llvm dev - Jan 2019 - Potential DWARF debug info bug: DW_TAG_label DIE has a DW_AT_prototyped attribute

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.

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().

Thoughts?

Todd Snider

Compiler Group
Texas Instruments Incorporated
Sugar Land, TX
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> 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.
>  
> 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

> -----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.
--paulr
> >
> > 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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