Index: lib/Target/ARM/ARMTargetMachine.cpp
===================================================================
--- lib/Target/ARM/ARMTargetMachine.cpp
+++ lib/Target/ARM/ARMTargetMachine.cpp
@@ -412,7 +412,10 @@
     // 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();
+    // Disable merging of extern globals for windows, since it would drop
+    // related dllexport attributes.
+    bool MergeExternalByDefault = !TM->getTargetTriple().isOSBinFormatMachO() &&
+                                  !TM->getTargetTriple().isOSWindows();
     addPass(createGlobalMergePass(TM, 127, OnlyOptimizeForSize,
                                   MergeExternalByDefault));
   }
Index: test/CodeGen/ARM/global-merge-external.ll
===================================================================
--- test/CodeGen/ARM/global-merge-external.ll
+++ test/CodeGen/ARM/global-merge-external.ll
@@ -3,6 +3,7 @@
 ; RUN: llc < %s -mtriple=arm-eabi  -arm-global-merge -global-merge-on-external=false | FileCheck %s --check-prefixes=CHECK,CHECK-NO-MERGE
 ; RUN: llc < %s -mtriple=arm-macho -arm-global-merge                                 | FileCheck %s --check-prefixes=CHECK,CHECK-NO-MERGE
 ; RUN: llc < %s -mtriple=arm-eabi  -arm-global-merge -relocation-model=pic           | FileCheck %s --check-prefixes=CHECK,CHECK-NO-MERGE
+; RUN llc < %s -mtriple=thumbv7-win32 -arm-global-merge                             | FileCheck %s --check-prefix=CHECK-WIN32-NO-MERGE
 
 @x = global i32 0, align 4
 @y = global i32 0, align 4
@@ -14,6 +15,9 @@
 ;CHECK:          [[LABEL1]]:
 ;CHECK-MERGE:    .long .L_MergedGlobals
 ;CHECK-NO-MERGE: .long {{_?x}}
+;CHECK-WIN32-NO-MERGE: f1:
+;CHECK-WIN32-NO-MERGE: movw: [[REG1:r[0-9]+]], :lower16:x
+;CHECK-WIN32-NO-MERGE: movt: [[REG1]], :upper16:x
   store i32 %a1, i32* @x, align 4
   store i32 %a2, i32* @y, align 4
   ret void
@@ -25,6 +29,9 @@
 ;CHECK:          [[LABEL2]]:
 ;CHECK-MERGE:    .long .L_MergedGlobals
 ;CHECK-NO-MERGE: .long {{_?y}}
+;CHECK-WIN32-NO-MERGE: g1:
+;CHECK-WIN32-NO-MERGE: movw: [[REG2:r[0-9]+]], :lower16:y
+;CHECK-WIN32-NO-MERGE: movt: [[REG2]], :upper16:y
   store i32 %a1, i32* @y, align 4
   store i32 %a2, i32* @z, align 4
   ret void