Question

我下面有两个类似的代码。

第一个代码：

unsigned long size = 256*1024*1024;
unsigned long stride = 256;
void *array = (void*)malloc(size);
for (unsigned long off = 0; off < size; off+=stride) {
    *(unsigned int*)(array+off) = off+stride;
}
*(unsigned int*)(array+off)=0;
int i=10000000;
struct timeval start, end;
gettimeofday(&start, NULL);
while (i>=1) {
    offset = *(unsigned int*)(array+off);
    i--;
}
gettimeofday(&end, NULL);
*(volatile unsigned int*)(array+offset);
printf("%.2f\n", (end.tv_sec-start.tv_sec)*1000000+(end.tv_usec-start.tv_usec));

第二个代码：

unsigned long size = 256*1024*1024;
unsigned long stride = 256;
void *array = (void*)malloc(size);
for (unsigned long off = 0; off < size; off+=stride) {
    *(unsigned int*)(array+off) = off+stride;
}
*(unsigned int*)(array+off)=0;
int i=10000000;
struct timeval start, end;
gettimeofday(&start, NULL);
#define ONE offset = *(unsigned int*)(array+off);
#define FIVE ONE ONE ONE ONE ONE
#define TEN FIVE FIVE
#define FIFTY TEN TEN TEN TEN TEN
#define HUNDRED FIFTY FIFTY
while (i>=1000) {
    HUNDRED
    HUNDRED
    HUNDRED
    HUNDRED
    HUNDRED
    HUNDRED
    HUNDRED
    HUNDRED
    HUNDRED
    HUNDRED
    i-=1000;
}
gettimeofday(&end, NULL);
*(volatile unsigned int*)(array+offset);
printf("%.2f\n", (end.tv_sec-start.tv_sec)*1000000+(end.tv_usec-start.tv_usec));

问题

两个代码之间的唯一区别是“ while循环”。它们都测量while循环的经过时间。

第一个代码的结果为779,851,000 ns，第二个代码的结果为1,624,344,000 ns。（大2.1倍）

我认为这种差异来自L1-i缓存未命中，所以我用性能来测量L1-i缓存未命中。

但是，第一代码的L1-i高速缓存未命中为34,541，第二代码的L1-i高速缓存未命中为43,078。（大1.2倍）

此结果无法完全解释while循环所经过时间的差异。

两个代码的经过时间之间有何大差异？

我想念什么吗？

为什么以下两个代码产生不同的结果？

0 个答案: