linux-kernel - RE: [PATCH] x86/uaccess: small optimization in unsafe_copy_to

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Date:   Sat, 17 Apr 2021 13:59:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Al Viro' <viro@...iv.linux.org.uk>,
        Eric Dumazet <eric.dumazet@...il.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH] x86/uaccess: small optimization in unsafe_copy_to_user()

From: Al Viro <viro@....linux.org.uk> On Behalf Of Al Viro
> Sent: 16 April 2021 20:44
> On Fri, Apr 16, 2021 at 12:24:13PM -0700, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > We have to loop only to copy u64 values.
> > After this first loop, we copy at most one u32, one u16 and one byte.
> 
> Does it actually yield a better code?
> 
> FWIW, this
> void bar(unsigned);
> void foo(unsigned n)
> {
> 	while (n >= 8) {
> 		bar(n);
> 		n -= 8;
> 	}
> 	while (n >= 4) {
> 		bar(n);
> 		n -= 4;
> 	}
> 	while (n >= 2) {
> 		bar(n);
> 		n -= 2;
> 	}
> 	while (n >= 1) {
> 		bar(n);
> 		n -= 1;
> 	}
> }

This variant might be better:

void foo(unsigned n)
{
	while (n >= 8) {
		bar(8);
		n -= 8;
	}
	if (likely(!n))
		return;
	if (n & 4)
		bar(4);
	if (n & 2)
		bar(2);
	if (n & 1)
		bar(1);
}

I think Al's version might have optimised down to this,
but Eric's asm contains the n -= 4/2/1;

OTOH gcc can make a real pig's breakfast of code like this!

	David

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