Index: lib/Target/ARM/AsmParser/ARMAsmParser.cpp
===================================================================
--- lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -10012,8 +10012,8 @@
     case 'w':
       break;
     default:
-      return Error(Loc, "cannot determine Thumb instruction size, "
-                        "use inst.n/inst.w instead");
+      Width = 0;
+      break;
     }
   } else {
     if (Suffix)
@@ -10029,6 +10029,7 @@
       return Error(Loc, "expected constant expression");
     }
 
+    char CurSuffix = Suffix;
     switch (Width) {
     case 2:
       if (Value->getValue() > 0xffff)
@@ -10039,11 +10040,21 @@
         return Error(Loc, StringRef(Suffix ? "inst.w" : "inst") +
                               " operand is too big");
       break;
+    case 0:
+      // Thumb mode, no width indicated. Guess from the opcode, if possible.
+      if (Value->getValue() < 0xe800)
+        CurSuffix = 'n';
+      else if (Value->getValue() >= 0xe8000000)
+        CurSuffix = 'w';
+      else
+        return Error(Loc, "cannot determine Thumb instruction size, "
+                          "use inst.n/inst.w instead");
+      break;
     default:
       llvm_unreachable("only supported widths are 2 and 4");
     }
 
-    getTargetStreamer().emitInst(Value->getValue(), Suffix);
+    getTargetStreamer().emitInst(Value->getValue(), CurSuffix);
     return false;
   };
 
Index: test/MC/ARM/inst-thumb-suffixes-auto.s
===================================================================
--- /dev/null
+++ test/MC/ARM/inst-thumb-suffixes-auto.s
@@ -0,0 +1,16 @@
+@ RUN: llvm-mc %s -triple armv7-linux-gnueabi -filetype asm -o - \
+@ RUN:   | FileCheck %s
+@ RUN: llvm-mc %s -triple armebv7-linux-gnueabi -filetype asm -o - \
+@ RUN:   | FileCheck %s
+
+	.syntax unified
+	.thumb
+
+	.align 2
+	.global inst_n
+	.type inst_n,%function
+inst_n:
+	@ bx lr, mov.w r0, #42
+	.inst 0x4770, 0xf04f002a
+@ CHECK: .inst.n 0x4770
+@ CHECK: .inst.w 0xf04f002a
Index: test/MC/ARM/inst-thumb-suffixes.s
===================================================================
--- test/MC/ARM/inst-thumb-suffixes.s
+++ test/MC/ARM/inst-thumb-suffixes.s
@@ -8,6 +8,6 @@
 	.global suffixes_required_in_thumb
 	.type suffixes_required_in_thumb,%function
 suffixes_required_in_thumb:
-	.inst 0x0000
+	.inst 0xff00
 @ CHECK-ERROR: cannot determine Thumb instruction size, use inst.n/inst.w instead