llvm dev - Nov 2018 - DebugInfo proposal: Emit an explicit empty address range on CUs with no code addresses

On Wed, Nov 28, 2018 at 9:47 AM Adrian Prantl <aprantl at apple.com>
wrote:
>
>
> > On Nov 28, 2018, at 9:40 AM, David Blaikie <dblaikie at
gmail.com> wrote:
> >
> > So I've been looking at a particular performance problem with
LLVM's
> symbolizer due to the use of ThinLTO, split DWARF, and split DWARF inlining
> info.
> >
> > This combination has a couple of problems:
> >
> > 1) it means multiple CUs in a single DWO, which isn't well
> defined/specified, and best avoided - so I'm working on fixing that
here
> (won't fix split DWARF+Full LTO) because we already don't use
cross-CU
> references in the split units (because there's no supported way to
express
> that in DWARF), so we clone/move any DIEs (like subprograms) referenced
> cross-CU into the CU that references them (eg: cross-CU inlining places the
> abstract subprogram definition for the inlined subroutine into the CU that
> has the inlining - rather than cross-CU referencing into the other CU)) -
> and in ThinLTO the only reason other units exist is to cross-CU
> optimize/inline, no code for imported CUs is ever emitted (except where
> it's been inlined) - so a ThinLTO compile has one primary unit, and
some
> other units it inlines from - so those other units never emit anything in
> the split unit, just a few DIEs in the skeleton unit if you're using
split
> DWARF inlining (or no unit at all if you aren't using that feature) -
so
> I'm working on making it so those units are non-split (rather than
having a
> degenerate/empty split unit)
> >
> > 2) symbolizer performance is hurt because whenever it sees a unit
> without ranges at the unit DIE, it assumes the producer just skipped those
> - and goes searching through the implementation DIEs (which may mean going
> over to the .dwo, or loading a whole .dwp) to see where their addresses
are.
> >
> > It's this second step that's a bit painfully unnecessary,
especially for
> a large DWP on a remote filesystem, etc.
> >
> > So, anyone have opinions on whether we should
> >
> > a) decide that a unit without ranges covers no ranges - and don't
do the
> search
>
> Are there compilers that do this ("forget" to emit ranges) that
we care to
> support with llvm-symbolizer?
>
I'm not specifically aware of any, though haven't gone looking.

>
> -- adrian
>
> >
> > b) emit zero-length ranges on any unit that has no code ranges
(low/high
> pc zero? Could pick anything, but that seems the most obvious)
> >
> > Thanks,
> > - Dave
>
>
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> On Nov 28, 2018, at 9:50 AM, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> 
> > a) decide that a unit without ranges covers no ranges - and don't
do the search
> 
> Are there compilers that do this ("forget" to emit ranges) that
we care to support with llvm-symbolizer?
> 
> I'm not specifically aware of any, though haven't gone looking.
Just in case this wasn't obvious in the sub-text:
I think we should figure out whether this assumption in llvm-symbolizer is
actually needed to support a compiler we care about and then potentially remove
it, or enforce it only when the CU is < DWARF 5 or something like that.

-- adrian
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Yeah, fair - I'll give it a week or something, see if Paul or anyone else
has ideas about why the existing behavior might be useful before I remove
it.

On Wed, Nov 28, 2018 at 10:27 AM Adrian Prantl <aprantl at apple.com>
wrote:
>
> On Nov 28, 2018, at 9:50 AM, David Blaikie <dblaikie at gmail.com>
wrote:
>
>
> > a) decide that a unit without ranges covers no ranges - and don't
do the
>> search
>>
>> Are there compilers that do this ("forget" to emit ranges)
that we care
>> to support with llvm-symbolizer?
>>
>
> I'm not specifically aware of any, though haven't gone looking.
>
>
> Just in case this wasn't obvious in the sub-text:
> I think we should figure out whether this assumption in llvm-symbolizer is
> actually needed to support a compiler we care about and then potentially
> remove it, or enforce it only when the CU is < DWARF 5 or something like
> that.
>
> -- adrian
>
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