[Threading] Move stack bounds fetching into the threading library.

Fetching the current stack bounds is done using threading functions, so
should be in the threading library.

rdar://90776105

lib/Threading/Pthreads.cpp

@@ -16,6 +16,10 @@
 
 #if SWIFT_THREADING_PTHREADS
 
+#if defined(__FreeBSD__) || defined(__OpenBSD__)
+#include <pthread_np.h>
+#endif
+
 #include "swift/Threading/Impl/Pthreads.h"
 #include "swift/Threading/Errors.h"
 
@@ -72,4 +76,43 @@ void swift::threading_impl::once_slow(once_t &predicate, void (*fn)(void *),
   pthread_mutex_unlock(&onceMutex);
 }
 
+#if defined(__OpenBSD__)
+swift::threading_impl::stack_bounds
+swift::threading_impl::thread_get_current_stack_bounds() {
+  stack_bounds result = { nullptr, nullptr };
+  stack_t sinfo;
+
+  if (!pthread_stackseg_np(pthread_self(), &sinfo)) {
+    result.low = (char *)sinfo.ss_sp - sinfo.ss_size;
+    result.high = sinfo.ss_sp;
+  }
+
+  return result;
+}
+#else
+swift::threading_impl::stack_bounds
+swift::threading_impl::thread_get_current_stack_bounds() {
+  stack_bounds result = { nullptr, nullptr };
+  pthread_attr_t attr;
+  size_t size = 0;
+  void *begin = nullptr;
+
+#if defined(__FreeBSD__)
+  if (pthread_attr_init(&attr))
+    return result;
+#endif
+
+  if (!pthread_getattr_np(pthread_self(), &attr)) {
+    if (!pthread_attr_getstack(&attr, &begin, &size)) {
+      result.low = begin;
+      result.high = (char *)begin + size;
+    }
+
+    pthread_attr_destroy(&attr);
+  }
+
+  return result;
+}
+#endif
+
 #endif // SWIFT_THREADING_PTHREADS