Object: Prepend __imp_ when mangling a dllimport symbol in IRObjectFile.

We cannot prepend __imp_ in the IR mangler because a function reference may
be emitted unmangled in a constant initializer. The linker is expected to
resolve such references to thunks. This is covered by the new test case.

Strictly speaking we ought to emit two undefined symbols, one with __imp_ and
one without, as we cannot know which symbol the final object file will refer
to. However, this would require rather intrusive changes to IRObjectFile,
and lld works fine without it for now.

This reimplements r239437, which was reverted in r239502.

Differential Revision: http://reviews.llvm.org/D10400

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239560 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Object/IRObjectFile.cpp

@@ -198,6 +198,9 @@ std::error_code IRObjectFile::printSymbolName(raw_ostream &OS,
     return std::error_code();
   }
 
+  if (GV->hasDLLImportStorageClass())
+    OS << "__imp_";
+
   if (Mang)
     Mang->getNameWithPrefix(OS, GV, false);
   else

test/CodeGen/X86/dllimport.ll

@@ -57,3 +57,7 @@ define void @use() nounwind {
 
   ret void
 }
+
+; CHECK: _fp:
+; CHECK-NEXT: .long _fun
+@fp = constant void ()* @fun

test/Object/dllimport.ll

@@ -0,0 +1,17 @@
+; RUN: llvm-as %s -o - | llvm-nm - | FileCheck %s
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+; CHECK: U __imp_f
+; CHECK: U __imp_v
+; CHECK: T g
+
+declare dllimport void @f()
+@v = external dllimport global i32
+
+define void @g() {
+  call void @f()
+  store i32 42, i32* @v
+  ret void
+}