Index: lib/Support/Triple.cpp
===================================================================
--- lib/Support/Triple.cpp
+++ lib/Support/Triple.cpp
@@ -1473,6 +1473,20 @@
 }
 
 bool Triple::isCompatibleWith(const Triple &Other) const {
+  // ARM and Thumb triples are compatible, if subarch, vendor and OS match.
+  if ((getArch() == Triple::thumb && Other.getArch() == Triple::arm) ||
+      (getArch() == Triple::arm && Other.getArch() == Triple::thumb) ||
+      (getArch() == Triple::thumbeb && Other.getArch() == Triple::armeb) ||
+      (getArch() == Triple::armeb && Other.getArch() == Triple::thumbeb))
+    if (getVendor() == Triple::Apple)
+      return getSubArch() == Other.getSubArch() &&
+             getVendor() == Other.getVendor() && getOS() == Other.getOS();
+    else
+      return getSubArch() == Other.getSubArch() &&
+             getVendor() == Other.getVendor() && getOS() == Other.getOS() &&
+             getEnvironment() == Other.getEnvironment() &&
+             getObjectFormat() == Other.getObjectFormat();
+
   // If vendor is apple, ignore the version number.
   if (getVendor() == Triple::Apple)
     return getArch() == Other.getArch() && getSubArch() == Other.getSubArch() &&
Index: test/LTO/ARM/Inputs/thumb.ll
===================================================================
--- /dev/null
+++ test/LTO/ARM/Inputs/thumb.ll
@@ -0,0 +1,15 @@
+target triple = "thumbv7-linux-gnueabihf"
+
+define i32 @foo(i32 %a, i32 %b) #0 {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
+
+define i32 @bar(i32 %a, i32 %b) #0 {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
+
+attributes #0 = { "target-features"="+thumb-mode" }
Index: test/LTO/ARM/link-arm-and-thumb.ll
===================================================================
--- /dev/null
+++ test/LTO/ARM/link-arm-and-thumb.ll
@@ -0,0 +1,29 @@
+; 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 2> %t3.out| FileCheck %s
+; RUN: FileCheck --allow-empty --input-file %t3.out --check-prefix STDERR %s
+
+target triple = "armv7-linux-gnueabihf"
+
+; CHECK: .code  32
+; CHECK-NEXT: main
+; CHECK-NEXT: .fnstart
+; CHECK-NEXT: mov r0, #30
+
+; CHECK: .code  16
+; CHECK-NEXT: .thumb_func
+; 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
+}
+
+; STDERR-NOT: warning: Linking two modules of different target triples:
Index: test/Linker/Inputs/thumb.ll
===================================================================
--- /dev/null
+++ test/Linker/Inputs/thumb.ll
@@ -0,0 +1,16 @@
+target triple = "thumbv7-linux-gnueabihf"
+
+define i32 @foo(i32 %a, i32 %b) #0 {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
+
+define i32 @bar(i32 %a, i32 %b) #1 {
+entry:
+  %add = add i32 %a, %b
+  ret i32 %add
+}
+
+attributes #0 = { "target-features"="-thumb-mode" }
+attributes #1 = { "target-features"="+thumb-mode" }
Index: test/Linker/link-arm-and-thumb.ll
===================================================================
--- /dev/null
+++ test/Linker/link-arm-and-thumb.ll
@@ -0,0 +1,26 @@
+; 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 2> %t3.out | llc | FileCheck %s
+; RUN: FileCheck --allow-empty --input-file %t3.out --check-prefix STDERR %s
+
+target triple = "armv7-linux-gnueabihf"
+
+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 @ @main
+; CHECK-NEXT: main
+
+; CHECK: .code  32 @ @foo
+; CHECK-NEXT: foo
+
+; CHECK: .code  16 @ @bar
+; CHECK-NEXT: .thumb_func
+; CHECK-NEXT: bar
+
+; STDERR-NOT: warning: Linking two modules of different target triples: