快速 ARM NEON memcpy [英] Fast ARM NEON memcpy

问题描述

我想在 ARMv7 内核上复制图像.最简单的实现是每行调用 memcpy.

I want to copy an image on an ARMv7 core. The naive implementation is to call memcpy per line.

for(i = 0; i < h; i++) {
  memcpy(d, s, w);
  s += sp;
  d += dp;
}

我知道以下内容

d, dp, s, sp, w

都是 32 字节对齐的，所以我的下一个(仍然很幼稚)实现是沿着

are all 32-byte aligned, so my next (still quite naive) implementation was along the lines of

for (int i = 0; i < h; i++) {
  uint8_t* dst = d;
  const uint8_t* src = s;
  int remaining = w;
  asm volatile (
    "1:                                               \n"
    "subs     %[rem], %[rem], #32                     \n"
    "vld1.u8  {d0, d1, d2, d3}, [%[src],:256]!        \n"
    "vst1.u8  {d0, d1, d2, d3}, [%[dst],:256]!        \n"
    "bgt      1b                                      \n"
    : [dst]"+r"(dst), [src]"+r"(src), [rem]"+r"(remaining)
    :
    : "d0", "d1", "d2", "d3", "cc", "memory"
  );
  d += dp;
  s += sp;
}

在大量迭代中比 memcpy 快约 150%(在不同的图像上，因此没有利用缓存).我觉得这应该离最佳状态还差得很远，因为我还没有使用预加载，但是当我使用预加载时，我似乎只能使性能变得更糟.有没有人对此有任何见解?

Which was ~150% faster than memcpy over a large number of iterations (on different images, so not taking advantage of caching). I feel like this should be nowhere near the optimum because I am yet to use preloading, but when I do I only seem to be able to make performance substantially worse. Does anyone have any insight here?

推荐答案

ARM 对此有很好的技术说明.

ARM has a great tech note on this.

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka13544.html

你的性能肯定会因微架构而异，ARM 的说明是在 A8 上，但我认为它会给你一个不错的主意，底部的总结是对各种利弊的很好的讨论不仅仅是常规数字，例如哪些方法导致最少的寄存器使用量等.

Your performance will definitely vary depending on the micro-architecture, ARM's note is on the A8 but I think it will give you a decent idea, and the summary at the bottom is a great discussion of the various pros and cons that go beyond just the regular numbers, such as which methods result in the least amount of register usage, etc.

是的，正如另一位评论者所提到的，预取是非常困难的，并且在不同的微架构下会以不同的方式工作，这取决于缓存有多大、每行有多大以及一堆其他细节关于缓存设计.如果你不小心，你最终可能会颠簸你需要的线路.我建议避免它用于可移植代码.

And yes, as another commenter mentions, pre-fetching is very difficult to get right, and will work differently with different micro-architectures, depending on how big the caches are and how big each line is and a bunch of other details about the cache design. You can end up thrashing lines you need if you aren't careful. I would recommend avoiding it for portable code.

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