[ThinLTO] Correctly resolve linkonce when importing aliasee

Summary:
When we have an aliasee that is linkonce, while we can't convert
the non-prevailing copies to available_externally, we still need to
convert the prevailing copy to weak. If a reference to the aliasee
is exported, not converting a copy to weak will result in undefined
references when the linkonce is removed in its original module.

Add a new test and update existing tests.

Reviewers: mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26076

llvm-svn: 285512

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index bfb8a5c76a2..654b86a0dfd 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -128,20 +128,25 @@ static void thinLTOResolveWeakForLinkerGUID(
     function_ref<void(StringRef, GlobalValue::GUID, GlobalValue::LinkageTypes)>
         recordNewLinkage) {
   for (auto &S : GVSummaryList) {
-    if (GlobalInvolvedWithAlias.count(S.get()))
-      continue;
     GlobalValue::LinkageTypes OriginalLinkage = S->linkage();
     if (!GlobalValue::isWeakForLinker(OriginalLinkage))
       continue;
     // We need to emit only one of these. The prevailing module will keep it,
     // but turned into a weak, while the others will drop it when possible.
+    // This is both a compile-time optimization and a correctness
+    // transformation. This is necessary for correctness when we have exported
+    // a reference - we need to convert the linkonce to weak to
+    // ensure a copy is kept to satisfy the exported reference.
+    // FIXME: We may want to split the compile time and correctness
+    // aspects into separate routines.
     if (isPrevailing(GUID, S.get())) {
       if (GlobalValue::isLinkOnceLinkage(OriginalLinkage))
         S->setLinkage(GlobalValue::getWeakLinkage(
             GlobalValue::isLinkOnceODRLinkage(OriginalLinkage)));
     }
-    // Alias can't be turned into available_externally.
+    // Alias and aliasee can't be turned into available_externally.
     else if (!isa<AliasSummary>(S.get()) &&
+             !GlobalInvolvedWithAlias.count(S.get()) &&
              (GlobalValue::isLinkOnceODRLinkage(OriginalLinkage) ||
               GlobalValue::isWeakODRLinkage(OriginalLinkage)))
       S->setLinkage(GlobalValue::AvailableExternallyLinkage);