[LLVMdev] [Debug Info PATCH] for support of ref_addr and removal of DIE duplication

[David Blaikie]

On Thu, Oct 17, 2013 at 11:11 AM, Manman Ren <manman.ren at gmail.com> wrote:

>
>
>
> On Wed, Oct 16, 2013 at 10:32 PM, David Blaikie <dblaikie at gmail.com>wrote:
>
>>
>>
>>
>> On Wed, Oct 16, 2013 at 9:10 PM, Manman Ren <manman.ren at gmail.com> wrote:
>>
>>>
>>>
>>>
>>> On Wed, Oct 16, 2013 at 1:58 PM, David Blaikie <dblaikie at gmail.com>wrote:
>>>
>>>>
>>>>
>>>>
>>>> On Wed, Oct 16, 2013 at 1:21 PM, Manman Ren <manman.ren at gmail.com>wrote:
>>>>
>>>>>
>>>>> There are a few places where we break the assumption:
>>>>> 1> formal_parameter constructed in DwarfDebug when adding attribute
>>>>> type
>>>>>      we call SPCU->addType(Arg, ATy), where Arg does not belong to
>>>>> SPCU.
>>>>> 2> inlined_subroutine constructed in DwarfDebug when adding attribute
>>>>> abstract_origin
>>>>>      The inlined_subroutine does not belong to the CU we call
>>>>> addDIEEntry on.
>>>>>      We create the children of inlined_subroutine and call addDIEEntry
>>>>> before we add it to an owner.
>>>>> 3> ...
>>>>>
>>>>> When building xalan with "lto -g", I saw 3158 violations of the
>>>>> assumption. Should we try to fix all these violations to make the
>>>>> assumption true?
>>>>>
>>>>
>>>> As you've said, this assumption (that DIEs are always constructed by
>>>> the CU that will own them) isn't necessary for this patch - it will be
>>>> necessary for cu-at-a-time emission. Perhaps we should discuss it in a
>>>> separate thread rather than trying to make a stronger assumption than is
>>>> needed for this patch.
>>>>
>>>> Of course without your patch the assumption is trivially correct (well,
>>>> given your point (1) above, perhaps it wasn't - maybe we do do things out
>>>> of order and from outside the CU) and we might want to say that it's so
>>>> important that your patch shouldn't violate it - but I'm not sure that's
>>>> necessary (Eric?). I haven't given it a great deal of thought.
>>>>
>>>
>>>
>>> Patches are reattached for convenience: remove DIE duplication with a
>>> worklist (name it patch_with), remove DIE duplication without a worklist
>>> but an assertion when we emit a ref4, we make sure the DIE and the
>>> referenced DIE belong to the same CU (name it patch_without).
>>>
>>
>> OK, we're going round in circles here and I'm not sure there are many
>> other ways I can communicate things.
>>
>>
>>> Without my patch, the assumption may be true, but it does not matter
>>> since we should always use ref4.
>>> I have provided some cases that the assertion fails with patch_without.
>>>
>>
>> Yes, because it's a narrow assertion based on an assumption I've
>> explained is not true (due to adding DIEs to types after-the-fact, due to
>> things like implicit special members, member templates, and nested types).
>>
>> The assertion you would need to check that a worklist is not required
>> would be a mapping listing the "assumptions" (ie: we found this parentless
>> DIE and assumed it was in the same DIE as this other DIE we're building)
>> and then, whenever a DIE is added to a parent that chains up to a CU we
>> would have to check that mapping to see if the assumption turned out to be
>> true.
>>
>> I don't mind if you implement this or not.
>>
>> What I do mind and will not accept is committing the worklist version of
>> this patch without justification, and by that I mean a demonstrated example
>> where it is needed, not just where it causes an overly-narrow assertion to
>> fail, but where it causes us to produce the wrong DWARF. If you have such
>> an example, let's look at that and figure out how to fix that - it might
>> (probably isn't) not be the right answer to use a worklist, but simply to
>> rework the order of construction/registration/parenting.
>>
>
> Let's agree on what assumptions you are trying to make:
> I have stated a few times: inside addDIEEntry, if a DIE does not have a
> parent, it should belong to "this" CU
> --> that is the assumption I made in the patch and I thought that you
> wanted to verify against
> Now you are talking about a different assumption: "we found this
> parentless DIE and assumed it was in the same DIE as this other DIE we're
> building", please be specific on where we should update the mapping list
>

I'm suggesting that we'd update the assumption mapping, if we wanted to add
assertions for this, at the time we rely on the assumption - ie: when we
choose the form to use.


> and what you mean by "this other DIE".
>

"this other DIE we're building" would be "Die" in the addDIEEntry function
- the one we're building, the one we're adding the attribute to.


>
> With my understanding of the assumption, I implemented "patch_without",
> where
>  +void CompileUnit::addDIEEntry(DIE *Die, uint16_t Attribute, DIEEntry
> *Entry) {
> +  DIE *DieCU = Die->getCompileUnitOrNull();
> +  DIE *EntryCU = Entry->getEntry()->getCompileUnitOrNull();
> +  if (!DieCU)
> +    // We assume that Die belongs to this CU, if it is not linked to any
> CU yet.
> +    DieCU = getCUDie();
>
+  if (!EntryCU)
> +    EntryCU = getCUDie();
> +  Die->addValue(Attribute, EntryCU == DieCU ? dwarf::DW_FORM_ref4 :
> +                                              dwarf::DW_FORM_ref_addr,
> Entry);
>  }
>
> If we made the correct assumptions inside addDIEEntry, we would make the
> correct decision between ref4 and ref_addr, so when we emitting a ref4, the
> two DIEs must belong to the same CU.
> If when emitting a ref4, the two DIEs do not belong to the same CU, it
> means we made the wrong assumptions inside addDIEEntry.
> If when emitting a ref4, the two DIEs do not belong to the same CU, that
> means we are generating wrong DWARF. Why is it an overly-narrow assertion?
>

Perhaps I'm not understanding the programmatic assertion you actually
wrote. Did you write a programmatic assertion (if so, what assertion did
you write?), or are you saying you examined the DWARF output and observed
that it was wrong?


> Since you are the one to ask for testing cases against some assumptions,
> please be specific on how you want addDIEEntry to look like without a
> worklist.
>

Perhaps we could approach this in a simpler way.

Start with the code you wrote, remove the worklist, choose an assumption
(such as the one in your code snippet here /\) then show me a test case
that produces incorrect DWARF. Then we'll talk about that concrete
situation (not an assertion failure, but actual incorrect DWARF) and see if
we should use a different assumption/model - try that, see if we have bogus
DWARF, look at all the test cases together and decide the right design as
we go.

It'll be much easier to discuss with concrete examples.

- David
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