dispatch_benchmark函数是libdispatch (Grand Central Dispatch) 的一部分，这个方法并没有被公开声明，所以必须要自己声明。

简介：

The dispatch_benchmark function executes the given block multiple times according to the count variable and then returns the average number of nanoseconds per execution. This function is for debugging and performance analysis work. For the best results, pass a high count value to dispatch_benchmark.

函数原型：

extern uint64_t dispatch_benchmark(size_t count, void (^block)(void))

实例代码：

#import <Foundation/Foundation.h>
    
// 声明GCD中dispatch_benchmark函数原型
extern uint64_t dispatch_benchmark(size_t count, void (^block)(void));
    
void test1(void);
void test2(void);
    
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        test1();
        test2();
    }
    return 0;
}
    
void test1(void) {
    // 执行次数
    size_t count = 1000;
    // 调用dispatch_benchmark函数，传入执行次数和待测试的代码块
    NSUInteger length = 1000000;
    uint64_t time = dispatch_benchmark(count, ^{
        @autoreleasepool {
            NSMutableArray *mutableArray = [NSMutableArray array];
            for (NSUInteger i = 0; i < length; i++) {
                [mutableArray addObject:@(i)];
            }
        }
    });
    NSLog(@"[[NSMutableArray array] addObject:] Avg. Runtime: %llu ns", time);
}
    
void test2(void) {
    // 执行次数
    size_t count = 1000;
    NSUInteger length = 1000000;
    // 调用dispatch_benchmark函数，传入执行次数和待测试的代码块
    uint64_t time = dispatch_benchmark(count, ^{
        @autoreleasepool {
            NSMutableArray *mutableArray = [NSMutableArray arrayWithCapacity:length];
            for (NSUInteger i = 0; i < length; i++) {
                [mutableArray addObject:@(i)];
            }
        }
    });
    NSLog(@"[[NSMutableArray arrayWithCapacity:] addObject:] Avg. Runtime: %llu ns", time);
}

输出：

[[NSMutableArray array] addObject:] Avg. Runtime: 35960141 ns
[[NSMutableArray arrayWithCapacity:] addObject:] Avg. Runtime: 32219393 ns