Index: lib/MC/MCObjectFileInfo.cpp
===================================================================
--- lib/MC/MCObjectFileInfo.cpp
+++ lib/MC/MCObjectFileInfo.cpp
@@ -598,7 +598,10 @@
   // Set the `IMAGE_SCN_MEM_16BIT` flag when compiling for thumb mode.  This is
   // used to indicate to the linker that the text segment contains thumb instructions
   // and to set the ISA selection bit for calls accordingly.
-  const bool IsThumb = T.getArch() == Triple::thumb;
+  // In practice, COFF currently is only supported on Windows, and Windows
+  // on ARM is thumb only. When invoked via the clang driver, the triple
+  // is set to arm-* since the assembler always should start in ARM mode.
+  const bool IsThumb = T.getArch() == Triple::arm || T.getArch() == Triple::thumb;
 
   CommDirectiveSupportsAlignment = true;
 
Index: test/MC/ARM/Windows/thumb-attributes.s
===================================================================
--- /dev/null
+++ test/MC/ARM/Windows/thumb-attributes.s
@@ -0,0 +1,25 @@
+@ RUN: llvm-mc -triple armv7-windows-itanium -filetype obj -o - %s \
+@ RUN:   | llvm-readobj -s - | FileCheck %s
+
+	.syntax unified
+	.thumb
+
+	.text
+
+	.global function
+	.thumb_func
+function:
+	bx lr
+
+@ CHECK: Sections [
+@ CHECK:   Section {
+@ CHECK:     Name: .text
+@ CHECK:     Characteristics [
+@ CHECK:       IMAGE_SCN_ALIGN_4BYTES
+@ CHECK:       IMAGE_SCN_CNT_CODE
+@ CHECK:       IMAGE_SCN_MEM_16BIT
+@ CHECK:       IMAGE_SCN_MEM_EXECUTE
+@ CHECK:       IMAGE_SCN_MEM_READ
+@ CHECK:     ]
+@ CHECK:   }
+@ CHECK: ]