Index: llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
===================================================================
--- llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
+++ llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -462,7 +462,21 @@
       EnableGlobalMerge == cl::BOU_TRUE) {
     bool OnlyOptimizeForSize = (TM->getOptLevel() < CodeGenOpt::Aggressive) &&
                                (EnableGlobalMerge == cl::BOU_UNSET);
-    addPass(createGlobalMergePass(TM, 4095, OnlyOptimizeForSize));
+
+    // Merging of extern globals is enabled by default on non-Mach-O as we
+    // expect it to be generally either beneficial or harmless. On Mach-O it
+    // is disabled as we emit the .subsections_via_symbols directive which
+    // means that merging extern globals is not safe.
+    bool MergeExternalByDefault = !TM->getTargetTriple().isOSBinFormatMachO();
+
+    // FIXME: global merging is only enabled when we optimise for size because
+    // with it also enabled for speed there is a performance regression in a
+    // popular benchmark.
+    if (!OnlyOptimizeForSize)
+      MergeExternalByDefault = false;
+
+    addPass(createGlobalMergePass(TM, 4095, OnlyOptimizeForSize,
+                                  MergeExternalByDefault));
   }
 
   return false;
Index: llvm/test/CodeGen/AArch64/global-merge-minsize.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/global-merge-minsize.ll
@@ -0,0 +1,21 @@
+; RUN: llc %s -o - -verify-machineinstrs | FileCheck %s
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-arm-none-eabi"
+
+@global0 = dso_local local_unnamed_addr global i32 0, align 4
+@global1 = dso_local local_unnamed_addr global i32 0, align 4
+
+define dso_local i32 @func() minsize optsize {
+; CHECK-LABEL: @func
+; CHECK:       adrp x8, .L_MergedGlobals
+; CHECK-NEXT:  add x8, x8, :lo12:.L_MergedGlobals
+; CHECK-NEXT:  ldp w9, w8, [x8]
+; CHECK-NEXT:  add w0, w8, w9
+; CHECK-NEXT:  ret
+entry:
+  %0 = load i32, i32* @global0, align 4
+  %1 = load i32, i32* @global1, align 4
+  %add = add nsw i32 %1, %0
+  ret i32 %add
+}