Index: lldb/trunk/include/lldb/Core/ArchSpec.h
===================================================================
--- lldb/trunk/include/lldb/Core/ArchSpec.h
+++ lldb/trunk/include/lldb/Core/ArchSpec.h
@@ -625,6 +625,7 @@
 
 protected:
   bool IsEqualTo(const ArchSpec &rhs, bool exact_match) const;
+  void UpdateCore();
 
   llvm::Triple m_triple;
   Core m_core = kCore_invalid;
Index: lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/TestLinuxCore.py
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/TestLinuxCore.py
+++ lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/TestLinuxCore.py
@@ -39,7 +39,6 @@
         super(LinuxCoreTestCase, self).tearDown()
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_i386(self):
         """Test that lldb can read the process information from an i386 linux core file."""
@@ -58,21 +57,18 @@
         self.do_test("linux-mips64el-gnuabi64", self._mips64_n64_pid, self._mips_regions)
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_x86_64(self):
         """Test that lldb can read the process information from an x86_64 linux core file."""
         self.do_test("linux-x86_64", self._x86_64_pid, self._x86_64_regions)
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_s390x(self):
         """Test that lldb can read the process information from an s390x linux core file."""
         self.do_test("linux-s390x", self._s390x_pid, self._s390x_regions)
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_same_pid_running(self):
         """Test that we read the information from the core correctly even if we have a running
@@ -102,7 +98,6 @@
             self.RemoveTempFile("linux-x86_64-pid.core")
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_two_cores_same_pid(self):
         """Test that we handle the situation if we have two core files with the same PID
@@ -132,7 +127,6 @@
         self.do_test("linux-x86_64", self._x86_64_pid, self._x86_64_regions)
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_FPR_SSE(self):
         # check x86_64 core file
Index: lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/gcore/TestGCore.py
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/gcore/TestGCore.py
+++ lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/gcore/TestGCore.py
@@ -23,14 +23,12 @@
     _x86_64_pid = 5669
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_i386(self):
         """Test that lldb can read the process information from an i386 linux core file."""
         self.do_test("linux-i386", self._i386_pid)
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_x86_64(self):
         """Test that lldb can read the process information from an x86_64 linux core file."""
Index: lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/thread_crash/TestLinuxCoreThreads.py
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/thread_crash/TestLinuxCoreThreads.py
+++ lldb/trunk/packages/Python/lldbsuite/test/functionalities/postmortem/elf-core/thread_crash/TestLinuxCoreThreads.py
@@ -27,14 +27,12 @@
     _x86_64_tid = 5250
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_i386(self):
         """Test that lldb can read the process information from an i386 linux core file."""
         self.do_test("linux-i386", self._i386_pid, self._i386_tid)
 
     @skipIf(oslist=['windows'])
-    @skipIfDarwin # <rdar://problem/31380097>, fails started happening with r299199
     @skipIf(triple='^mips')
     def test_x86_64(self):
         """Test that lldb can read the process information from an x86_64 linux core file."""
Index: lldb/trunk/source/Core/ArchSpec.cpp
===================================================================
--- lldb/trunk/source/Core/ArchSpec.cpp
+++ lldb/trunk/source/Core/ArchSpec.cpp
@@ -834,19 +834,7 @@
 
 bool ArchSpec::SetTriple(const llvm::Triple &triple) {
   m_triple = triple;
-
-  llvm::StringRef arch_name(m_triple.getArchName());
-  const CoreDefinition *core_def = FindCoreDefinition(arch_name);
-  if (core_def) {
-    m_core = core_def->core;
-    // Set the byte order to the default byte order for an architecture.
-    // This can be modified if needed for cases when cores handle both
-    // big and little endian
-    m_byte_order = core_def->default_byte_order;
-  } else {
-    Clear();
-  }
-
+  UpdateCore();
   return IsValid();
 }
 
@@ -994,8 +982,10 @@
     GetTriple().setVendor(other.GetTriple().getVendor());
   if (TripleOSIsUnspecifiedUnknown() && !other.TripleOSIsUnspecifiedUnknown())
     GetTriple().setOS(other.GetTriple().getOS());
-  if (GetTriple().getArch() == llvm::Triple::UnknownArch)
+  if (GetTriple().getArch() == llvm::Triple::UnknownArch) {
     GetTriple().setArch(other.GetTriple().getArch());
+    UpdateCore();
+  }
   if (GetTriple().getEnvironment() == llvm::Triple::UnknownEnvironment &&
       !TripleVendorWasSpecified()) {
     if (other.TripleVendorWasSpecified())
@@ -1190,6 +1180,20 @@
   return false;
 }
 
+void ArchSpec::UpdateCore() {
+  llvm::StringRef arch_name(m_triple.getArchName());
+  const CoreDefinition *core_def = FindCoreDefinition(arch_name);
+  if (core_def) {
+    m_core = core_def->core;
+    // Set the byte order to the default byte order for an architecture.
+    // This can be modified if needed for cases when cores handle both
+    // big and little endian
+    m_byte_order = core_def->default_byte_order;
+  } else {
+    Clear();
+  }
+}
+
 //===----------------------------------------------------------------------===//
 // Helper methods.
 
Index: lldb/trunk/unittests/Core/ArchSpecTest.cpp
===================================================================
--- lldb/trunk/unittests/Core/ArchSpecTest.cpp
+++ lldb/trunk/unittests/Core/ArchSpecTest.cpp
@@ -134,3 +134,22 @@
   AS = ArchSpec();
   EXPECT_FALSE(AS.SetTriple(""));
 }
+
+TEST(ArchSpecTest, MergeFrom) {
+  ArchSpec A;
+  ArchSpec B("x86_64-pc-linux");
+
+  EXPECT_FALSE(A.IsValid());
+  ASSERT_TRUE(B.IsValid());
+  EXPECT_EQ(llvm::Triple::ArchType::x86_64, B.GetTriple().getArch());
+  EXPECT_EQ(llvm::Triple::VendorType::PC, B.GetTriple().getVendor());
+  EXPECT_EQ(llvm::Triple::OSType::Linux, B.GetTriple().getOS());
+  EXPECT_EQ(ArchSpec::eCore_x86_64_x86_64, B.GetCore());
+
+  A.MergeFrom(B);
+  ASSERT_TRUE(A.IsValid());
+  EXPECT_EQ(llvm::Triple::ArchType::x86_64, A.GetTriple().getArch());
+  EXPECT_EQ(llvm::Triple::VendorType::PC, A.GetTriple().getVendor());
+  EXPECT_EQ(llvm::Triple::OSType::Linux, A.GetTriple().getOS());
+  EXPECT_EQ(ArchSpec::eCore_x86_64_x86_64, A.GetCore());
+}