[LLVMdev] metadata linking bug or by design

[Xi Wang]

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.

- 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
>