Index: lib/Target/ARM/ARMTargetMachine.cpp
===================================================================
--- lib/Target/ARM/ARMTargetMachine.cpp
+++ lib/Target/ARM/ARMTargetMachine.cpp
@@ -348,7 +348,10 @@
     // tricky when doing code gen per function.
     bool OnlyOptimizeForSize = (TM->getOptLevel() < CodeGenOpt::Aggressive) &&
                                (EnableGlobalMerge == cl::BOU_UNSET);
-    addPass(createGlobalMergePass(TM, 127, OnlyOptimizeForSize));
+    // Merging of extern globals is enabled by default as we expect it to be
+    // generally either beneficial or harmless.
+    addPass(createGlobalMergePass(TM, 127, OnlyOptimizeForSize,
+                                  /*MergeExternalByDefault=*/true));
   }
 
   return false;
Index: test/CodeGen/ARM/global-merge-external.ll
===================================================================
--- /dev/null
+++ test/CodeGen/ARM/global-merge-external.ll
@@ -0,0 +1,47 @@
+; RUN: llc < %s -mtriple=arm-eabi -arm-global-merge                                 | FileCheck %s --check-prefix=CHECK-MERGE
+; RUN: llc < %s -mtriple=arm-eabi -arm-global-merge -global-merge-on-external=true  | FileCheck %s --check-prefix=CHECK-MERGE
+; RUN: llc < %s -mtriple=arm-eabi -arm-global-merge -global-merge-on-external=false | FileCheck %s --check-prefix=CHECK-NO-MERGE
+
+@x = global i32 0, align 4
+@y = global i32 0, align 4
+@z = global i32 0, align 4
+
+define void @f1(i32 %a1, i32 %a2) {
+;CHECK:          f1:
+;CHECK:          ldr {{r[0-9]+}}, [[LABEL1:\.LCPI[0-9]+_[0-9]]]
+;CHECK:          [[LABEL1]]:
+;CHECK-MERGE:    .long _MergedGlobals_x
+;CHECK-NO-MERGE: .long x
+  store i32 %a1, i32* @x, align 4
+  store i32 %a2, i32* @y, align 4
+  ret void
+}
+
+define void @g1(i32 %a1, i32 %a2) {
+;CHECK:          g1:
+;CHECK:          ldr {{r[0-9]+}}, [[LABEL2:\.LCPI[0-9]+_[0-9]]]
+;CHECK:          [[LABEL2]]:
+;CHECK-MERGE:    .long _MergedGlobals_x
+;CHECK-NO-MERGE: .long y
+  store i32 %a1, i32* @y, align 4
+  store i32 %a2, i32* @z, align 4
+  ret void
+}
+
+;CHECK-NO-MERGE-NOT: .globl _MergedGlobals_x
+
+;CHECK-MERGE:	.type	_MergedGlobals_x,%object
+;CHECK-MERGE:	.globl	_MergedGlobals_x
+;CHECK-MERGE:	.align	2
+;CHECK-MERGE: _MergedGlobals_x:
+;CHECK-MERGE:	.size	_MergedGlobals_x, 12
+
+;CHECK-MERGE:	.globl	x
+;CHECK-MERGE: x = _MergedGlobals_x
+;CHECK-MERGE: .size x, 4
+;CHECK-MERGE:	.globl	y
+;CHECK-MERGE: y = _MergedGlobals_x+4
+;CHECK-MERGE: .size y, 4
+;CHECK-MERGE:	.globl	z
+;CHECK-MERGE: z = _MergedGlobals_x+8
+;CHECK-MERGE: .size z, 4