Index: cfe/trunk/lib/Driver/ToolChain.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChain.cpp
+++ cfe/trunk/lib/Driver/ToolChain.cpp
@@ -541,11 +541,30 @@
           << tools::arm::getARMArch(MArch, getTriple()) << "ARM";
     }
 
-    // Assembly files should start in ARM mode, unless arch is M-profile.
-    // Windows is always thumb.
-    if ((InputType != types::TY_PP_Asm && Args.hasFlag(options::OPT_mthumb,
-         options::OPT_mno_thumb, ThumbDefault)) || IsMProfile ||
-         getTriple().isOSWindows()) {
+    // Check to see if an explicit choice to use thumb has been made via
+    // -mthumb. For assembler files we must check for -mthumb in the options
+    // passed to the assember via -Wa or -Xassembler.
+    bool IsThumb = false;
+    if (InputType != types::TY_PP_Asm)
+      IsThumb = Args.hasFlag(options::OPT_mthumb, options::OPT_mno_thumb,
+                              ThumbDefault);
+    else {
+      // Ideally we would check for these flags in
+      // CollectArgsForIntegratedAssembler but we can't change the ArchName at
+      // that point. There is no assembler equivalent of -mno-thumb, -marm, or
+      // -mno-arm.
+      for (const Arg *A :
+           Args.filtered(options::OPT_Wa_COMMA, options::OPT_Xassembler)) {
+        for (StringRef Value : A->getValues()) {
+          if (Value == "-mthumb")
+            IsThumb = true;
+        }
+      }
+    }
+    // Assembly files should start in ARM mode, unless arch is M-profile, or
+    // -mthumb has been passed explicitly to the assembler. Windows is always
+    // thumb.
+    if (IsThumb || IsMProfile || getTriple().isOSWindows()) {
       if (IsBigEndian)
         ArchName = "thumbeb";
       else
Index: cfe/trunk/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChains/Clang.cpp
+++ cfe/trunk/lib/Driver/ToolChains/Clang.cpp
@@ -1889,6 +1889,15 @@
       switch (C.getDefaultToolChain().getArch()) {
       default:
         break;
+      case llvm::Triple::thumb:
+      case llvm::Triple::thumbeb:
+      case llvm::Triple::arm:
+      case llvm::Triple::armeb:
+        if (Value == "-mthumb")
+          // -mthumb has already been processed in ComputeLLVMTriple()
+          // recognize but skip over here.
+          continue;
+
       case llvm::Triple::mips:
       case llvm::Triple::mipsel:
       case llvm::Triple::mips64:
Index: cfe/trunk/test/Driver/arm-target-as-mthumb.s
===================================================================
--- cfe/trunk/test/Driver/arm-target-as-mthumb.s
+++ cfe/trunk/test/Driver/arm-target-as-mthumb.s
@@ -0,0 +1,17 @@
+// Make sure -mthumb does not affect assembler triple, but -Wa,-mthumb or
+// -Xassembler -mthumb does. Also check that -Wa,-mthumb or -Xassembler -mthumb
+// does not affect non assembler files.
+
+// RUN: %clang -target armv7a-linux-gnueabi -### -c -mthumb %s 2>&1 | \
+// RUN: FileCheck -check-prefix=TRIPLE-ARM %s
+// RUN: %clang -target armv7a-linux-gnueabi -### -c -Wa,-mthumb \
+// RUN: %S/Inputs/wildcard1.c  2>&1 | FileCheck -check-prefix=TRIPLE-ARM %s
+
+// TRIPLE-ARM: "-triple" "armv7--linux-gnueabi"
+
+// RUN: %clang -target armv7a-linux-gnueabi -### -c -Wa,-mthumb %s 2>&1 | \
+// RUN: FileCheck -check-prefix=TRIPLE-THUMB %s
+// RUN: %clang -target armv7a-linux-gnueabi -### -c -Xassembler -mthumb %s \
+// RUN: 2>&1 | FileCheck -check-prefix=TRIPLE-THUMB %s
+
+// TRIPLE-THUMB: "-triple" "thumbv7--linux-gnueabi"