Initial import.

1 files changed, 44 insertions, 0 deletions

diff --git a/src/guard.cc b/src/guard.cc
new file mode 100644
index 0000000..172af47
--- /dev/null
+++ b/src/guard.cc
@@ -0,0 +1,44 @@
+#include <stdint.h>
+#include <pthread.h>
+
+static int32_t *low_32_bits(int64_t *ptr)
+{
+	int32_t *low= (int32_t*)ptr;
+	// Test if the machine is big endian - constant propagation at compile time
+	// should eliminate this completely.
+	int one = 1;
+	if (*(char*)&one != 1)
+	{
+		low++;
+	}
+	return low;
+}
+
+extern "C" int __cxa_guard_acquire(int64_t *guard_object)
+{
+	char first_byte = (*guard_object) >> 56;
+	if (1 == first_byte) { return 0; }
+	int32_t *lock = low_32_bits(guard_object);
+	while (!__sync_bool_compare_and_swap(lock, 0, 1))
+	{
+		// TODO: Use some of the remaining 24 bits to define a mutex to sleep
+		// on.  Whether this is actually worth bothering with depends on
+		// whether there is likely to be any contention.
+		sched_yield();
+	}
+	return 1;
+}
+
+extern "C" void __cxa_guard_abort(int64_t *guard_object)
+{
+	int32_t *lock = low_32_bits(guard_object);
+	*lock = 0;
+}
+extern "C" void __cxa_guard_release(int64_t *guard_object)
+{
+	// Set the first byte to 1
+	*guard_object |= ((int64_t)1) << 57;
+	__cxa_guard_abort(guard_object);
+}
+
+