1 files changed, 151 insertions, 0 deletions
diff --git a/libgo/runtime/go-semacquire.c b/libgo/runtime/go-semacquire.c
new file mode 100644
index 000000000..24c6a7388
--- /dev/null
+++ b/libgo/runtime/go-semacquire.c
@@ -0,0 +1,151 @@
+/* go-semacquire.c -- implement runtime.Semacquire and runtime.Semrelease.
+
+   Copyright 2009 The Go Authors. All rights reserved.
+   Use of this source code is governed by a BSD-style
+   license that can be found in the LICENSE file.  */
+
+#include <stdint.h>
+
+#include <pthread.h>
+
+#include "go-assert.h"
+#include "runtime.h"
+
+/* We use a single global lock and condition variable.  This is
+   painful, since it will cause unnecessary contention, but is hard to
+   avoid in a portable manner.  On Linux we can use futexes, but they
+   are unfortunately not exposed by libc and are thus also hard to use
+   portably.  */
+
+static pthread_mutex_t sem_lock = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t sem_cond = PTHREAD_COND_INITIALIZER;
+
+/* If the value in *ADDR is positive, and we are able to atomically
+   decrement it, return true.  Otherwise do nothing and return
+   false.  */
+
+static _Bool
+acquire (uint32 *addr)
+{
+  while (1)
+    {
+      uint32 val;
+
+      val = *addr;
+      if (val == 0)
+	return 0;
+      if (__sync_bool_compare_and_swap (addr, val, val - 1))
+	return 1;
+    }
+}
+
+/* Implement runtime.Semacquire.  ADDR points to a semaphore count.
+   We have acquired the semaphore when we have decremented the count
+   and it remains nonnegative.  */
+
+void
+semacquire (uint32 *addr)
+{
+  while (1)
+    {
+      int i;
+
+      /* If the current count is positive, and we are able to atomically
+	 decrement it, then we have acquired the semaphore.  */
+      if (acquire (addr))
+	return;
+
+      /* Lock the mutex.  */
+      i = pthread_mutex_lock (&sem_lock);
+      __go_assert (i == 0);
+
+      /* Check the count again with the mutex locked.  */
+      if (acquire (addr))
+	{
+	  i = pthread_mutex_unlock (&sem_lock);
+	  __go_assert (i == 0);
+	  return;
+	}
+
+      /* The count is zero.  Even if a call to runtime.Semrelease
+	 increments it to become positive, that call will try to
+	 acquire the mutex and block, so we are sure to see the signal
+	 of the condition variable.  */
+      i = pthread_cond_wait (&sem_cond, &sem_lock);
+      __go_assert (i == 0);
+
+      /* Unlock the mutex and try again.  */
+      i = pthread_mutex_unlock (&sem_lock);
+      __go_assert (i == 0);
+    }
+}
+
+/* Implement runtime.Semrelease.  ADDR points to a semaphore count.  We
+   must atomically increment the count.  If the count becomes
+   positive, we signal the condition variable to wake up another
+   process.  */
+
+void
+semrelease (uint32 *addr)
+{
+  int32_t val;
+
+  val = __sync_fetch_and_add (addr, 1);
+
+  /* VAL is the old value.  It should never be negative.  If it is
+     negative, that implies that Semacquire somehow decremented a zero
+     value, or that the count has overflowed.  */
+  __go_assert (val >= 0);
+
+  /* If the old value was zero, then we have now released a count, and
+     we signal the condition variable.  If the old value was positive,
+     then nobody can be waiting.  We have to use
+     pthread_cond_broadcast, not pthread_cond_signal, because
+     otherwise there would be a race condition when the count is
+     incremented twice before any locker manages to decrement it.  */
+  if (val == 0)
+    {
+      int i;
+
+      i = pthread_mutex_lock (&sem_lock);
+      __go_assert (i == 0);
+
+      i = pthread_cond_broadcast (&sem_cond);
+      __go_assert (i == 0);
+
+      i = pthread_mutex_unlock (&sem_lock);
+      __go_assert (i == 0);
+    }
+}
+
+
+#ifndef HAVE_SYNC_FETCH_AND_ADD_4
+
+/* For targets which don't have the required sync support.  Really
+   this should be provided by gcc itself.  FIXME.  */
+
+static pthread_mutex_t sync_lock = PTHREAD_MUTEX_INITIALIZER;
+
+uint32
+__sync_fetch_and_add_4(uint32*, uint32)
+  __attribute__((visibility("hidden")));
+
+uint32
+__sync_fetch_and_add_4(uint32* ptr, uint32 add)
+{
+  int i;
+  uint32 ret;
+
+  i = pthread_mutex_lock(&sync_lock);
+  __go_assert(i == 0);
+
+  ret = *ptr;
+  *ptr += add;
+
+  i = pthread_mutex_unlock(&sync_lock);
+  __go_assert(i == 0);
+
+  return ret;
+}
+
+#endif