Index: lib/CodeGen/AsmPrinter/AsmPrinter.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1349,9 +1349,13 @@
     const TargetLoweringObjectFile &Obj = getObjFileLowering();
     const MCSymbol *KeySym = nullptr;
     const MCSection *KeySec = nullptr;
-    if (S.ComdatKey) {
-      KeySym = getSymbol(S.ComdatKey);
-      KeySec = getObjFileLowering().SectionForGlobal(S.ComdatKey, *Mang, TM);
+    if (GlobalValue *GV = S.ComdatKey) {
+      if (GV->hasAvailableExternallyLinkage())
+        // Don't dynamically initialize available_externally vars.
+        continue;
+
+      KeySym = getSymbol(GV);
+      KeySec = getObjFileLowering().SectionForGlobal(GV, *Mang, TM);
     }
     const MCSection *OutputSection =
         (isCtor ? Obj.getStaticCtorSection(S.Priority, KeySym, KeySec)
Index: test/MC/COFF/global_ctors_dtors.ll
===================================================================
--- test/MC/COFF/global_ctors_dtors.ll
+++ test/MC/COFF/global_ctors_dtors.ll
@@ -11,9 +11,10 @@
 
 %ini = type { i32, void()*, i8* }
 
-@llvm.global_ctors = appending global [2 x %ini ] [
+@llvm.global_ctors = appending global [3 x %ini ] [
   %ini { i32 65535, void ()* @a_global_ctor, i8* null },
-  %ini { i32 65535, void ()* @b_global_ctor, i8* bitcast (i32* @b to i8*) }
+  %ini { i32 65535, void ()* @b_global_ctor, i8* bitcast (i32* @b to i8*) },
+  %ini { i32 65535, void ()* @c_global_ctor, i8* bitcast (i32* @c to i8*) }
 ]
 @llvm.global_dtors = appending global [1 x %ini ] [%ini { i32 65535, void ()* @a_global_dtor, i8* null }]
 
@@ -26,11 +27,18 @@
 
 @b = global i32 zeroinitializer
 
+@c = available_externally dllimport global i32 zeroinitializer
+
 define void @b_global_ctor() nounwind {
   store i32 42, i32* @b
   ret void
 }
 
+define void @c_global_ctor() nounwind {
+  store i32 42, i32* @c
+  ret void
+}
+
 define void @a_global_dtor() nounwind {
   %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8]* @.str2, i32 0, i32 0))
   ret void
@@ -45,11 +53,13 @@
 ; WIN32: a_global_ctor
 ; WIN32: .section .CRT$XCU,"rd",associative .bss,{{_?}}b
 ; WIN32: b_global_ctor
+; WIN32-NOT: c_global_ctor
 ; WIN32: .section .CRT$XTX,"rd"
 ; WIN32: a_global_dtor
 ; MINGW32: .section .ctors,"wd"
 ; MINGW32: a_global_ctor
 ; MINGW32: .section .ctors,"wd",associative .bss,{{_?}}b
 ; MINGW32: b_global_ctor
+; MINGW32-NOT: c_global_ctor
 ; MINGW32: .section .dtors,"wd"
 ; MINGW32: a_global_dtor