Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_stoptheworld_mac.cc
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_stoptheworld_mac.cc
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_stoptheworld_mac.cc
@@ -48,6 +48,29 @@
   CHECK(0 && "unimplemented");
 }
 
+#if defined(__x86_64__)
+typedef x86_thread_state64_t regs_struct;
+
+#define SP_REG __rsp
+
+#elif defined(__aarch64__)
+typedef arm_thread_state64_t regs_struct;
+
+# if __DARWIN_UNIX03
+#  define SP_REG __sp
+# else
+#  define SP_REG sp
+# endif
+
+#elif defined(__i386)
+typedef x86_thread_state32_t regs_struct;
+
+#define SP_REG __esp
+
+#else
+#error "Unsupported architecture"
+#endif
+
 tid_t SuspendedThreadsListMac::GetThreadID(uptr index) const {
   CHECK_LT(index, threads_.size());
   return threads_[index].tid;
@@ -83,8 +106,26 @@
 
 PtraceRegistersStatus SuspendedThreadsListMac::GetRegistersAndSP(
     uptr index, uptr *buffer, uptr *sp) const {
-  CHECK(0 && "unimplemented");
-  return REGISTERS_UNAVAILABLE_FATAL;
+  thread_t thread = GetThread(index);
+  regs_struct regs;
+  int err;
+  mach_msg_type_number_t reg_count = MACHINE_THREAD_STATE_COUNT;
+  err = thread_get_state(thread, MACHINE_THREAD_STATE, (thread_state_t)&regs,
+                         &reg_count);
+  if (err != KERN_SUCCESS) {
+    VReport(1, "Error - unable to get registers for a thread\n");
+    // KERN_INVALID_ARGUMENT indicates that either the flavor is invalid,
+    // or the thread does not exist. The other possible error case,
+    // MIG_ARRAY_TOO_LARGE, means that the state is too large, but it's
+    // still safe to proceed.
+    return err == KERN_INVALID_ARGUMENT ? REGISTERS_UNAVAILABLE_FATAL
+                                        : REGISTERS_UNAVAILABLE;
+  }
+
+  internal_memcpy(buffer, &regs, sizeof(regs));
+  *sp = regs.SP_REG;
+
+  return REGISTERS_AVAILABLE;
 }
 
 uptr SuspendedThreadsListMac::RegisterCount() const {