iOS 计算代码块执行时间

#import <mach/mach_time.h> // for mach_absolute_time

double MachTimeToSecs(uint64_t time)
{
    mach_timebase_info_data_t timebase;
    mach_timebase_info(&timebase);
    return (double)time * (double)timebase.numer /  (double)timebase.denom / 1e9;
}

- (void)profileDoSomething
{  
    uint64_t begin = mach_absolute_time();
    [self doSomething];
    uint64_t end = mach_absolute_time();
    NSLog(@"Time taken to doSomething %g s", MachTimeToSecs(end - begin));
}