[lldb-dev] Question on assert

[Tamas Berghammer via lldb-dev]

Hi Todd,

The 64 bit ID of a DIE is built up in the following way:
* The offset of the DIE is in the lower 32 bit
* If we are using SymbolFileDWARF then the higher 32 bit is the offset of
the compile unit this DIE belongs to
* If we are using SymbolFileDWARFDwo then the higher 32 bit is the offset
of the base compile unit in the parent SymbolFileDWARF
* If we are using SymbolFileDWARFDebugMap then the higher 32 bit is the ID
of the SymbolFileDWARF this DIE belongs to
* If the higher 32 bit is 0 then that means that the source of the DIE
isn't specified

The assert then tries to verify that one of the following conditions holds:
* The higher 32 bit of "id" is 0 what means that we don't have a symbol
file pointer (AFAIK shouldn't happen) or we are coming from a
SymbolFileDWARF
* The higher 32 bit of "cu_id" is 0 what means that the compile unit is at
0 offset what is the case for the single compile units in
SymbolFileDWARFDwo (and I think for SymbolFileDWARFDebugMap)
* The higher 32 bit of "id" (what is the ID of the SymbolFileDWARF we are
belonging to) matches with the higher 32 bit of "cu_id" (what is the offset
of the compile unit in the base object file)

After thinking a bit more about the assert I think the problem is that the
way I calculate cu_id is incompatible for the case when we are using
SymbolFileDWARFDebugMap.

I think changing line 188 to the following should fix the issue:
lldb::user_id_t cu_id = m_cu->GetID()&0xffffffff00000000ull;

Please give it a try on OSX and let me know if it helps. I tested it on
Linux and it isn't cause any regression there.

Thanks,
Tamas

On Wed, Oct 14, 2015 at 9:13 PM Todd Fiala <todd.fiala at gmail.com> wrote:

> Hi Tamas,
>
> There is an assert in DWARFDIE.cpp (lines 189 - 191) that we're hitting on
> the OS X side somewhat frequently nowadays:
>
>             assert ((id&0xffffffff00000000ull) == 0 ||
>
>                     (cu_id&0xffffffff00000000ll) == 0 ||
>
>                     (id&0xffffffff00000000ull) == (cu_id&
> 0xffffffff00000000ll));
>
>
> It does not seem to get hit consistently.  We're trying to tease apart
> what it is trying to do.  It's a bit strange since it is saying that the
> assert should not fire if any one of three clauses is true.  But it's hard
> to figure out what exactly is going on there.
>
>
> Can you elucidate what this is trying to do?  Thanks!
>
> --
> -Todd
>
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