Move metadata linking after lazy global materialization/linking.

[tejohnson]

Summary

Currently, named metadata is linked before the LazilyLinkGlobalValues
list is walked and materialized/linked. As a result, references
from DISubprogram and DIGlobalVariable metadata to yet unmaterialized
functions and variables cause them to be added to the lazy linking
list and their definitions are materialized and linked.

This makes the llvm-link -only-needed option not have the intended
effect when debug information is present, as the otherwise unneeded
functions/variables are still linked in.

Additionally, for ThinLTO I have implemented a mechanism to only link
in debug metadata needed by imported functions. Moving named metadata
linking after lazy GV linking will facilitate applying this mechanism
to the LTO and "llvm-link -only-needed" cases as well.

[tejohnson]

I can't seem to get Phab to reopen this issue since the previous
commit closed it. So ignore this and please look at the new revision I
just created in D14447.

llvm/trunk/lib/Linker/LinkModules.cpp

bool ModuleLinker::run() {
 
   // Resolve all uses of aliases with aliasees.
   for (GlobalAlias &Src : SrcM->aliases()) {
     if (DoNotLinkFromSource.count(&Src))
       continue;
     linkGlobalValueBody(Src);
   }
 
-  // Remap all of the named MDNodes in Src into the DstM module. We do this
-  // after linking GlobalValues so that MDNodes that reference GlobalValues
-  // are properly remapped.
-  linkNamedMDNodes();
-
-  // Merge the module flags into the DstM module.
-  if (linkModuleFlagsMetadata())
-    return true;
-
   // Update the initializers in the DstM module now that all globals that may
   // be referenced are in DstM.
   for (GlobalVariable &Src : SrcM->globals()) {
     // Only process initialized GV's or ones not already in dest.
     if (!Src.hasInitializer() || DoNotLinkFromSource.count(&Src))
       continue;
     linkGlobalValueBody(Src);
   }
while (!LazilyLinkGlobalValues.empty()) {
     if (shouldLinkOnlyNeeded() && SGV->isDeclaration())
       continue;
 
     assert(!SGV->isDeclaration() && "users should not pass down decls");
     if (linkGlobalValueBody(*SGV))
       return true;
   }
 
+  // Remap all of the named MDNodes in Src into the DstM module. We do this
+  // after linking GlobalValues so that MDNodes that reference GlobalValues
+  // are properly remapped.
+  linkNamedMDNodes();
+
+  // Merge the module flags into the DstM module.
+  if (linkModuleFlagsMetadata())
+    return true;
+
   return false;
 }
 
 Linker::StructTypeKeyInfo::KeyTy::KeyTy(ArrayRef<Type *> E, bool P)
     : ETypes(E), IsPacked(P) {}
 
 Linker::StructTypeKeyInfo::KeyTy::KeyTy(const StructType *ST)
     : ETypes(ST->elements()), IsPacked(ST->isPacked()) {}

llvm/trunk/test/Linker/Inputs/only-needed-named-metadata.ll

+@X = external global i32
+
+declare i32 @foo()
+
+define void @bar() {
+  load i32, i32* @X
+  call i32 @foo()
+  ret void
+}

llvm/trunk/test/Linker/only-needed-named-metadata.ll

+; RUN: llvm-as %S/only-needed-named-metadata.ll -o %t.bc
+; RUN: llvm-as %S/Inputs/only-needed-named-metadata.ll -o %t2.bc
+; RUN: llvm-link -S -only-needed %t2.bc %t.bc | FileCheck %s
+; RUN: llvm-link -S -internalize -only-needed %t2.bc %t.bc | FileCheck %s
+
+; CHECK: @U = external global i32
+; CHECK: declare i32 @unused()
+
+@X = global i32 5
+@U = global i32 6
+define i32 @foo() { ret i32 7 }
+define i32 @unused() { ret i32 8 }
+
+!llvm.named = !{!0, !1}
+!0 = !{i32 ()* @unused}
+!1 = !{i32* @U}