diff --git a/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp b/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp
--- a/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp
+++ b/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp
@@ -11,12 +11,14 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ToolDrivers/llvm-dlltool/DlltoolDriver.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Object/COFFImportFile.h"
 #include "llvm/Object/COFFModuleDefinition.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/Option.h"
+#include "llvm/Support/Host.h"
 #include "llvm/Support/Path.h"
 
 #include <vector>
@@ -51,10 +53,8 @@
   DllOptTable() : OptTable(InfoTable, false) {}
 };
 
-} // namespace
-
 // Opens a file. Path has to be resolved already.
-static std::unique_ptr<MemoryBuffer> openFile(const Twine &Path) {
+std::unique_ptr<MemoryBuffer> openFile(const Twine &Path) {
   ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MB = MemoryBuffer::getFile(Path);
 
   if (std::error_code EC = MB.getError()) {
@@ -65,7 +65,7 @@
   return std::move(*MB);
 }
 
-static MachineTypes getEmulation(StringRef S) {
+MachineTypes getEmulation(StringRef S) {
   return StringSwitch<MachineTypes>(S)
       .Case("i386", IMAGE_FILE_MACHINE_I386)
       .Case("i386:x86-64", IMAGE_FILE_MACHINE_AMD64)
@@ -74,6 +74,47 @@
       .Default(IMAGE_FILE_MACHINE_UNKNOWN);
 }
 
+MachineTypes getMachine(Triple T) {
+  switch (T.getArch()) {
+  case Triple::x86:
+    return COFF::IMAGE_FILE_MACHINE_I386;
+  case Triple::x86_64:
+    return COFF::IMAGE_FILE_MACHINE_AMD64;
+  case Triple::arm:
+    return COFF::IMAGE_FILE_MACHINE_ARMNT;
+  case Triple::aarch64:
+    return COFF::IMAGE_FILE_MACHINE_ARM64;
+  default:
+    return COFF::IMAGE_FILE_MACHINE_UNKNOWN;
+  }
+}
+
+MachineTypes getDefaultMachine() {
+  return getMachine(Triple(sys::getDefaultTargetTriple()));
+}
+
+static bool consume_back_lower(StringRef &S, const char *Str) {
+  if (!S.endswith_lower(Str))
+    return false;
+  S = S.drop_back(strlen(Str));
+  return true;
+}
+
+Optional<std::string> getPrefix(StringRef Argv0) {
+  StringRef ProgName = llvm::sys::path::stem(Argv0);
+  // x86_64-w64-mingw32-dlltool -> x86_64-w64-mingw32
+  // llvm-dlltool -> None
+  // aarch64-w64-mingw32-llvm-dlltool-10.exe -> aarch64-w64-mingw32
+  ProgName = ProgName.rtrim("0123456789.-");
+  if (!consume_back_lower(ProgName, "dlltool"))
+    return None;
+  consume_back_lower(ProgName, "llvm-");
+  consume_back_lower(ProgName, "-");
+  return ProgName.str();
+}
+
+} // namespace
+
 int llvm::dlltoolDriverMain(llvm::ArrayRef<const char *> ArgsArr) {
   DllOptTable Table;
   unsigned MissingIndex;
@@ -94,12 +135,6 @@
     return 1;
   }
 
-  if (!Args.hasArgNoClaim(OPT_m) && Args.hasArgNoClaim(OPT_d)) {
-    llvm::errs() << "error: no target machine specified\n"
-                 << "supported targets: i386, i386:x86-64, arm, arm64\n";
-    return 1;
-  }
-
   for (auto *Arg : Args.filtered(OPT_UNKNOWN))
     llvm::errs() << "ignoring unknown argument: " << Arg->getAsString(Args)
                  << "\n";
@@ -119,7 +154,12 @@
     return 1;
   }
 
-  COFF::MachineTypes Machine = IMAGE_FILE_MACHINE_UNKNOWN;
+  COFF::MachineTypes Machine = getDefaultMachine();
+  if (Optional<std::string> Prefix = getPrefix(ArgsArr[0])) {
+    Triple T(*Prefix);
+    if (T.getArch() != Triple::UnknownArch)
+      Machine = getMachine(T);
+  }
   if (auto *Arg = Args.getLastArg(OPT_m))
     Machine = getEmulation(Arg->getValue());
 
diff --git a/llvm/test/tools/llvm-dlltool/triple-prefix.def b/llvm/test/tools/llvm-dlltool/triple-prefix.def
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-dlltool/triple-prefix.def
@@ -0,0 +1,27 @@
+;; Don't make symlinks on Windows.
+; UNSUPPORTED: system-windows
+
+; RUN: rm -rf %t
+; RUN: mkdir %t
+; RUN: ln -s llvm-dlltool %t/i686-w64-mingw32-llvm-dlltool-10.0.exe
+; RUN: ln -s llvm-dlltool %t/x86_64-w64-mingw32-llvm-dlltool-10
+; RUN: ln -s llvm-dlltool %t/armv7-w64-mingw32-dlltool
+; RUN: ln -s llvm-dlltool %t/aarch64-w64-mingw32-DLLTOOL.exe
+
+; RUN: %t/i686-w64-mingw32-llvm-dlltool-10.0.exe -d %s -l %t.a
+; RUN: llvm-readobj %t.a | FileCheck --check-prefix=I386 %s
+; RUN: %t/x86_64-w64-mingw32-llvm-dlltool-10 -d %s -l %t.a
+; RUN: llvm-readobj %t.a | FileCheck --check-prefix=X86_64 %s
+; RUN: %t/armv7-w64-mingw32-dlltool -d %s -l %t.a
+; RUN: llvm-readobj %t.a | FileCheck --check-prefix=ARM %s
+; RUN: %t/aarch64-w64-mingw32-DLLTOOL.exe -d %s -l %t.a
+; RUN: llvm-readobj %t.a | FileCheck --check-prefix=ARM64 %s
+
+LIBRARY test.dll
+EXPORTS
+TestFunction
+
+; I386: Format: COFF-i386
+; X86_64: Format: COFF-x86-64
+; ARM: Format: COFF-ARM{{$}}
+; ARM64: Format: COFF-ARM64