Adding support for arm mrc instruction.

Timing reference is here:
https://github.com/google/benchmark/blob/v1.1.0/src/cycleclock.h#L116

Cargo.toml

@@ -11,6 +11,8 @@ edition = "2018"
 
 [dependencies]
 libc = "0.2"
+time = "0.1.42"
+chrono = "*"
 
 [build-dependencies]
 cc = "1"

src/cpucounter.c

@@ -1,8 +1,17 @@
 #include <stdint.h>
 
+#if defined(__x86_64__) || defined(__amd64__)
 uint64_t cpucounter(void)
 {
     uint64_t low, high;
     __asm__ __volatile__ ("rdtscp" : "=a" (low), "=d" (high) : : "%ecx");
     return (high << 32) | low;
 }
+#elif defined(__aarch64__)
+uint64_t cpucounter(void)
+{
+    uint64_t virtual_timer_value;
+    __asm__ __volatile__ ("mrs %0, cntvct_el0" : "=r"(virtual_timer_value));
+    return virtual_timer_value;
+}
+#endif

src/cpucounter.rs

@@ -1,15 +1,26 @@
 use super::timestamp::*;
-
 pub(crate) struct CPUCounter;
 
 #[cfg(asm)]
 #[inline]
+#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 unsafe fn cpucounter() -> u64 {
     let (low, high): (u64, u64);
     asm!("rdtscp" :  "={eax}" (low), "={edx}" (high) : : "ecx");
     (high << 32) | low
 }
 
+
+// https://github.com/google/benchmark/blob/v1.1.0/src/cycleclock.h#L116
+#[cfg(asm)]
+#[inline]
+#[cfg(any(target_arch = "aarch64"))]
+unsafe fn cpucounter() -> u64 {
+    let (vtm): (u64);
+    asm!("mrs %0, cntvct_el0" : "=r"(vtm));
+    vtm
+}
+
 #[cfg(not(asm))]
 extern "C" {
     fn cpucounter() -> u64;