Add aarch64 support to src/cycleclock.h.
From google3's base/cycleclock.
diff --git a/src/cycleclock.h b/src/cycleclock.h
index 42541da..3110804 100644
--- a/src/cycleclock.h
+++ b/src/cycleclock.h
@@ -99,6 +99,14 @@
_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;
