linux-kernel - Re: [PATCH] x86/boot: Avoid redundant address overlap tests in memcpy().

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Date:   Tue, 25 Jan 2022 07:14:47 +0900
From:   Kuniyuki Iwashima <kuniyu@...zon.co.jp>
To:     <dave.hansen@...el.com>
CC:     <benh@...zon.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
        <kuni1840@...il.com>, <kuniyu@...zon.co.jp>,
        <linux-kernel@...r.kernel.org>, <mingo@...hat.com>,
        <tglx@...utronix.de>, <x86@...nel.org>
Subject: Re: [PATCH] x86/boot: Avoid redundant address overlap tests in memcpy().

From:   Dave Hansen <dave.hansen@...el.com>
Date:   Mon, 24 Jan 2022 09:38:40 -0800
> On 1/22/22 17:58, Kuniyuki Iwashima wrote:
> > -void *memmove(void *dest, const void *src, size_t n)
> > +void *____memmove(void *dest, const void *src, size_t n)
> >   {
> >   	unsigned char *d = dest;
> >   	const unsigned char *s = src;
> >   
> > -	if (d <= s || d - s >= n)
> > -		return ____memcpy(dest, src, n);
> > -
> >   	while (n-- > 0)
> >   		d[n] = s[n];
> >   
> >   	return dest;
> >   }
> 
> The ___ naming is pretty cruel.  Could we call it memmove_no_overlap() 
> or memmove_unsafe()?  Surely we can put some *useful* bytes in the name 
> rather than padding it out with _'s.  No need to perpetuate the 
> ____memcpy() naming.

Sure.  I will rename it to memmove_unsafe() because it supports another
overlap case. (d > s)


> 
> Also, is this worth the churn?  It probably saves less than 10 
> instructions, all of which are ridiculously cheap.  Is there a *reason* 
> for this other than being a pure cleanup?

This is just for cleanup.