Index: llvm/trunk/lib/LTO/LTO.cpp
===================================================================
--- llvm/trunk/lib/LTO/LTO.cpp
+++ llvm/trunk/lib/LTO/LTO.cpp
@@ -542,8 +542,12 @@
     };
 
     auto ModuleID = MBRef.getBufferIdentifier();
-    if (!Cache || !CombinedIndex.modulePaths().count(ModuleID))
-      // Cache disabled or no entry for this module in the combined index
+
+    if (!Cache || !CombinedIndex.modulePaths().count(ModuleID) ||
+        all_of(CombinedIndex.getModuleHash(ModuleID),
+               [](uint32_t V) { return V == 0; }))
+      // Cache disabled or no entry for this module in the combined index or
+      // no module hash.
       return RunThinBackend(AddStream);
 
     SmallString<40> Key;
Index: llvm/trunk/lib/LTO/ThinLTOCodeGenerator.cpp
===================================================================
--- llvm/trunk/lib/LTO/ThinLTOCodeGenerator.cpp
+++ llvm/trunk/lib/LTO/ThinLTOCodeGenerator.cpp
@@ -243,6 +243,13 @@
     // export list, the hash for every single module in the import list, the
     // list of ResolvedODR for the module, and the list of preserved symbols.
 
+    // Include the hash for the current module
+    auto ModHash = Index.getModuleHash(ModuleID);
+
+    if (all_of(ModHash, [](uint32_t V) { return V == 0; }))
+      // No hash entry, no caching!
+      return;
+
     SHA1 Hasher;
 
     // Start with the compiler revision
@@ -251,8 +258,6 @@
     Hasher.update(LLVM_REVISION);
 #endif
 
-    // Include the hash for the current module
-    auto ModHash = Index.getModuleHash(ModuleID);
     Hasher.update(ArrayRef<uint8_t>((uint8_t *)&ModHash[0], sizeof(ModHash)));
     for (auto F : ExportList)
       // The export list can impact the internalization, be conservative here
Index: llvm/trunk/test/ThinLTO/X86/cache.ll
===================================================================
--- llvm/trunk/test/ThinLTO/X86/cache.ll
+++ llvm/trunk/test/ThinLTO/X86/cache.ll
@@ -1,6 +1,28 @@
+; Verify first that *without* hash, we don't use the cache.
+
 ; RUN: opt -module-summary %s -o %t.bc
 ; RUN: opt -module-summary %p/Inputs/cache.ll -o %t2.bc
 
+; Verify that enabling caching is ignoring module without hash
+; RUN: rm -Rf %t.cache && mkdir %t.cache
+; RUN: llvm-lto -thinlto-action=run -exported-symbol=globalfunc %t2.bc  %t.bc -thinlto-cache-dir %t.cache
+; RUN: ls %t.cache/llvmcache.timestamp
+; RUN: ls %t.cache | count 1
+
+; Verify that enabling caching is ignoring module without hash with llvm-lto2
+; RUN: rm -Rf %t.cache && mkdir %t.cache
+; RUN: llvm-lto2 -o %t.o %t2.bc  %t.bc -cache-dir %t.cache \
+; RUN:  -r=%t2.bc,_main,plx \
+; RUN:  -r=%t2.bc,_globalfunc,lx \
+; RUN:  -r=%t.bc,_globalfunc,plx
+; RUN: ls %t.cache | count 0
+
+
+; Repeat again, *with* hash this time.
+
+; RUN: opt -module-hash -module-summary %s -o %t.bc
+; RUN: opt -module-hash -module-summary %p/Inputs/cache.ll -o %t2.bc
+
 ; Verify that enabling caching is working
 ; RUN: rm -Rf %t.cache && mkdir %t.cache
 ; RUN: llvm-lto -thinlto-action=run -exported-symbol=globalfunc %t2.bc  %t.bc -thinlto-cache-dir %t.cache
Index: llvm/trunk/test/ThinLTO/X86/empty_module_with_cache.ll
===================================================================
--- llvm/trunk/test/ThinLTO/X86/empty_module_with_cache.ll
+++ llvm/trunk/test/ThinLTO/X86/empty_module_with_cache.ll
@@ -22,13 +22,13 @@
 ; RUN: rm -Rf %t.cache && mkdir %t.cache
 ; RUN: llvm-lto -thinlto-action=run %t2.bc  %t.bc -exported-symbol=main -thinlto-cache-dir %t.cache
 ; RUN: ls %t.cache/llvmcache.timestamp
-; RUN: ls %t.cache | count 2
+; RUN: ls %t.cache | count 1
 
 ; Verify that caching is disabled for module without hash, with llvm-lto2
 ; RUN: rm -Rf %t.cache && mkdir %t.cache
 ; RUN: llvm-lto2 -o %t.o %t2.bc  %t.bc -cache-dir %t.cache \
 ; RUN:  -r=%t2.bc,_main,plx
-; RUN: ls %t.cache | count 1
+; RUN: ls %t.cache | count 0
 
 
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"