1 files changed, 134 insertions, 0 deletions
diff --git a/contrib/libcxxrt/guard.cc b/contrib/libcxxrt/guard.cc
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+/**
+ * guard.cc: Functions for thread-safe static initialisation.
+ *
+ * Static values in C++ can be initialised lazily their first use.  This file
+ * contains functions that are used to ensure that two threads attempting to
+ * initialize the same static do not call the constructor twice.  This is
+ * important because constructors can have side effects, so calling the
+ * constructor twice may be very bad.
+ *
+ * Statics that require initialisation are protected by a 64-bit value.  Any
+ * platform that can do 32-bit atomic test and set operations can use this
+ * value as a low-overhead lock.  Because statics (in most sane code) are
+ * accessed far more times than they are initialised, this lock implementation
+ * is heavily optimised towards the case where the static has already been
+ * initialised.  
+ */
+#include <stdint.h>
+#include <pthread.h>
+#include <assert.h>
+
+#ifdef __arm__
+// ARM ABI - 32-bit guards.
+
+/**
+ * Acquires a lock on a guard, returning 0 if the object has already been
+ * initialised, and 1 if it has not.  If the object is already constructed then
+ * this function just needs to read a byte from memory and return.
+ */
+extern "C" int __cxa_guard_acquire(volatile int32_t *guard_object)
+{
+	if ((1<<31) == *guard_object) { return 0; }
+	// If we can atomically move the value from 0 -> 1, then this is
+	// uninitialised.
+	if (__sync_bool_compare_and_swap(guard_object, 0, 1))
+	{
+		return 1;
+	}
+	// If the value is not 0, some other thread was initialising this.  Spin
+	// until it's finished.
+	while (__sync_bool_compare_and_swap(guard_object, (1<<31), (1<<31)))
+	{
+		// If the other thread aborted, then we grab the lock
+		if (__sync_bool_compare_and_swap(guard_object, 0, 1))
+		{
+			return 1;
+		}
+		sched_yield();
+	}
+	return 0;
+}
+
+/**
+ * Releases the lock without marking the object as initialised.  This function
+ * is called if initialising a static causes an exception to be thrown.
+ */
+extern "C" void __cxa_guard_abort(int32_t *guard_object)
+{
+	assert(__sync_bool_compare_and_swap(guard_object, 1, 0));
+}
+/**
+ * Releases the guard and marks the object as initialised.  This function is
+ * called after successful initialisation of a static.
+ */
+extern "C" void __cxa_guard_release(int32_t *guard_object)
+{
+	assert(__sync_bool_compare_and_swap(guard_object, 1, (1<<31)));
+}
+
+
+#else
+// Itanium ABI: 64-bit guards
+
+/**
+ * Returns a pointer to the low 32 bits in a 64-bit value, respecting the
+ * platform's byte order.
+ */
+static int32_t *low_32_bits(volatile 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;
+}
+
+/**
+ * Acquires a lock on a guard, returning 0 if the object has already been
+ * initialised, and 1 if it has not.  If the object is already constructed then
+ * this function just needs to read a byte from memory and return.
+ */
+extern "C" int __cxa_guard_acquire(volatile int64_t *guard_object)
+{
+	char first_byte = (*guard_object) >> 56;
+	if (1 == first_byte) { return 0; }
+	int32_t *lock = low_32_bits(guard_object);
+	// Simple spin lock using the low 32 bits.  We assume that concurrent
+	// attempts to initialize statics are very rare, so we don't need to
+	// optimise for the case where we have lots of threads trying to acquire
+	// the lock at the same time.
+	while (!__sync_bool_compare_and_swap_4(lock, 0, 1))
+	{
+		sched_yield();
+	}
+	// We have to test the guard again, in case another thread has performed
+	// the initialisation while we were trying to acquire the lock.
+	first_byte = (*guard_object) >> 56;
+	return (1 != first_byte);
+}
+
+/**
+ * Releases the lock without marking the object as initialised.  This function
+ * is called if initialising a static causes an exception to be thrown.
+ */
+extern "C" void __cxa_guard_abort(int64_t *guard_object)
+{
+	int32_t *lock = low_32_bits(guard_object);
+	*lock = 0;
+}
+/**
+ * Releases the guard and marks the object as initialised.  This function is
+ * called after successful initialisation of a static.
+ */
+extern "C" void __cxa_guard_release(int64_t *guard_object)
+{
+	// Set the first byte to 1
+	*guard_object |= ((int64_t)1) << 56;
+	__cxa_guard_abort(guard_object);
+}
+
+#endif