[ThinLTO] Auto-hide prevailing linkonce_odr only when all copies eligible

Summary:
We hit undefined references building with ThinLTO when one source file
contained explicit instantiations of a template method (weak_odr) but
there were also implicit instantiations in another file (linkonce_odr),
and the latter was the prevailing copy. In this case the symbol was
marked hidden when the prevailing linkonce_odr copy was promoted to
weak_odr. It led to unsats when the resulting shared library was linked
with other code that contained a reference (expecting to be resolved due
to the explicit instantiation).

Add a CanAutoHide flag to the GV summary to allow the thin link to
identify when all copies are eligible for auto-hiding (because they were
all originally linkonce_odr global unnamed addr), and only do the
auto-hide in that case.

Most of the changes here are due to plumbing the new flag through the
bitcode and llvm assembly, and resulting test changes. I augmented the
existing auto-hide test to check for this situation.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, eraman, dexonsmith, arphaman, dang, llvm-commits, steven_wu, wmi

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59709

llvm-svn: 360466

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/IR/ModuleSummaryIndex.cpp b/llvm/lib/IR/ModuleSummaryIndex.cpp
index 4704215800b..18b7ac09388 100644
--- a/llvm/lib/IR/ModuleSummaryIndex.cpp
+++ b/llvm/lib/IR/ModuleSummaryIndex.cpp
@@ -26,6 +26,7 @@ STATISTIC(ReadOnlyLiveGVars,
 
 FunctionSummary FunctionSummary::ExternalNode =
     FunctionSummary::makeDummyFunctionSummary({});
+
 bool ValueInfo::isDSOLocal() const {
   // Need to check all summaries are local in case of hash collisions.
   return getSummaryList().size() &&
@@ -35,6 +36,15 @@ bool ValueInfo::isDSOLocal() const {
                       });
 }
 
+bool ValueInfo::canAutoHide() const {
+  // Can only auto hide if all copies are eligible to auto hide.
+  return getSummaryList().size() &&
+         llvm::all_of(getSummaryList(),
+                      [](const std::unique_ptr<GlobalValueSummary> &Summary) {
+                        return Summary->canAutoHide();
+                      });
+}
+
 // Gets the number of immutable refs in RefEdgeList
 unsigned FunctionSummary::immutableRefCount() const {
   // Here we take advantage of having all readonly references
@@ -399,6 +409,10 @@ void ModuleSummaryIndex::exportToDot(raw_ostream &OS) const {
         if (Flags.Live && hasReadOnlyFlag(SummaryIt.second))
           A.addComment("immutable");
       }
+      if (Flags.DSOLocal)
+        A.addComment("dsoLocal");
+      if (Flags.CanAutoHide)
+        A.addComment("canAutoHide");
 
       auto VI = getValueInfo(SummaryIt.first);
       A.add("label", getNodeLabel(VI, SummaryIt.second));