Index: lldb/trunk/packages/Python/lldbsuite/test/functionalities/register/register_command/TestRegisters.py
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/functionalities/register/register_command/TestRegisters.py
+++ lldb/trunk/packages/Python/lldbsuite/test/functionalities/register/register_command/TestRegisters.py
@@ -272,14 +272,18 @@
         target = self.dbg.CreateTarget(exe)
         self.assertTrue(target, VALID_TARGET)
 
-        lldbutil.run_break_set_by_symbol(
-            self, "main", num_expected_locations=-1)
-
-        # Launch the process, and do not stop at the entry point.
-        process = target.LaunchSimple(
-            None, None, self.get_process_working_directory())
+        # Launch the process, stop at the entry point.
+        error = lldb.SBError()
+        process = target.Launch(
+                lldb.SBListener(),
+                None, None, # argv, envp
+                None, None, None, # stdin/out/err
+                self.get_process_working_directory(),
+                0, # launch flags
+                True, # stop at entry
+                error)
+        self.assertTrue(error.Success(), "Launch succeeds. Error is :" + str(error))
 
-        process = target.GetProcess()
         self.assertTrue(
             process.GetState() == lldb.eStateStopped,
             PROCESS_STOPPED)
Index: lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.h
===================================================================
--- lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.h
+++ lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.h
@@ -136,6 +136,8 @@
   bool CopyMPXtoXSTATE(uint32_t reg);
 
   bool IsMPX(uint32_t reg_index) const;
+
+  void UpdateXSTATEforWrite(uint32_t reg_index);
 };
 
 } // namespace process_linux
Index: lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.cpp
===================================================================
--- lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.cpp
+++ lldb/trunk/source/Plugins/Process/Linux/NativeRegisterContextLinux_x86_64.cpp
@@ -528,6 +528,22 @@
   return error;
 }
 
+void NativeRegisterContextLinux_x86_64::UpdateXSTATEforWrite(
+    uint32_t reg_index) {
+  XSAVE_HDR::XFeature &xstate_bv = m_fpr.xstate.xsave.header.xstate_bv;
+  if (IsFPR(reg_index)) {
+    // IsFPR considers both %st and %xmm registers as floating point, but these
+    // map to two features. Set both flags, just in case.
+    xstate_bv |= XSAVE_HDR::XFeature::FP | XSAVE_HDR::XFeature::SSE;
+  } else if (IsAVX(reg_index)) {
+    // Lower bytes of some %ymm registers are shared with %xmm registers.
+    xstate_bv |= XSAVE_HDR::XFeature::YMM | XSAVE_HDR::XFeature::SSE;
+  } else if (IsMPX(reg_index)) {
+    // MPX registers map to two XSAVE features.
+    xstate_bv |= XSAVE_HDR::XFeature::BNDREGS | XSAVE_HDR::XFeature::BNDCSR;
+  }
+}
+
 Status NativeRegisterContextLinux_x86_64::WriteRegister(
     const RegisterInfo *reg_info, const RegisterValue &reg_value) {
   assert(reg_info && "reg_info is null");
@@ -538,6 +554,8 @@
                                                ? reg_info->name
                                                : "<unknown register>");
 
+  UpdateXSTATEforWrite(reg_index);
+
   if (IsGPR(reg_index))
     return WriteRegisterRaw(reg_index, reg_value);
 
Index: lldb/trunk/source/Plugins/Process/Utility/RegisterContextPOSIX_x86.h
===================================================================
--- lldb/trunk/source/Plugins/Process/Utility/RegisterContextPOSIX_x86.h
+++ lldb/trunk/source/Plugins/Process/Utility/RegisterContextPOSIX_x86.h
@@ -149,9 +149,10 @@
   RegInfo m_reg_info;
   FPRType
       m_fpr_type; // determines the type of data stored by union FPR, if any.
-  FPR m_fpr;      // floating-point registers including extended register sets.
-  IOVEC m_iovec;  // wrapper for xsave.
-  YMM m_ymm_set;  // copy of ymmh and xmm register halves.
+  lldb_private::FPR m_fpr;     // floating-point registers including extended
+                               // register sets.
+  lldb_private::IOVEC m_iovec; // wrapper for xsave.
+  lldb_private::YMM m_ymm_set; // copy of ymmh and xmm register halves.
   std::unique_ptr<lldb_private::RegisterInfoInterface>
       m_register_info_ap; // Register Info Interface (FreeBSD or Linux)
 
Index: lldb/trunk/source/Plugins/Process/Utility/RegisterContext_x86.h
===================================================================
--- lldb/trunk/source/Plugins/Process/Utility/RegisterContext_x86.h
+++ lldb/trunk/source/Plugins/Process/Utility/RegisterContext_x86.h
@@ -13,8 +13,10 @@
 #include <cstddef>
 #include <cstdint>
 
+#include "llvm/ADT/BitmaskEnum.h"
 #include "llvm/Support/Compiler.h"
 
+namespace lldb_private {
 //---------------------------------------------------------------------------
 // i386 ehframe, dwarf regnums
 //---------------------------------------------------------------------------
@@ -313,13 +315,28 @@
 
 LLVM_PACKED_START
 struct XSAVE_HDR {
-  uint64_t xstate_bv; // OS enabled xstate mask to determine the extended states
+  enum class XFeature : uint64_t {
+    FP = 1,
+    SSE = FP << 1,
+    YMM = SSE << 1,
+    BNDREGS = YMM << 1,
+    BNDCSR = BNDREGS << 1,
+    OPMASK = BNDCSR << 1,
+    ZMM_Hi256 = OPMASK << 1,
+    Hi16_ZMM = ZMM_Hi256 << 1,
+    PT = Hi16_ZMM << 1,
+    PKRU = PT << 1,
+    LLVM_MARK_AS_BITMASK_ENUM(/*LargestValue*/ PKRU)
+  };
+
+  XFeature xstate_bv; // OS enabled xstate mask to determine the extended states
                       // supported by the processor
-  uint64_t xcomp_bv;  // Mask to indicate the format of the XSAVE area and of
+  XFeature xcomp_bv;  // Mask to indicate the format of the XSAVE area and of
                       // the XRSTOR instruction
   uint64_t reserved1[1];
   uint64_t reserved2[5];
 };
+static_assert(sizeof(XSAVE_HDR) == 64, "XSAVE_HDR layout incorrect");
 LLVM_PACKED_END
 
 // x86 extensions to FXSAVE (i.e. for AVX and MPX processors)
@@ -355,4 +372,8 @@
   size_t iov_len; // sizeof(XSAVE)
 };
 
+LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE();
+
+} // namespace lldb_private
+
 #endif