3 files changed, 25 insertions, 8 deletions
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 33f1a7e454e..d45360f0049 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -6097,13 +6097,18 @@ std::error_code ModuleSummaryIndexBitcodeReader::parseModule() {
           return error("Invalid record");
         break;
       case bitc::GLOBALVAL_SUMMARY_BLOCK_ID:
-        assert(VSTOffset > 0 && "Expected non-zero VST offset");
         assert(!SeenValueSymbolTable &&
                "Already read VST when parsing summary block?");
-        if (std::error_code EC =
-                parseValueSymbolTable(VSTOffset, ValueIdToLinkageMap))
-          return EC;
-        SeenValueSymbolTable = true;
+        // We might not have a VST if there were no values in the
+        // summary. An empty summary block generated when we are
+        // performing ThinLTO compiles so we don't later invoke
+        // the regular LTO process on them.
+        if (VSTOffset > 0) {
+          if (std::error_code EC =
+                  parseValueSymbolTable(VSTOffset, ValueIdToLinkageMap))
+            return EC;
+          SeenValueSymbolTable = true;
+        }
         SeenGlobalValSummary = true;
         if (std::error_code EC = parseEntireSummary())
           return EC;
diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index df700a8aba1..a353edf7aec 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -3341,13 +3341,15 @@ static const uint64_t INDEX_VERSION = 1;
 /// Emit the per-module summary section alongside the rest of
 /// the module's bitcode.
 void ModuleBitcodeWriter::writePerModuleGlobalValueSummary() {
-  if (Index->begin() == Index->end())
-    return;
-
   Stream.EnterSubblock(bitc::GLOBALVAL_SUMMARY_BLOCK_ID, 4);
 
   Stream.EmitRecord(bitc::FS_VERSION, ArrayRef<uint64_t>{INDEX_VERSION});
 
+  if (Index->begin() == Index->end()) {
+    Stream.ExitBlock();
+    return;
+  }
+
   // Abbrev for FS_PERMODULE.
   BitCodeAbbrev *Abbv = new BitCodeAbbrev();
   Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE));
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 0d47cd49a17..017a45ac7ba 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -713,6 +713,16 @@ Error LTO::runThinLTO(AddOutputFn AddOutput, bool HasRegularLTO) {
       ModuleToDefinedGVSummaries(ThinLTO.ModuleMap.size());
   ThinLTO.CombinedIndex.collectDefinedGVSummariesPerModule(
       ModuleToDefinedGVSummaries);
+  // Create entries for any modules that didn't have any GV summaries
+  // (either they didn't have any GVs to start with, or we suppressed
+  // generation of the summaries because they e.g. had inline assembly
+  // uses that couldn't be promoted/renamed on export). This is so
+  // InProcessThinBackend::start can still launch a backend thread, which
+  // is passed the map of summaries for the module, without any special
+  // handling for this case.
+  for (auto &Mod : ThinLTO.ModuleMap)
+    if (!ModuleToDefinedGVSummaries.count(Mod.first))
+      ModuleToDefinedGVSummaries.try_emplace(Mod.first);
 
   StringMap<FunctionImporter::ImportMapTy> ImportLists(
       ThinLTO.ModuleMap.size());