_AT_prototyped attribute is not a valid attribute for the DW_TAG_label
> debug information entry (DIE). In this case, EmitGenDwarfAbbrev() is
> clearly in violation of the DWARF 4 specification.

DWARF is a "permissive" standard which means it's not illegal to do this,
so it might be instructive to look at the change history to see if the
commit(s) that introduced it have any explanation.

Offhand, though, I agree it appears to be a complete waste of space and
worth taking out.

> >
> > However, I notice that one of the MachO lit tests is expecting the
> DW_TAG_label DIE to contain a DW_AT_prototyped attribute whose value is
> false (llvm/test/MC/MachO/gen-dwarf.s).
> >
> > Does the DW_AT_prototyped attribute sever any purpose or provide any
> value in the context of a DW_TAG_label DIE?
> >
> > If not, I propose to remove the call to EmitAbbrev() on behalf of the
> DW_AT_prototyped attribute for a DW_TAG_label in EmitGenDwarfAbbrev().
> I don't think that any Mach-O DWARF consumer depends on the presence of
> DW_TAG_prototyped inside of a DW_TAG_label. The only occurrence of
> DW_TAG_prototyped inside of LLDB for example is in a switch statement of
> attributes that are ignored...
> Feel free to add me (and the "debug-info" group) as a reviewer for the
> patch in phabricator.
> thanks,
> adrian
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