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

[pcc]

Summary

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.

[rnk]

lgtm

llvm/trunk/lib/Object/IRObjectFile.cpp

std::error_code IRObjectFile::printSymbolName(raw_ostream &OS,
   const GlobalValue *GV = getGV(Symb);
   if (!GV) {
     unsigned Index = getAsmSymIndex(Symb);
     assert(Index <= AsmSymbols.size());
     OS << AsmSymbols[Index].first;
     return std::error_code();
   }
 
+  if (GV->hasDLLImportStorageClass())
+    OS << "__imp_";
+
   if (Mang)
     Mang->getNameWithPrefix(OS, GV, false);
   else
     OS << GV->getName();
 
   return std::error_code();
 }
 

llvm/trunk/test/CodeGen/X86/dllimport.ll

 ; CHECK-DAG: movl __imp__Var1, [[R1:%[a-z]{3}]]
 ; CHECK-DAG: movl __imp__Var2, [[R2:%[a-z]{3}]]
   %1 = load i32, i32* @Var1
   %2 = load i32, i32* @Var2
   call void(...) @dummy(i32 %1, i32 %2)
 
   ret void
 }
+
+; CHECK: _fp:
+; CHECK-NEXT: .long _fun
+@fp = constant void ()* @fun

llvm/trunk/test/Object/dllimport.ll

+; 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
+}