diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp b/llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
--- a/llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
@@ -445,7 +445,7 @@
     // Force the use of an ELF container.
     MAI = new X86ELFMCAsmInfo(TheTriple);
   } else if (TheTriple.isWindowsMSVCEnvironment() ||
-             TheTriple.isWindowsCoreCLREnvironment()) {
+             TheTriple.isWindowsCoreCLREnvironment() || TheTriple.isUEFI()) {
     if (Options.getAssemblyLanguage().equals_insensitive("masm"))
       MAI = new X86MCAsmInfoMicrosoftMASM(TheTriple);
     else
@@ -453,8 +453,6 @@
   } else if (TheTriple.isOSCygMing() ||
              TheTriple.isWindowsItaniumEnvironment()) {
     MAI = new X86MCAsmInfoGNUCOFF(TheTriple);
-  } else if (TheTriple.isUEFI()) {
-    MAI = new X86MCAsmInfoGNUCOFF(TheTriple);
   } else {
     // The default is ELF.
     MAI = new X86ELFMCAsmInfo(TheTriple);
diff --git a/llvm/lib/Target/X86/X86CallingConv.td b/llvm/lib/Target/X86/X86CallingConv.td
--- a/llvm/lib/Target/X86/X86CallingConv.td
+++ b/llvm/lib/Target/X86/X86CallingConv.td
@@ -1127,6 +1127,9 @@
   // Mingw64 and native Win64 use Win64 CC
   CCIfSubtarget<"isTargetWin64()", CCDelegateTo<CC_X86_Win64_C>>,
 
+  // UEFI64 uses Win64 CC
+  CCIfSubtarget<"isTargetUEFI64()", CCDelegateTo<CC_X86_Win64_C>>,
+
   // Otherwise, drop to normal X86-64 CC
   CCDelegateTo<CC_X86_64_C>
 ]>;
diff --git a/llvm/lib/Target/X86/X86RegisterInfo.h b/llvm/lib/Target/X86/X86RegisterInfo.h
--- a/llvm/lib/Target/X86/X86RegisterInfo.h
+++ b/llvm/lib/Target/X86/X86RegisterInfo.h
@@ -27,7 +27,7 @@
   ///
   bool Is64Bit;
 
-  /// IsWin64 - Is the target on of win64 flavours
+  /// IsWin64 - Is the target one of win64 flavours
   ///
   bool IsWin64;
 
diff --git a/llvm/lib/Target/X86/X86Subtarget.h b/llvm/lib/Target/X86/X86Subtarget.h
--- a/llvm/lib/Target/X86/X86Subtarget.h
+++ b/llvm/lib/Target/X86/X86Subtarget.h
@@ -334,6 +334,10 @@
 
   bool isOSWindows() const { return TargetTriple.isOSWindows(); }
 
+  bool isUEFI() const { return TargetTriple.isUEFI(); }
+
+  bool isTargetUEFI64() const { return Is64Bit && isUEFI(); }
+
   bool isTargetWin64() const { return Is64Bit && isOSWindows(); }
 
   bool isTargetWin32() const { return !Is64Bit && isOSWindows(); }
@@ -349,7 +353,8 @@
 
   bool isCallingConvWin64(CallingConv::ID CC) const {
     switch (CC) {
-    // On Win64, all these conventions just use the default convention.
+    // On Win64 and UEFI64, all these conventions just use the default
+    // convention.
     case CallingConv::C:
     case CallingConv::Fast:
     case CallingConv::Tail:
@@ -360,7 +365,7 @@
     case CallingConv::X86_ThisCall:
     case CallingConv::X86_VectorCall:
     case CallingConv::Intel_OCL_BI:
-      return isTargetWin64();
+      return isTargetWin64() || isTargetUEFI64();
     // This convention allows using the Win64 convention on other targets.
     case CallingConv::Win64:
       return true;