Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -457,18 +457,35 @@
 
     if (Sym.getFlags() & object::BasicSymbolRef::SF_Undefined)
       continue;
-    if (Res.Prevailing && Sym.isGV()) {
+    if (Sym.isGV()) {
       GlobalValue *GV = Sym.getGV();
-      Keep.push_back(GV);
-      switch (GV->getLinkage()) {
-      default:
-        break;
-      case GlobalValue::LinkOnceAnyLinkage:
-        GV->setLinkage(GlobalValue::WeakAnyLinkage);
-        break;
-      case GlobalValue::LinkOnceODRLinkage:
-        GV->setLinkage(GlobalValue::WeakODRLinkage);
-        break;
+      if (Res.Prevailing) {
+        Keep.push_back(GV);
+        switch (GV->getLinkage()) {
+        default:
+          break;
+        case GlobalValue::LinkOnceAnyLinkage:
+          GV->setLinkage(GlobalValue::WeakAnyLinkage);
+          break;
+        case GlobalValue::LinkOnceODRLinkage:
+          GV->setLinkage(GlobalValue::WeakODRLinkage);
+          break;
+        }
+      } else {
+        // This definition is not prevailing, but we can still use it for
+        // optimization if it has ODR linkage. This is done by setting its
+        // linkage to AvailableExternallyLinkage.
+        // This causes a declaration to be emitted, and declarations cannot
+        // have comdats. Therefore, we can only set AvailableExternallyLinkage
+        // on symbols that don't have a comdat or from which we can remove
+        // the comdat.
+        if (GV->hasLinkOnceODRLinkage() || GV->hasWeakODRLinkage()) {
+          if (auto *G = dyn_cast<GlobalObject>(GV))
+            G->setComdat(nullptr);
+          if (!GV->hasComdat()) {
+            GV->setLinkage(GlobalValue::AvailableExternallyLinkage);
+          }
+        }
       }
     }
     // Common resolution: collect the maximum size/alignment over all commons.