Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -547,6 +547,18 @@
   }
 }
 
+static void writeDefFile(StringRef Name) {
+  std::error_code EC;
+  raw_fd_ostream OS(Name, EC, sys::fs::F_None);
+  if (EC)
+    fatal("cannot open " + Name + ": " + EC.message());
+
+  OS << "EXPORTS\n";
+  for (Export &E : Config->Exports)
+    OS << "    " << E.ExportName << " "
+       << "@" << E.Ordinal << "\n";
+}
+
 // A helper function for filterBitcodeFiles.
 static bool needsRebuilding(MemoryBufferRef MB) {
   // The MSVC linker doesn't support thin archives, so if it's a thin
@@ -1197,6 +1209,53 @@
       return;
   }
 
+  if (Config->DLL && ((Config->MinGW && Config->Exports.empty()) ||
+                      Args.hasArg(OPT_export_all_symbols))) {
+    std::vector<std::string> ExcludeSymbols = {
+        "_NULL_IMPORT_DESCRIPTOR",
+        // Runtime pseudo-reloc.
+        "_pei386_runtime_relocator",
+        "do_pseudo_reloc",
+        // Global vars that should not be exported.
+        "impure_ptr",
+        "_impure_ptr",
+        "_fmode",
+        "environ",
+        "__dso_handle",
+        // Entry point symbols, and entry hooks; these are the MinGW
+        // names that differ from the standard ones (lacking an extra
+        // underscore).
+        "DllMain@12",
+        "DllEntryPoint@0",
+        "DllMainCRTStartup@12",
+    };
+
+    if (Config->Machine == I386) {
+      // For I386, prepend the underscore - all the symbols in the exclude list
+      // are cdecl/stdcall that should have an underscore.
+      for (auto &Str : ExcludeSymbols)
+        Str = "_" + Str;
+    } else {
+      // For non-I386, drop the stdcall decoration.
+      for (auto &Str : ExcludeSymbols)
+        Str = Str.substr(0, Str.find('@'));
+    }
+    std::set<StringRef> ExcludeSymbolSet(ExcludeSymbols.begin(),
+                                         ExcludeSymbols.end());
+
+    Symtab->forEachSymbol([=](Symbol *S) {
+      auto *Def = dyn_cast<Defined>(S->body());
+      if (Def && Def->isLive() && Def->getChunk()) {
+        if (ExcludeSymbolSet.find(Def->getName()) != ExcludeSymbolSet.end())
+          return;
+        Export E;
+        E.Name = Def->getName();
+        E.Sym = Def;
+        Config->Exports.push_back(E);
+      }
+    });
+  }
+
   // Windows specific -- when we are creating a .dll file, we also
   // need to create a .lib file.
   if (!Config->Exports.empty() || Config->DLL) {
@@ -1205,6 +1264,10 @@
     assignExportOrdinals();
   }
 
+  // Handle /output-def
+  if (auto *Arg = Args.getLastArg(OPT_output_def))
+    writeDefFile(Arg->getValue());
+
   // Set extra alignment for .comm symbols
   for (auto Pair : Config->AlignComm) {
     StringRef Name = Pair.first;
Index: COFF/Options.td
===================================================================
--- COFF/Options.td
+++ COFF/Options.td
@@ -102,10 +102,12 @@
 def help_q : Flag<["/?", "-?"], "">, Alias<help>;
 
 // LLD extensions
+def export_all_symbols : F<"export-all-symbols">;
 def nopdb : F<"nopdb">, HelpText<"Disable PDB generation for DWARF users">;
 def nosymtab : F<"nosymtab">;
 def lldmingw : F<"lldmingw">;
 def msvclto : F<"msvclto">;
+def output_def : Joined<["/", "-"], "output-def:">;
 def rsp_quoting : Joined<["--"], "rsp-quoting=">,
   HelpText<"Quoting style for response files, 'windows' (default) or 'posix'">;
 
Index: test/COFF/export-all.s
===================================================================
--- /dev/null
+++ test/COFF/export-all.s
@@ -0,0 +1,38 @@
+# REQEUIRES: x86
+
+# RUN: llvm-mc -triple=i686-windows-gnu %s -filetype=obj -o %t.obj
+
+# RUN: lld-link -lldmingw -dll -out:%t.dll -entry:DllMainCRTStartup@12 %t.obj -implib:%t.lib
+# RUN: llvm-readobj -coff-exports %t.dll | FileCheck %s
+
+# CHECK-NOT: Name: DllMainCRTStartup
+# CHECK: Name: foobar
+
+.global _foobar
+.global _DllMainCRTStartup@12
+.text
+_DllMainCRTStartup@12:
+  ret
+_foobar:
+  ret
+
+# Test specifying -export-all-symbols, on an object file that contains axi
+# dllexport directive for some of the symbols.
+
+# RUN: yaml2obj < %p/Inputs/export.yaml > %t.obj
+#
+# RUN: lld-link -out:%t.dll -dll %t.obj -lldmingw -export-all-symbols -output-def:%t.def
+# RUN: llvm-readobj -coff-exports %t.dll | FileCheck -check-prefix=CHECK2 %s
+# RUN: cat %t.def | FileCheck -check-prefix=CHECK2-DEF %s
+
+# Note, this will actually export _DllMainCRTStartup as well, since
+# it uses the standard spelling in this object file, not the MinGW one.
+
+# CHECK2: Name: exportfn1
+# CHECK2: Name: exportfn2
+# CHECK2: Name: exportfn3
+
+# CHECK2-DEF: EXPORTS
+# CHECK2-DEF: exportfn1 @3
+# CHECK2-DEF: exportfn2 @4
+# CHECK2-DEF: exportfn3 @5