From d9630aee882b7f36aeca5fb797ebd2a4aea81f03 Mon Sep 17 00:00:00 2001
From: Elliott Hughes <enh@google.com>
Date: Wed, 10 Feb 2016 15:48:27 -0800
Subject: [PATCH] Add aarch64 support to src/cycleclock.h.

From google3's base/cycleclock.
---
 src/cycleclock.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/cycleclock.h b/src/cycleclock.h
index 42541daf..31108046 100644
--- a/src/cycleclock.h
+++ b/src/cycleclock.h
@@ -99,6 +99,14 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
   _asm rdtsc
 #elif defined(COMPILER_MSVC)
   return __rdtsc();
+#elif defined(__aarch64__)
+  // System timer of ARMv8 runs at a different frequency than the CPU's.
+  // The frequency is fixed, typically in the range 1-50MHz.  It can be
+  // read at CNTFRQ special register.  We assume the OS has set up
+  // the virtual timer properly.
+  int64_t virtual_timer_value;
+  asm volatile("mrs %0, cntvct_el0" : "=r"(virtual_timer_value));
+  return virtual_timer_value;
 #elif defined(__ARM_ARCH)
 #if (__ARM_ARCH >= 6)  // V6 is the earliest arch that has a standard cyclecount
   uint32_t pmccntr;