[LLVMdev] metadata linking bug or by design

[Xi Wang]

Great!  Thanks!

I noticed this when I was creating my own metadata nodes, which
contained references to functions --- after linking some elements that
referred to functions magically became null.

- xi

On Thu, Aug 4, 2011 at 2:42 PM, Devang Patel <dpatel at apple.com> wrote:
>
> On Aug 4, 2011, at 11:30 AM, Xi Wang wrote:
>
>> I looked at an earlier version.  NamedMDNodes were linked after global
>> values.  Current trunk version links NamedMDNodes before that, though
>> the comment says otherwise: "We do this after linking GlobalValues so
>> that MDNodes that reference GlobalValues are properly remapped".  see
>> ModuleLinker::run in lib/Linker/LinkModules.cpp.
>>
>> If I move the call to linkNamedMDNodes in ModuleLinker::run to the
>> last step (just before return false), everything looks good.  Not sure
>> if linkNamedMDNodes must be called earlier.
>
> I made  that change in response to Chris's change ValueMapper. Metadata mapping is being investigated as part of PR 10553 anyway. So Ill fix this particular regression you have found.
> Thanks!
> -
> Devang
>
>>
>> - xi
>>
>> On Thu, Aug 4, 2011 at 1:22 PM, Devang Patel <dpatel at apple.com> wrote:
>>>
>>> On Aug 4, 2011, at 8:21 AM, Xi Wang wrote:
>>>
>>>> Hi,
>>>>
>>>> It seems that current linker "drops" updated llvm::Value * references
>>>> in metadata.  Here goes an example.  We have a two-line source file
>>>> a.c:
>>>>
>>>> static int x;
>>>> void foo() { x = 1; }
>>>>
>>>> and a similar b.c:
>>>>
>>>> static int x;
>>>> void bar() { x = 1; }
>>>>
>>>> They both have an internal variable x; one of them will be renamed (to
>>>> x1) during linking.
>>>>
>>>> clang -emit-llvm -c -g a.c
>>>> clang -emit-llvm -c -g b.c
>>>> llvm-link -o t.bc a.o b.o
>>>>
>>>> Before linking we can see the following lines in both bitcode files
>>>> from a.c and b.c, where the last element of !5 refers to the internal
>>>> variable x.
>>>>
>>>> !llvm.dbg.sp = !{!1}
>>>> !5 = metadata !{i32 655412, i32 0, metadata !0, metadata !"x",
>>>> metadata !"x", metadata !"", metadata !2, i32 1, metadata !6, i32 1,
>>>> i32 1, i32* @x} ; [ DW_TAG_variable ]
>>>>
>>>> However after linking the last element of !11 becomes null.  I was
>>>> expecting that it would refer to the renamed x1.
>>>>
>>>> !llvm.dbg.gv = !{!9, !11}
>>>> !9 = metadata !{i32 655412, i32 0, metadata !0, metadata !"x",
>>>> metadata !"x", metadata !"", metadata !3, i32 1, metadata !10, i32 1,
>>>> i32 1, i32* @x} ; [ DW_TAG_variable ]
>>>> !11 = metadata !{i32 655412, i32 0, metadata !1, metadata !"x",
>>>> metadata !"x", metadata !"", metadata !7, i32 1, metadata !12, i32 1,
>>>> i32 1, null} ; [ DW_TAG_variable ]
>>>>
>>>> Is it intentional or a bug?  Thanks.
>>>
>>> It's a bug, probably a regression. Please file a bugzilla report.
>>> Thanks,
>>> -
>>> Devang
>>>
>
>