Index: include/clang/Basic/Attr.td
===================================================================
--- include/clang/Basic/Attr.td
+++ include/clang/Basic/Attr.td
@@ -299,6 +299,9 @@
 def BlocksSupported : LangOpt<"Blocks">;
 def ObjCAutoRefCount : LangOpt<"ObjCAutoRefCount">;
 
+// Language option for CMSE extensions
+def Cmse : LangOpt<"Cmse">;
+
 // Defines targets for target-specific attributes. Empty lists are unchecked.
 class TargetSpec {
   // Specifies Architectures for which the target applies, based off the
Index: include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- include/clang/Basic/DiagnosticCommonKinds.td
+++ include/clang/Basic/DiagnosticCommonKinds.td
@@ -252,6 +252,8 @@
   "the %0 sub-architecture does not support unaligned accesses">;
 def err_target_unsupported_execute_only : Error<
   "execute only is not supported for the %0 sub-architecture">;
+def err_target_unsupported_mcmse : Error<
+  "-mcmse is not supported for %0">;
 def err_opt_not_valid_with_opt : Error<
   "option '%0' cannot be specified with '%1'">;
 def err_opt_not_valid_without_opt : Error<
Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -300,6 +300,8 @@
                                            "field padding (0: none, 1:least "
                                            "aggressive, 2: more aggressive)")
 
+LANGOPT(Cmse, 1, 0, "ARM Security extensions support")
+
 LANGOPT(XRayInstrument, 1, 0, "controls whether to do XRay instrumentation")
 LANGOPT(XRayAlwaysEmitCustomEvents, 1, 0,
         "controls whether to always emit intrinsic calls to "
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -2143,6 +2143,9 @@
   HelpText<"Disallow use of CRC instructions (ARM only)">;
 def mno_neg_immediates: Flag<["-"], "mno-neg-immediates">, Group<m_arm_Features_Group>,
   HelpText<"Disallow converting instructions with negative immediates to their negation or inversion.">;
+def mcmse : Flag<["-"], "mcmse">, Group<m_arm_Features_Group>,
+  Flags<[DriverOption,CC1Option]>,
+  HelpText<"Allow use of CMSE (Armv8-M Security Extensions)">;
 
 def mgeneral_regs_only : Flag<["-"], "mgeneral-regs-only">, Group<m_aarch64_Features_Group>,
   HelpText<"Generate code which only uses the general purpose registers (AArch64 only)">;
Index: lib/Basic/Targets/ARM.cpp
===================================================================
--- lib/Basic/Targets/ARM.cpp
+++ lib/Basic/Targets/ARM.cpp
@@ -434,6 +434,11 @@
       DSP = 1;
     } else if (Feature == "+fp-only-sp") {
       HW_FP_remove |= HW_FP_DP;
+    } else if (Feature == "+8msecext") {
+      if (CPUProfile != "M" || ArchVersion != 8) {
+        Diags.Report(diag::err_target_unsupported_mcmse) << CPU;
+        return false;
+      }
     } else if (Feature == "+strict-align") {
       Unaligned = 0;
     } else if (Feature == "+fp16") {
@@ -713,6 +718,10 @@
 
   Builder.defineMacro("__ARM_SIZEOF_MINIMAL_ENUM", Opts.ShortEnums ? "1" : "4");
 
+  // CMSE
+  if (ArchVersion == 8 && ArchProfile == llvm::ARM::ProfileKind::M)
+    Builder.defineMacro("__ARM_FEATURE_CMSE", Opts.Cmse ? "3" : "1");
+
   if (ArchVersion >= 6 && CPUAttr != "6M" && CPUAttr != "8M_BASE") {
     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1");
     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2");
Index: lib/Driver/ToolChains/Arch/ARM.cpp
===================================================================
--- lib/Driver/ToolChains/Arch/ARM.cpp
+++ lib/Driver/ToolChains/Arch/ARM.cpp
@@ -469,6 +469,10 @@
     }
   }
 
+  // CMSE: Check for target 8M (for -mcmse to be applicable) is performed later.
+  if (Args.getLastArg(options::OPT_mcmse))
+    Features.push_back("+8msecext");
+
   // Look for the last occurrence of -mlong-calls or -mno-long-calls. If
   // neither options are specified, see if we are compiling for kernel/kext and
   // decide whether to pass "+long-calls" based on the OS and its version.
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -1423,6 +1423,9 @@
   if (!Args.hasFlag(options::OPT_mimplicit_float,
                     options::OPT_mno_implicit_float, true))
     CmdArgs.push_back("-no-implicit-float");
+
+  if (Args.getLastArg(options::OPT_mcmse))
+    CmdArgs.push_back("-mcmse");
 }
 
 void Clang::RenderTargetOptions(const llvm::Triple &EffectiveTriple,
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -2729,6 +2729,7 @@
   Opts.HalfArgsAndReturns = Args.hasArg(OPT_fallow_half_arguments_and_returns)
                             | Opts.NativeHalfArgsAndReturns;
   Opts.GNUAsm = !Args.hasArg(OPT_fno_gnu_inline_asm);
+  Opts.Cmse = Args.hasArg(OPT_mcmse); // Armv8-M Security Extensions
 
   // __declspec is enabled by default for the PS4 by the driver, and also
   // enabled for Microsoft Extensions or Borland Extensions, here.
Index: test/Preprocessor/arm-target-features.c
===================================================================
--- test/Preprocessor/arm-target-features.c
+++ test/Preprocessor/arm-target-features.c
@@ -198,6 +198,7 @@
 // V8M_BASELINE: #define __ARM_ARCH_ISA_THUMB 1
 // V8M_BASELINE: #define __ARM_ARCH_PROFILE 'M'
 // V8M_BASELINE-NOT: __ARM_FEATURE_CRC32
+// V8M_BASELINE: #define __ARM_FEATURE_CMSE 1
 // V8M_BASELINE-NOT: __ARM_FEATURE_DSP
 // V8M_BASELINE-NOT: __ARM_FP 0x{{.*}}
 // V8M_BASELINE-NOT: __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1
@@ -210,6 +211,7 @@
 // V8M_MAINLINE: #define __ARM_ARCH_ISA_THUMB 2
 // V8M_MAINLINE: #define __ARM_ARCH_PROFILE 'M'
 // V8M_MAINLINE-NOT: __ARM_FEATURE_CRC32
+// V8M_MAINLINE: #define __ARM_FEATURE_CMSE 1
 // V8M_MAINLINE-NOT: __ARM_FEATURE_DSP
 // V8M_MAINLINE-NOT: #define __ARM_FP 0x
 // V8M_MAINLINE: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
@@ -675,6 +677,24 @@
 // M7-THUMB-ALLOW-FP-INSTR:#define __ARM_FP 0xe
 // M7-THUMB-ALLOW-FP-INSTR:#define __ARM_FPV5__ 1
 
+// Check that -mcmse (security extension) option works correctly for v8-M targets
+// RUN: %clang -target armv8m.base-none-linux-gnu -mcmse -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=V8M_CMSE %s
+// RUN: %clang -target armv8m.main-none-linux-gnu -mcmse -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=V8M_CMSE %s
+// RUN: %clang -target arm-none-linux-gnu -mcpu=cortex-m33 -mcmse -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=V8M_CMSE %s
+// RUN: %clang -target arm -mcpu=cortex-m23 -mcmse -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=V8M_CMSE %s
+// V8M_CMSE-NOT: __ARM_FEATURE_CMSE 1
+// V8M_CMSE: #define __ARM_FEATURE_CMSE 3
+
+// Check that CMSE is not defined on architectures w/o support for security extension
+// RUN: %clang -target arm-arm-none-gnueabi -mcpu=cortex-a5 -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=NOTV8M_CMSE %s
+// RUN: %clang -target armv8a-none-linux-gnu -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=NOTV8M_CMSE %s
+// RUN: %clang -target armv8.1a-none-none-eabi -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=NOTV8M_CMSE %s
+// NOTV8M_CMSE-NOT: __ARM_FEATURE_CMSE
+
+// Check that -mcmse option gives error on non v8-M targets
+// RUN: not %clang -target arm-arm-none-eabi -mthumb -mcmse -mcpu=cortex-m7 -x c -E -dM %s -o - 2>&1 | FileCheck -match-full-lines --check-prefix=NOTV8MCMSE_OPT %s
+// NOTV8MCMSE_OPT: error: -mcmse is not supported for cortex-m7
+
 // Test whether predefines are as expected when targeting v8m cores
 // RUN: %clang -target arm -mcpu=cortex-m23 -x c -E -dM %s -o - | FileCheck -match-full-lines --check-prefix=M23 %s
 // M23: #define __ARM_ARCH 8