linux-kernel - RE: [RFC PATCH v2 1/4] tools/nolibc: x86-64: Use `rep movsb` for `memcpy()` and `memmove()`

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Message-ID: <0f885fa14710453c9512315d6cfeee35@AcuMS.aculab.com>
Date:   Sun, 3 Sep 2023 20:35:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ammar Faizi' <ammarfaizi2@...weeb.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>
CC:     Willy Tarreau <w@....eu>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        Nicholas Rosenberg <inori@...x.org>,
        "Michael William Jonathan" <moe@...weeb.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH v2 1/4] tools/nolibc: x86-64: Use `rep movsb` for
 `memcpy()` and `memmove()`

From: Ammar Faizi
> Sent: 02 September 2023 13:37
> On 2023/09/02 午後7:29, Alviro Iskandar Setiawan wrote:
> > On Sat, Sep 2, 2023 at 1:38 PM Ammar Faizi wrote:
> >> Ok, I'll do that.
> >
> > Another micro-optimization. Since the likely case is the forward copy,
> > make it the case that doesn't take the jump.
> >
> > Pseudo C:
> > if (unlikely(dst - src < n))
> >          backward_copy();
> > else
> >          forward_copy();
> 
> Point taken.

I believe it makes almost no difference.
I'm sure I read that modern (Intel at least) cpu never do
static branch prediction.
So 'cold cache' there is 50% chance of the branch being taken.
Nothing the compiler can do will affect it.
OTOH having
	if (likely(dst - src >= n))
		forwards();
	else
		backwards();
(and in that order) probably makes the code a bit easier to read.

	David

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