[llvm] r191792 - Debug Info: remove duplication of DIEs when a DIE is part of the type system
David Blaikie
dblaikie at gmail.com
Thu Oct 3 13:48:15 PDT 2013
On Thu, Oct 3, 2013 at 1:33 PM, Manman Ren <manman.ren at gmail.com> wrote:
>
>
>
> On Thu, Oct 3, 2013 at 11:38 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>>
>> On Thu, Oct 3, 2013 at 11:26 AM, Manman Ren <manman.ren at gmail.com> wrote:
>>
>>>
>>>
>>>
>>> On Wed, Oct 2, 2013 at 5:55 PM, Eric Christopher <echristo at gmail.com>wrote:
>>>
>>>> >
>>>> >>
>>>> >> I would like to revisit this (maybe revert this) later on when type
>>>> units
>>>> >> are working.
>>>> >
>>>> >
>>>> > I'd rather revert this before type units (& then revisit this
>>>> afterwards) as
>>>> > it may complicate the type unit work. I think the extra benefit (using
>>>> > direct ref_addr, rather than signatures) of this approach over type
>>>> units
>>>> > may be implementable via an incremental improvement to type units.
>>>> >
>>>>
>>>> FWIW I've been following along with the thread and agree here. David
>>>> is actively working on type units and we should have something here
>>>> shortly. Let's let that work proceed and revert this for now.
>>>>
>>>
>>> I think we do agree that this approach has some extra benefit (using
>>> direct ref_addr, rather than signatures) over type units.
>>> The type-unit work can achieve what this approach does in removing DIE
>>> duplication, but using signatures rather than ref_addr.
>>> It may be possible to get the extra benefit via an incremental
>>> improvement to type units, but we are not certain about that.
>>>
>>
>> Agreed.
>>
>>
>>>
>>> I also want to emphasize this patch after wrapping up CU::getDIE as
>>> David sugggested is not big, the list of changes can be found at the end of
>>> this email.
>>>
>>
>> I don't quite understand this. The list of changes looks incomplete (the
>> insert*DIE functions in DwarfDebug and the maps they use, etc, are not
>> shown here) - are you describing this as "changes" as distinct from
>> "additions"? Even adding completely new code comes at a maintenance burden
>> that should be weighed against the value added.
>>
>
> Oops, I forgot to add the change to the header file: a single map added
> to DwarfDebug and the corresponding access functions. (5 lines of code)
>
>
>> & this doesn't look like how I would expect the code to look after my
>> suggestion (for one thing, I think I was suggesting having one map in
>> DwarfDebug, rather than one per kind of thing - and simply having getDIE
>> (and insertDIE, for that matter) delegate based on a list of known tag
>> types that should be cross-CU versus CU-local)
>>
>
> Yes, that is what I am going to implement, the list I gave earlier is
> based on the current design, but the number of changes will be the same.
> Change to header file: 5 lines of code
> Change to CU::getDIE to delegate tags to DwarfDebug::getDIE: 5 lines of
> code
> The changes list in my earlier message: 20 lines of code
> I guess for type units, we need to wrap up addDIEEntry to use
> ref_signature, so part of the change can be reused by type units.
>
I think it might be easier to discuss this in terms of reverting your
current patch and then looking at the complete new patch we're
discussing/proposing.
>
>
>>
>>
>>> I don't quite get the complication caused by this approach to type
>>> units. When we start constructing a DIE for a type MDNode, we either use
>>> the type unit or we use
>>> the standard approach of creating a DIE, my changes touch the standard
>>> approach a little by moving the map from CU to DwarfDebug.
>>>
>>
>> Yep, it might come out this cleanly, though I'm a little cautious about
>> putting all these things in together then trying to clean it up/evaluate
>> the changes appropriately.
>>
>>
>>> When referring to the type, we either use ref_signature or ref_addr or
>>> ref4.
>>>
>>> Even after type units start working, people can still choose to use
>>> either type units or ref_addr|ref4 for a period of time before their tool
>>> chains are updated to support type units.
>>> As for the tool chains, extra time may be required to set up a map from
>>> the type signature to the actual address in the Dwarf.
>>>
>>
>> Certainly - as I said, I don't doubt your change has some value over type
>> units. There's a question as to how much value and how best to fit it in,
>> if we do, to the existing code.
>>
>>
>>> The performance improvement of this patch is kind of straight-forward,
>>> if we have X CUs refer to the same type MDNode, X copies of type DIEs will
>>> be created
>>> without this patch.
>>>
>>
>> Yes, I agree that it has value.
>>
>>
>>> I do have some numbers collected before, I will dig that out.
>>>
>>
>> Thanks, that'd be very helpful.
>>
> 3GB memory reduction for xalan built with lto -g (without removing DIE
> duplication, the memory usage is 7GB)
>
OK, so you're looking at memory usage during LTO, not the size of the
resulting debug info? (that's fine, just trying to understand exactly what
metrics you're targeting)
& this is peak total memory usage (how/what exactly are you measuring?)
during LTO?
>
>
>>
>> (but I'm also particularly interested in how much value this has over
>> type units - which is one reason I'd like to do this after type units,
>> either that or we'll need to add flags or somesuch to disable this
>> optimization so we can compare type units with and without this change)
>>
> The value will be the advantage of ref_addr over ref_signature after type
> units are fully working.
>
Yes, I understand the mechanical benefit, when I speak of this I mean
looking at the actual numbers. The % benefit in whatever metrics we care
about.
> If I revert this change, we have a choice of modifying the standard route
> of ref4 to support ref_addr, as it is currently implemented;
>
I don't quite understand what you're saying here.
> or implementing this on top of type units (but we are not certain that we
> can implement this via an incremental improvement to type units, and what
> benefit did we gain compared
> to modifying the standard route).
>
I'm not necessarily suggesting that this feature would be implemented in a
highly interrelated way with type units, but that seeing how it layers on
top of type units may be useful.
> But it provides us huge memory reduction before type units are full
> working and tool chains are updated.
>
Tool chain support is a fair point, if lldb doesn't and won't support type
units any time soon, then the value of this feature over today's behavior
(or tomorrow's behavior with type units disabled, etc) is a relevant metric
to consider.
I'm still inclined to prefer backing this patch out and discussing the
right design rather than trying to coerce this into the right design from
where it is now. It'll make the patch history and impact clearer.
- David
>
> Manman
>
>
>>
>> The other problem is that removing these patches will be somewhat more
>> difficult after I've spent the next couple of weeks cleaning code up and
>> adding in type units. I think there's enough design discussion to have with
>> regards to how best to implement your feature that I'd rather have that
>> design discussion on top of type units rather than the other way around -
>> though it's possible we can come to a reasonable/good design in either
>> order.
>>
>>
>>>
>>> Thanks,
>>> Manman
>>>
>>> 15 - insertDIE(Ty, TyDIE);
>>> 16 + DD->insertTypeDIE(Ty, TyDIE);
>>>
>>> 31 - insertDIE(SP, SPDie);
>>> 32 + DD->insertSPDIE(SP, SPDie);
>>>
>>> 40 - insertDIE(DT, StaticMemberDIE);
>>> 41 + DD->insertStaticMemberDIE(DT, StaticMemberDIE);
>>>
>>> 52 + // Process the worklist to add attributes with the correct form
>>> (ref_addr or
>>> 53 + // ref4).
>>> 54 + for (unsigned I = 0, E = DIEEntryWorklist.size(); I < E; I++) {
>>> 55 + addDIEEntry(DIEEntryWorklist[I].Die,
>>> DIEEntryWorklist[I].Attribute,
>>> 56 + dwarf::DW_FORM_ref4, DIEEntryWorklist[I].Entry);
>>> 57 + assert(E == DIEEntryWorklist.size() &&
>>> 58 + "We should not add to the worklist during
>>> finalization.");
>>> 59 + }
>>> 60 +
>>>
>>> 68 +
>>> 69 +/// When we don't know whether the correct form is ref4 or
>>> ref_addr, we create
>>> 70 +/// a worklist item and insert it to DIEEntryWorklist.
>>> 71 +void DwarfDebug::addDIEEntry(DIE *Die, uint16_t Attribute, uint16_t
>>> Form,
>>> 72 + DIEEntry *Entry) {
>>> 73 + /// Early exit when we only have a single CU.
>>> 74 + if (GlobalCUIndexCount == 1 || Form != dwarf::DW_FORM_ref4) {
>>> 75 + Die->addValue(Attribute, Form, Entry);
>>> 76 + return;
>>> 77 + }
>>> 78 + DIE *DieCU = Die->checkCompileUnit();
>>> 79 + DIE *EntryCU = Entry->getEntry()->checkCompileUnit();
>>> 80 + if (!DieCU || !EntryCU) {
>>> 81 + // Die or Entry is not added to an owner yet.
>>> 82 + insertDIEEntryWorklist(Die, Attribute, Entry);
>>> 83 + return;
>>> 84 + }
>>> 85 + Die->addValue(Attribute,
>>> 86 + EntryCU == DieCU ? dwarf::DW_FORM_ref4 :
>>> dwarf::DW_FORM_ref_addr,
>>> 87 + Entry);
>>> 88 +}
>>>
>>>
>>>
>>>>
>>>> (and no, computing the hash isn't enough to matter time wise on
>>>> anything)
>>>>
>>>> -eric
>>>>
>>>
>>>
>>
>
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