C/C++遍历二维数组，列优先(column-major)比行优先(row-major)慢，why？

C/C++遍历二维数组，列优先(column-major)比行优先(row-major)慢，why？

简单粗暴的答案：存在Cache机制！

稍微啰嗦一点：CPU访问内存（读/写，遍历数组的话主要是读），不是每次都直接从内存上操作，而是先看Cache里是否有所指定地址的值！

这个问题，stackoverflow上有人问过的，结论是：CPU读取内存某地址处的值，并不是每次都去内存中取出来，有时候会从cache里读取。当初次访问数组的时候，会把连续一块(chunk)内存地址上的值都读到cache里（比如，64字节），后续CPU接受到一个内存地址要读取数据时，先看cache里有没有，没有的话再去内存上取。

原文：
https://stackoverflow.com/questions/33722520/why-is-iterating-2d-array-row-major-faster-than-column-major

关键句：The important factor is that the cache doesn't contain individual bytes or words, it holds chunks of adjacent memory, known as a cache-line, typically 64-bytes in size. So when address X is in the cache the CPU probably doesn't need to pre-emptively fetch X+1 because it has probably already got it (except in the case where X is the last byte in a cache-line, in which case it probably will have pre-fetched the next cache-line).