linux-kernel - Re: [PATCH 2/3] lib: zlib_inflate: improves decompression performance

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Date:   Thu, 12 Nov 2020 00:06:09 +0800
From:   Zhaoxiu Zeng <zengzhaoxiu@....com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
Subject: Re: [PATCH 2/3] lib: zlib_inflate: improves decompression performance

在 2020/11/11 11:46, Jann Horn 写道:
> On Mon, Nov 9, 2020 at 8:27 PM <zengzhaoxiu@....com> wrote:
>> This patch does:
>> 1. Cleanup code and reduce branches
>> 2. Use copy_from_back to copy the matched bytes from the back output buffer
> 
> What exactly is copy_from_back()? Is it like memmove()? If yes, have
> you tried using memmove() instead of the code added in patch 1/3?
> 

If use memcpy(or memmove), the code will be like this:
	while (dist < len) {
		memcpy(out, out - dist, dist);
		out += dist;
		len -= dist;
	}
	memcpy(out, out - dist, len);
or:
	const u8 * const from = out - dist;
	while (dist < len) {
		memcpy(out, from, dist);
		out += dist;
		len -= dist;
		dist *= 2;
	}
	memcpy(out, from, len);

In addition, the len is small in most cases, so the function calls are expensive.