Index: llvm/tools/dsymutil/DwarfLinker.cpp
===================================================================
--- llvm/tools/dsymutil/DwarfLinker.cpp
+++ llvm/tools/dsymutil/DwarfLinker.cpp
@@ -271,12 +271,30 @@
   Info.Prune &= (DIE.getTag() == dwarf::DW_TAG_module) ||
                 dwarf::toUnsigned(DIE.find(dwarf::DW_AT_declaration), 0);
 
-  // Don't prune it if there is no definition for the DIE.
-  Info.Prune &= Info.Ctxt && Info.Ctxt->getCanonicalDIEOffset();
-
   return Info.Prune;
 }
 
+/// The canonical DIE offset is calculated when cloning a DIE. Therefore we
+/// cannot do it during analysis, but rather have to fix it up at the beginning
+/// of the clone step.
+static void updatePruning(CompileUnit::DIEInfo &Info, CompileUnit &CU) {
+  // Don't prune forward declarations inside a DW_TAG_module if no definition
+  // exists elsewhere.
+  if (!Info.Prune || !Info.Ctxt || !Info.Ctxt->getCanonicalDIEOffset())
+    return;
+
+  Info.Prune = false;
+
+  unsigned int ParentIdx = Info.ParentIdx;
+  while (ParentIdx != 0) {
+    CompileUnit::DIEInfo &ParentInfo = CU.getInfo(ParentIdx);
+    if (!ParentInfo.Prune)
+      return;
+    ParentInfo.Prune = false;
+    ParentIdx = ParentInfo.ParentIdx;
+  }
+}
+
 static bool dieNeedsChildrenToBeMeaningful(uint32_t Tag) {
   switch (Tag) {
   default:
@@ -840,6 +858,9 @@
     if (!Worklist.empty())
       Worklist.back().ChildInfo = &MyInfo;
 
+    if (!Current.Flags)
+      updatePruning(MyInfo, CU);
+
     if (MyInfo.Prune)
       continue;
 
@@ -2512,14 +2533,13 @@
               *CU, UnitID++, !Options.NoODR && !Options.Update, ""));
         }
       }
-      
+
       // Now build the DIE parent links that we will use during the next phase.
       for (auto &CurrentUnit : LinkContext.CompileUnits) {
         auto CUDie = CurrentUnit->getOrigUnit().getUnitDIE();
         if (!CUDie)
           continue;
-        analyzeContextInfo(CurrentUnit->getOrigUnit().getUnitDIE(), 0,
-                           *CurrentUnit, &ODRContexts.getRoot(),
+        analyzeContextInfo(CUDie, 0, *CurrentUnit, &ODRContexts.getRoot(),
                            UniquingStringPool, ODRContexts);
       }