[LLVMdev] LTO type uniquing: ODR assertion failure

Manman Ren
Mon, 21 Jul 2014 22:35:37 -0000

On Mon, Jul 21, 2014 at 1:14 PM, David Blaikie <dblaikie at gmail.com>
wrote:

> On Mon, Jul 21, 2014 at 10:39 AM, Manman Ren <manman.ren at
gmail.com> wrote:
> >
> >
> >
> > On Mon, Jul 14, 2014 at 11:32 AM, Manman Ren <manman.ren at
gmail.com>
> wrote:
> >>
> >>
> >> We still have access to types via MDNodes directly and the
assertion
> that
> >> assumes all accesses to DITypes are accessing the resolved DIType
will
> fire
> >>
> >> i.e assert(Ty == resolve(Ty.getRef()))
> >>
> >> One example is the access to DIType via DIArray in SubroutineType.
If
> all
> >> elements in the type array are DITypes we can create a DITypeArray
and
> use
> >> that for SubroutineType's type array instead. But we currently
have
> >> unspecified parameter in the type array and it is not a DIType.
> >
> >
> > I am going to work on a patch that adds DITypeArray (each element will
be
> > DITypeRef, SubroutineType's type array will be DITypeArray) and
adds
> > DITrivialType that extends from DIType (unspecified parameter will be
> > DITrivialType).
> > If you  have opinions against it, please let me know,
>
> We haven't bothered using typed arrays in DebugInfo yet (as you say,
> we just have DIArray) so I have two thoughts
>
> 1) why does this one case need fixing/changing? Is it because we have
> things that aren't DIDescriptors inside the DIArray? (the strings that
> refer to types). Given how loosely typed DIDescriptor is (it doesn't
> check that it's a valid DIDescriptor) I assume this doesn't
actually
> cause a problem, though it's certainly not nice. So we could just
> leave it as-is, pass DIArray's element to "resolve" (it'd
implicitly
> convert the DIDescriptor back to a raw MDNode*), then perhaps we'd
> need to make DITypeRef's ctor public so it could be used here. Not
> suggesting that's ideal, though.
>

I should have provided an example to help understanding the issue :)

When processing the following type node, we throw an assertion failure
assert(Ty == resolve(Ty.getRef()))
!{i32 786436, metadata <badref>, null, metadata
!"SpuPacketType", i32 102,
i64 32, i64 32, i32 0, i32 0, null, metadata <badref>, i32 0, null, null,
metadata !"_ZTS13SpuPacketType"} ; [ DW_TAG_enumeration_type ]
[SpuPacketType] [line 102, size 32, align 32, offset 0] [def] [from ]

There are two type nodes with the same identifier:
!473 = metadata !{i32 786436, metadata !474, null, metadata
!"SpuPacketType", i32 102, i64 32, i64 32, i32 0, i32 0, null,
metadata
!475, i32 0, null, null, metadata !"_ZTS13SpuPacketType"} ; [
DW_TAG_enumeration_type ] [SpuPacketType] [line 102, size 32, align 32,
offset 0] [def] [from ]
!6695 = metadata !{i32 786436, metadata !6696, null, metadata
!"SpuPacketType", i32 102, i64 32, i64 32, i32 0, i32 0, null,
metadata
!475, i32 0, null, null, metadata !"_ZTS13SpuPacketType"} ; [
DW_TAG_enumeration_type ] [SpuPacketType] [line 102, size 32, align 32,
offset 0] [def] [from ]

The only difference between these two is the file node
!474 = metadata !{metadata
!"/Users/manmanren/swb/rdar_17628609/AppleSPUFirmware-71/SPU/SPUPacket.h",
metadata !"/Users/manmanren/swb/rdar_17628609/AppleSPUFirmware-71"}
!6696 = metadata !{metadata
!"/Users/manmanren/swb/rdar_17628609/AppleSPUFirmware-71/SPU/../SPU/SPUPacket.h",
metadata !"/Users/manmanren/swb/rdar_17628609/AppleSPUFirmware-71"}

We have direct access to 473 via 580's type array.
!580 = metadata !{i32 786453, i32 0, null, metadata !"", i32 0, i64 0,
i64
0, i64 0, i32 0, null, metadata !581, i32 0, null, null, null} ; [
DW_TAG_subroutine_type ] [line 0, size 0, align 0, offset 0] [from ]
!581 = metadata !{metadata !124, metadata !575, metadata !473, metadata
!582, metadata !212, metadata !128, metadata !584}

MDNode 473 will be resolved to MDNode 6695 and the assertion "assert(Ty
=resolve(Ty.getRef()))" will fire.

-------------------------------------------------
To fix the problem, we need to remove the direct access to MDNode 473 by
replacing MDNode 581 from
metadata !{metadata !124, metadata !575, metadata !473, metadata !582,
metadata !212, metadata !128, metadata !584}
to
metadata !{metadata !124, metadata !575, metadata
!"_ZTS13SpuPacketType",
metadata !582, metadata !212, metadata !128, metadata !584}

And treat the field {metadata !"_ZTS13SpuPacketType"} as DITypeRef.

-------------------------------------------------
If we have DIDescriptorRef and all elements currently inside DIArray are
DIDescirptors, we can make DIArray an array of DIDescriptorRef.
I don't think it is a good idea to add DIDescriptorRef (it makes our types
loose) and am not sure about the 2nd condition.

So I proposed to add DITypeArray (or DITypedArray<DITypeRef> as David
suggested, where all elements are DITypeRef),
DICompositeType::getTypeArray() will return DITypeArray and
DITypeArray::getElement(unsigned) will return DITypeRef.

This is actually more complicated than I thought, not all DICompositeType's
getTypeArray() can return an array of DITypeRefs. For example,
getTypeArray() of ArrayType and VectorType can not return an array of
DITypeRefs.
We can fix that by extending DICompositeType to DISubroutineType and only
DISubroutineType::getTypeArray() will return DITypeArray.
Even for SubroutineType, elements of the type array can be unspecified
parameters which can't be DITypeRefs. That is why I was thinking about
making unspecified parameters trivial DITypes.

Thanks a lot,
Manman


>
> 2) If we're going to fix DIArray apparent type safety (it's not
safe -
> just convenient), perhaps we could just template it? (to avoid churn,
> we could leave DIArray as a typedef of DITypedArray<DIDescriptor> for
> example, and then have DITypedArray<DITypeRef> which is your
> DITypeArray (again, provided via typedef)). It's so small though, that
> I'm not too fussed if we write it out again as you've proposed.
>
> - David
>
> >
> > Thanks,
> > Manman
> >
> >>
> >> What are your thoughts? Suggestions are welcome.
> >>
> >> Is it a good idea to canonicalize file names (i.e dA/B.h should be
> >> equivalent to dA/../dA/B.h)? This will reduce the chance of having
two
> >> DITypes that should be equivalent with equivalent file names.
> >>
> >> Thanks,
> >> Manman
> >
> >
>
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