Index: llvm/trunk/lib/Transforms/IPO/GlobalOpt.cpp
===================================================================
--- llvm/trunk/lib/Transforms/IPO/GlobalOpt.cpp
+++ llvm/trunk/lib/Transforms/IPO/GlobalOpt.cpp
@@ -1980,10 +1980,12 @@
 static bool isSimpleEnoughValueToCommitHelper(Constant *C,
                                    SmallPtrSet<Constant*, 8> &SimpleConstants,
                                    const DataLayout *DL) {
-  // Simple integer, undef, constant aggregate zero, global addresses, etc are
-  // all supported.
-  if (C->getNumOperands() == 0 || isa<BlockAddress>(C) ||
-      isa<GlobalValue>(C))
+  // Simple global addresses are supported, do not allow dllimport globals.
+  if (auto *GV = dyn_cast<GlobalValue>(C))
+    return !GV->hasDLLImportStorageClass();
+
+  // Simple integer, undef, constant aggregate zero, etc are all supported.
+  if (C->getNumOperands() == 0 || isa<BlockAddress>(C))
     return true;
 
   // Aggregate values are safe if all their elements are.
@@ -2054,8 +2056,7 @@
     return false;
 
   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C))
-    // Do not allow weak/*_odr/linkonce/dllimport/dllexport linkage or
-    // external globals.
+    // Do not allow weak/*_odr/linkonce linkage or external globals.
     return GV->hasUniqueInitializer();
 
   if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
Index: llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll
===================================================================
--- llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll
+++ llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll
@@ -50,7 +50,31 @@
   ret void
 }
 
+; PR19955
+
+@dllimportptr = global i32* null, align 4
+; CHECK: @dllimportptr = global i32* null, align 4
+@dllimportvar = external dllimport global i32
+define internal void @test3() {
+entry:
+  store i32* @dllimportvar, i32** @dllimportptr, align 4
+  ret void
+}
+
+@dllexportptr = global i32* null, align 4
+; CHECK: @dllexportptr = global i32* @dllexportvar, align 4
+@dllexportvar = dllexport global i32 0, align 4
+; CHECK: @dllexportvar = dllexport global i32 20, align 4
+define internal void @test4() {
+entry:
+  store i32 20, i32* @dllexportvar, align 4
+  store i32* @dllexportvar, i32** @dllexportptr, align 4
+  ret void
+}
+
 @llvm.global_ctors = appending constant
-  [2 x { i32, void ()* }]
+  [4 x { i32, void ()* }]
   [{ i32, void ()* } { i32 65535, void ()* @test1 },
-   { i32, void ()* } { i32 65535, void ()* @test2 }]
+   { i32, void ()* } { i32 65535, void ()* @test2 },
+   { i32, void ()* } { i32 65535, void ()* @test3 },
+   { i32, void ()* } { i32 65535, void ()* @test4 }]