[LLVMdev] metadata linking bug or by design

[Devang Patel]

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
>>