Index: lib/CodeGen/GlobalMerge.cpp
===================================================================
--- lib/CodeGen/GlobalMerge.cpp
+++ lib/CodeGen/GlobalMerge.cpp
@@ -615,7 +615,7 @@
     // It's not safe to merge globals that may be preempted
     if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
       continue;
-
+    
     if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
         !GV.hasInternalLinkage())
       continue;
Index: lib/Target/AArch64/AArch64TargetMachine.cpp
===================================================================
--- lib/Target/AArch64/AArch64TargetMachine.cpp
+++ lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -461,7 +461,14 @@
       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();
+    addPass(createGlobalMergePass(TM, 4095, OnlyOptimizeForSize, 
+                                  MergeExternalByDefault));
   }
 
   return false;
Index: test/CodeGen/AArch64/global_merge_aarc64_ac6.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/global_merge_aarc64_ac6.ll
@@ -0,0 +1,35 @@
+;RUN: llc -O3 %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
+
+; Function Attrs: minsize norecurse nounwind optsize readonly
+
+;CHECK-LABEL: @func
+;CHECK: adrp x8, .L_MergedGlobals 
+;CHECK: add x8, x8, :lo12:.L_MergedGlobals
+;CHECK-NEXT: ldp
+;CHECK-NEXT: add
+;CHECK-NEXT: ret
+define dso_local i32 @func() local_unnamed_addr #0 {
+entry:
+  %0 = load i32, i32* @global0, align 4, !tbaa !2
+  %1 = load i32, i32* @global1, align 4, !tbaa !2
+  %add = add nsw i32 %1, %0
+  ret i32 %add
+}
+
+attributes #0 = { minsize norecurse nounwind optsize readonly "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="cortex-a53" "target-features"="+aes,+crc,+crypto,+fp-armv8,+neon,+sha2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 9.0.0 (https://git.llvm.org/git/clang.git/ 7a6d690bb52df60e979a7c7f9d03a774009bb443) (https://git.llvm.org/git/llvm.git/ 71abe189dddb95b0d5782e0866058b9a92ff2402)"}
+!2 = !{!3, !3, i64 0}
+!3 = !{!"int", !4, i64 0}
+!4 = !{!"omnipotent char", !5, i64 0}
+!5 = !{!"Simple C/C++ TBAA"}