Index: include/llvm/Transforms/IPO/PassManagerBuilder.h
===================================================================
--- include/llvm/Transforms/IPO/PassManagerBuilder.h
+++ include/llvm/Transforms/IPO/PassManagerBuilder.h
@@ -133,6 +133,8 @@
   bool VerifyOutput;
   bool MergeFunctions;
   bool PrepareForLTO;
+  bool PrepareForThinLTO;
+  bool PerformThinLTO;
 
   /// Profile data file name that the instrumentation will be written to.
   std::string PGOInstrGen;
@@ -170,6 +172,7 @@
   /// populateModulePassManager - This sets up the primary pass manager.
   void populateModulePassManager(legacy::PassManagerBase &MPM);
   void populateLTOPassManager(legacy::PassManagerBase &PM);
+  void populateThinLTOPassManager(legacy::PassManagerBase &PM);
 };
 
 /// Registers a function for adding a standard set of passes.  This should be
Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -140,6 +140,8 @@
     PrepareForLTO = false;
     PGOInstrGen = RunPGOInstrGen;
     PGOInstrUse = RunPGOInstrUse;
+    PrepareForThinLTO = false;
+    PerformThinLTO = false;
 }
 
 PassManagerBuilder::~PassManagerBuilder() {
@@ -233,6 +235,11 @@
   MPM.add(createTailCallEliminationPass()); // Eliminate tail calls
   MPM.add(createCFGSimplificationPass());     // Merge & remove BBs
   MPM.add(createReassociatePass());           // Reassociate expressions
+  if (PrepareForThinLTO) {
+    MPM.add(createAggressiveDCEPass());        // Delete dead instructions
+    MPM.add(createInstructionCombiningPass()); // Combine silly seq's
+    return;
+  }
   // Rotate Loop - disable header duplication at -Oz
   MPM.add(createLoopRotatePass(SizeLevel == 2 ? 0 : -1));
   MPM.add(createLICMPass());                  // Hoist loop invariants
@@ -346,6 +353,12 @@
 
     MPM.add(createIPSCCPPass());              // IP SCCP
     MPM.add(createGlobalOptimizerPass());     // Optimize out global vars
+
+    if (PerformThinLTO)
+      // Linking modules together can lead to duplicated global constants, only
+      // keep one copy of each constant.
+      MPM.add(createConstantMergePass());
+
     // Promote any localized global vars
     MPM.add(createPromoteMemoryToRegisterPass());
 
@@ -378,6 +391,12 @@
 
   addFunctionSimplificationPasses(MPM);
 
+  // If we are planning to perform THinLTO later, let's not bloat the code with
+  // unrolling/vectorization/... now. We'll first run the inliner + CGSCC passes
+  // during ThinLTO and performs the rest of the optimizations afterward.
+  if (PrepareForThinLTO)
+    return;
+
   // FIXME: This is a HACK! The inliner pass above implicitly creates a CGSCC
   // pass manager that we are specifically trying to avoid. To prevent this
   // we must insert a no-op module pass to reset the pass manager.
@@ -396,7 +415,7 @@
   if (!DisableUnitAtATime)
     MPM.add(createReversePostOrderFunctionAttrsPass());
 
-  if (!DisableUnitAtATime && OptLevel > 1 && !PrepareForLTO) {
+  if (!DisableUnitAtATime && OptLevel > 1 && !PrepareForLTO)
     // Remove avail extern fns and globals definitions if we aren't
     // compiling an object file for later LTO. For LTO we want to preserve
     // these so they are eligible for inlining at link-time. Note if they
@@ -407,6 +426,15 @@
     // globals referenced by available external functions dead
     // and saves running remaining passes on the eliminated functions.
     MPM.add(createEliminateAvailableExternallyPass());
+
+  if (PerformThinLTO) {
+    // Remove dead fns and globals. Removing unreferenced functions could lead
+    // to more opportunities for globalopt
+    MPM.add(createGlobalDCEPass());
+    MPM.add(createGlobalOptimizerPass());
+    // Remove dead fns and globals after globalopt
+    MPM.add(createGlobalDCEPass());
+    addFunctionSimplificationPasses(MPM);
   }
 
   if (EnableNonLTOGlobalsModRef)
@@ -682,6 +710,20 @@
     PM.add(createMergeFunctionsPass());
 }
 
+void PassManagerBuilder::populateThinLTOPassManager(
+    legacy::PassManagerBase &PM) {
+  PerformThinLTO = true;
+
+  if (VerifyInput)
+    PM.add(createVerifierPass());
+
+  populateModulePassManager(PM);
+
+  if (VerifyOutput)
+    PM.add(createVerifierPass());
+  PerformThinLTO = false;
+}
+
 void PassManagerBuilder::populateLTOPassManager(legacy::PassManagerBase &PM) {
   if (LibraryInfo)
     PM.add(new TargetLibraryInfoWrapperPass(*LibraryInfo));