From 3a97bb08f4ee75635b9932f4d603932aff67054a Mon Sep 17 00:00:00 2001
From: Frederik Seiffert <frederik@algoriddim.com>
Date: Tue, 7 May 2019 17:38:54 +0200
Subject: [PATCH] Added basic spinlock implementation using builtins.

This will be used on platforms without pthread_spin_lock(), e.g. Android targeting API level < 24.

Implementation lifted from spinlock.h in libobjc2.
---
 Source/NSThread.m | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/Source/NSThread.m b/Source/NSThread.m
index 9419e28b3..c7b35c7ff 100644
--- a/Source/NSThread.m
+++ b/Source/NSThread.m
@@ -38,21 +38,40 @@
 // Dummy implementatation
 // cleaner than IFDEF'ing the code everywhere
 #if !(HAVE_PTHREAD_SPIN_LOCK)
-#warning no spin_locks, using dummy versions
-typedef int pthread_spinlock_t;
+typedef volatile int pthread_spinlock_t;
 int pthread_spin_init(pthread_spinlock_t *lock, int pshared)
 {
-#if DEBUG
+#if DEBUG && !__has_builtin(__sync_bool_compare_and_swap)
   fprintf(stderr,"NSThread.m: Warning this platform does not support spin locks - init.\n");
 #endif
   return 0;
 }
 int pthread_spin_lock(pthread_spinlock_t *lock)
 {
+#if __has_builtin(__sync_bool_compare_and_swap)
+  int count = 0;
+  // Set the spin lock value to 1 if it is 0.
+  while(!__sync_bool_compare_and_swap(lock, 0, 1))
+  {
+    count++;
+    if (0 == count % 10)
+    {
+      // If it is already 1, let another thread play with the CPU for a
+      // bit then try again.
+      sleep(0);
+    }
+  }
+#else
+  #warning no spin_locks, using dummy versions
+#endif
   return 0;
 }
 int pthread_spin_unlock(pthread_spinlock_t *lock)
 {
+#if __has_builtin(__sync_bool_compare_and_swap)
+  __sync_synchronize();
+  *lock = 0;
+#endif
   return 0;
 }
 int pthread_spin_destroy(pthread_spinlock_t *lock)