Index: clang/include/clang/Driver/Driver.h
===================================================================
--- clang/include/clang/Driver/Driver.h
+++ clang/include/clang/Driver/Driver.h
@@ -578,6 +578,10 @@
   /// compilation based on which -f(no-)?lto(=.*)? option occurs last.
   void setLTOMode(const llvm::opt::ArgList &Args);
 
+  /// Parse the \p Inputs list for LTO inputs. LTO inputs are object files
+  /// that contain LLVM bitcode
+  void setLTOModeFromInputFiles(const InputList *Inputs);
+
   /// Retrieves a ToolChain for a particular \p Target triple.
   ///
   /// Will cache ToolChains for the life of the driver object, and create them
Index: clang/lib/Driver/CMakeLists.txt
===================================================================
--- clang/lib/Driver/CMakeLists.txt
+++ clang/lib/Driver/CMakeLists.txt
@@ -3,6 +3,7 @@
   Option
   ProfileData
   Support
+  BitReader
   )
 
 if(WIN32)
Index: clang/lib/Driver/Driver.cpp
===================================================================
--- clang/lib/Driver/Driver.cpp
+++ clang/lib/Driver/Driver.cpp
@@ -64,6 +64,7 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
@@ -586,6 +587,43 @@
   return Target;
 }
 
+static LTOKind
+getInputLTOMode(const std::pair<types::ID, const llvm::opt::Arg *> Input) {
+  // Must be an object file
+  if (Input.first != types::TY_Object)
+    return LTOK_None;
+
+  // ... and contain LLVM bitcode
+  for (const char *Filename : Input.second->getValues()) {
+    auto Buff = llvm::MemoryBuffer::getFile(Filename);
+    if (Buff) {
+      auto R = (*Buff)->getMemBufferRef();
+      Expected<llvm::BitcodeLTOInfo> LTOInfo = llvm::getBitcodeLTOInfo(R);
+
+      if (LTOInfo)
+        return LTOInfo->IsThinLTO ? LTOK_Thin : LTOK_Full;
+
+      llvm::consumeError(LTOInfo.takeError());
+    }
+  }
+
+  return LTOK_None;
+}
+
+void Driver::setLTOModeFromInputFiles(const InputList *Inputs) {
+  if (LTOMode != LTOK_None)
+    return;
+
+  for (auto Input : *Inputs) {
+    auto InputLTOMode = getInputLTOMode(Input);
+
+    if (InputLTOMode != LTOK_None) {
+      LTOMode = InputLTOMode;
+      break;
+    }
+  }
+}
+
 // Parse the LTO options and record the type of LTO compilation
 // based on which -f(no-)?lto(=.*)? option occurs last.
 void Driver::setLTOMode(const llvm::opt::ArgList &Args) {
@@ -1164,6 +1202,8 @@
   InputList Inputs;
   BuildInputs(C->getDefaultToolChain(), *TranslatedArgs, Inputs);
 
+  setLTOModeFromInputFiles(&Inputs);
+
   // Populate the tool chains for the offloading devices, if any.
   CreateOffloadingDeviceToolChains(*C, Inputs);
 
Index: clang/test/Driver/thinlto_backend.c
===================================================================
--- clang/test/Driver/thinlto_backend.c
+++ clang/test/Driver/thinlto_backend.c
@@ -24,3 +24,13 @@
 // RUN: not %clang -O2 -o %t1.o %s -c -fthinlto-index=%t.thinlto.bc 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-WARNING
 // CHECK-WARNING: error: option '-fthinlto-index={{.*}}' requires input to be LLVM bitcode
+
+// thinlto enabled without -flto=thin explicitly passed
+// RUN: %clang -O2 %s -c -flto=thin -o %r.o
+// RUN: %clang %r.o -### 2>&1 | FileCheck %s -check-prefix=CHECK-THIN-LTO-ENABLED
+// CHECK-THIN-LTO-ENABLED: -plugin-opt=thinlto
+
+// Same for full LTO
+// RUN: %clang -O2 %s -target x86_64-unknown-linux-gnu -c -flto=full -o %r.o
+// RUN: %clang %r.o -### 2>&1 | FileCheck %s -check-prefix=CHECK-FULL-LTO-ENABLED
+// CHECK-FULL-LTO-ENABLED: LLVMgold