Index: lib/Linker/IRMover.cpp
===================================================================
--- lib/Linker/IRMover.cpp
+++ lib/Linker/IRMover.cpp
@@ -1250,6 +1250,13 @@
     return T0.getArch() == T1.getArch() && T0.getSubArch() == T1.getSubArch() &&
            T0.getVendor() == T1.getVendor() && T0.getOS() == T1.getOS();
 
+  // Allow linking ARM and Thumb modules together without warning, if subarch,
+  // vendor and OS match.
+  if ((T0.getArch() == Triple::thumb && T1.getArch() == Triple::arm) ||
+      (T0.getArch() == Triple::arm && T1.getArch() == Triple::thumb))
+    return T0.getSubArch() == T1.getSubArch() &&
+           T0.getVendor() == T1.getVendor() && T0.getOS() == T1.getOS();
+
   return T0 == T1;
 }
 
@@ -1264,6 +1271,42 @@
   return DstTriple.str();
 }
 
+// Add thumb-mode to target-features if we link ARM and Thumb code together.
+void maybeAddARMSubtargetFeatures(const Triple &SrcTriple,
+                                  const Triple &DstTriple,
+                                  GlobalValue *GV) {
+  // Only consider triples with Arm or Thumb architecture.
+  if (SrcTriple.getArch() == DstTriple.getArch())
+    return;
+  if (SrcTriple.getArch() != Triple::thumb && SrcTriple.getArch() != Triple::arm)
+    return;
+  if (DstTriple.getArch() != Triple::thumb && DstTriple.getArch() != Triple::arm)
+    return;
+
+  const std::string thumbMode = SrcTriple.getArch() == Triple::arm ?
+                                  "-thumb-mode" : "+thumb-mode";
+
+  llvm::Function *Fn = dyn_cast<llvm::Function>(GV);
+  if (!Fn)
+    return;
+
+  Attribute targetFeatures;
+  if (Fn->hasFnAttribute("target-features")) {
+    targetFeatures = Fn->getFnAttribute("target-features");
+  }
+
+  std::string newFeatures = targetFeatures.getValueAsString();
+  // Do not add thumb-mode if it is already part of the target-features.
+  if (newFeatures.find("thumb-mode") != std::string::npos)
+    return;
+
+  if (newFeatures.size() > 0)
+    newFeatures += ",";
+
+  newFeatures += thumbMode;
+  Fn->addFnAttr("target-features", newFeatures);
+}
+
 Error IRLinker::run() {
   // Ensure metadata materialized before value mapping.
   if (SrcM->getMaterializer())
@@ -1315,6 +1358,8 @@
     GlobalValue *GV = Worklist.back();
     Worklist.pop_back();
 
+    maybeAddARMSubtargetFeatures(SrcTriple, DstTriple, GV);
+
     // Already mapped.
     if (ValueMap.find(GV) != ValueMap.end() ||
         AliasValueMap.find(GV) != AliasValueMap.end())
Index: test/LTO/ARM/Inputs/thumb.ll
===================================================================
--- /dev/null
+++ test/LTO/ARM/Inputs/thumb.ll
@@ -0,0 +1,13 @@
+target triple = "thumbv7-apple-ios"
+
+define i32 @foo(i32 %a, i32 %b) {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
+
+define i32 @bar(i32 %a, i32 %b) {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
Index: test/LTO/ARM/link-arm-and-thumb.ll
===================================================================
--- /dev/null
+++ test/LTO/ARM/link-arm-and-thumb.ll
@@ -0,0 +1,25 @@
+; RUN: llvm-as %s -o %t1.bc
+; RUN: llvm-as %p/Inputs/thumb.ll -o %t2.bc
+; RUN: llvm-lto -exported-symbol main \
+; RUN:          -exported-symbol bar \
+; RUN:          -filetype=asm \
+; RUN:          -o - \
+; RUN:          %t1.bc %t2.bc | FileCheck %s
+
+target triple = "armv7-apple-ios"
+
+; CHECK: .code  32
+; CHECK-NEXT: _main
+; CHECK-NEXT: mov  r0, #30
+
+; CHECK: .code  16
+; CHECK-NEXT: .thumb_func   _bar
+; CHECK-NEXT: _bar
+
+declare i32 @foo(i32 %a, i32 %b);
+
+define i32 @main() {
+entry:
+  %add = call i32 @foo(i32 10, i32 20)
+  ret i32 %add
+}
Index: test/Linker/Inputs/thumb.ll
===================================================================
--- /dev/null
+++ test/Linker/Inputs/thumb.ll
@@ -0,0 +1,15 @@
+target triple = "thumbv7-apple-ios"
+
+define i32 @foo(i32 %a, i32 %b) #0 {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
+
+define i32 @bar(i32 %a, i32 %b) {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
+
+attributes #0 = { "target-features"="-thumb-mode" }
Index: test/Linker/link-arm-and-thumb.ll
===================================================================
--- /dev/null
+++ test/Linker/link-arm-and-thumb.ll
@@ -0,0 +1,23 @@
+; RUN: llvm-as %s -o %t1.bc
+; RUN: llvm-as %p/Inputs/thumb.ll -o %t2.bc
+; RUN: llvm-link %t1.bc %t2.bc -S | llc | FileCheck %s
+
+target triple = "armv7-apple-ios"
+
+declare i32 @foo(i32 %a, i32 %b);
+
+define i32 @main() {
+entry:
+  %add = call i32 @foo(i32 10, i32 20)
+  ret i32 %add
+}
+
+; CHECK: .code  32
+; CHECK-NEXT: _main
+
+; CHECK: .code  32
+; CHECK-NEXT: _foo
+
+; CHECK: .code  16
+; CHECK-NEXT: .thumb_func   _bar
+; CHECK-NEXT: _bar