4 files changed, 21 insertions, 16 deletions
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 101e8eba6b1..df9ab1c496c 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -6147,8 +6147,8 @@ std::error_code ModuleSummaryIndexBitcodeReader::parseModule() {
           if (!TheIndex)
             break;
           if (TheIndex->modulePaths().empty())
-            // Does not have any summary emitted.
-            break;
+            // We always seed the index with the module.
+            TheIndex->addModulePath(Buffer->getBufferIdentifier(), 0);
           if (TheIndex->modulePaths().size() != 1)
             return error("Don't expect multiple modules defined?");
           auto &Hash = TheIndex->modulePaths().begin()->second.second;
diff --git a/llvm/lib/IR/ModuleSummaryIndex.cpp b/llvm/lib/IR/ModuleSummaryIndex.cpp
index 6107cf40a08..9072f4bc7b1 100644
--- a/llvm/lib/IR/ModuleSummaryIndex.cpp
+++ b/llvm/lib/IR/ModuleSummaryIndex.cpp
@@ -20,8 +20,17 @@ using namespace llvm;
 // per-module instances.
 void ModuleSummaryIndex::mergeFrom(std::unique_ptr<ModuleSummaryIndex> Other,
                                    uint64_t NextModuleId) {
+  if (Other->modulePaths().empty())
+    return;
+
+  assert(Other->modulePaths().size() == 1 &&
+         "Can only merge from an single-module index at that time");
+
+  StringRef OtherModPath = Other->modulePaths().begin()->first();
+  StringRef ModPath = addModulePath(OtherModPath, NextModuleId,
+                                    Other->getModuleHash(OtherModPath))
+                          ->first();
 
-  StringRef ModPath;
   for (auto &OtherGlobalValSummaryLists : *Other) {
     GlobalValue::GUID ValueGUID = OtherGlobalValSummaryLists.first;
     GlobalValueSummaryList &List = OtherGlobalValSummaryLists.second;
@@ -31,16 +40,6 @@ void ModuleSummaryIndex::mergeFrom(std::unique_ptr<ModuleSummaryIndex> Other,
     assert(List.size() == 1);
     std::unique_ptr<GlobalValueSummary> Summary = std::move(List.front());
 
-    // Add the module path string ref for this module if we haven't already
-    // saved a reference to it.
-    if (ModPath.empty()) {
-      auto Path = Summary->modulePath();
-      ModPath = addModulePath(Path, NextModuleId, Other->getModuleHash(Path))
-                    ->first();
-    } else
-      assert(ModPath == Summary->modulePath() &&
-             "Each module in the combined map should have a unique ID");
-
     // Note the module path string ref was copied above and is still owned by
     // the original per-module index. Reset it to the new module path
     // string reference owned by the combined index.
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index b2d42f4d2a6..94a4abfb896 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -541,13 +541,15 @@ public:
                          ImportList, DefinedGlobals, ModuleMap);
     };
 
-    if (!Cache)
+    auto ModuleID = MBRef.getBufferIdentifier();
+    if (!Cache || !CombinedIndex.modulePaths().count(ModuleID))
+      // Cache disabled or no entry for this module in the combined index
       return RunThinBackend(AddStream);
 
     SmallString<40> Key;
     // The module may be cached, this helps handling it.
-    computeCacheKey(Key, CombinedIndex, MBRef.getBufferIdentifier(),
-                    ImportList, ExportList, ResolvedODR, DefinedGlobals);
+    computeCacheKey(Key, CombinedIndex, ModuleID, ImportList, ExportList,
+                    ResolvedODR, DefinedGlobals);
     if (AddStreamFn CacheAddStream = Cache(Task, Key))
       return RunThinBackend(CacheAddStream);
 
diff --git a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
index f4232dc2f89..8afe13d8e16 100644
--- a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
+++ b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
@@ -234,6 +234,10 @@ public:
     if (CachePath.empty())
       return;
 
+    if (!Index.modulePaths().count(ModuleID))
+      // The module does not have an entry, it can't have a hash at all
+      return;
+
     // Compute the unique hash for this entry
     // This is based on the current compiler version, the module itself, the
     // export list, the hash for every single module in the import list, the