arm64: add cycle counter support

The ISB (instruction synchronization barrier) might be too heavy for
START/STOPTIMER use but should be more accurate in checkasm where the
timing overhead is subtracted.

libavutil/aarch64/timer.h

+/*
+ * Copyright (c) 2015 Janne Grunau <janne-libav@jannau.net>
+ *
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVUTIL_AARCH64_TIMER_H
+#define AVUTIL_AARCH64_TIMER_H
+
+#include <stdint.h>
+#include "config.h"
+
+#if HAVE_INLINE_ASM
+
+#define AV_READ_TIME read_time
+
+static inline uint64_t read_time(void)
+{
+    uint64_t cycle_counter;
+    __asm__ volatile(
+        "isb                   \t\n"
+        "mrs %0, pmccntr_el0       "
+        : "=r"(cycle_counter) :: "memory" );
+
+    return cycle_counter;
+}
+
+#endif /* HAVE_INLINE_ASM */
+
+#endif /* AVUTIL_AARCH64_TIMER_H */

libavutil/timer.h

@@ -38,7 +38,9 @@
 
 #include "log.h"
 
-#if   ARCH_ARM
+#if   ARCH_AARCH64
+#   include "aarch64/timer.h"
+#elif ARCH_ARM
 #   include "arm/timer.h"
 #elif ARCH_BFIN
 #   include "bfin/timer.h"