linux-kernel - Re: [PATCH 03/15] print_integer: new and improved way of printing integers

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Message-ID: <20200421165948.GO5820@bombadil.infradead.org>
Date:   Tue, 21 Apr 2020 09:59:48 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, pmladek@...e.com,
        rostedt@...dmis.org, sergey.senozhatsky@...il.com,
        andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk
Subject: Re: [PATCH 03/15] print_integer: new and improved way of printing
 integers

On Mon, Apr 20, 2020 at 11:57:31PM +0300, Alexey Dobriyan wrote:
> 1) memcpy is done in forward direction
> 	it can be done backwards but nobody does that,

If you're determined to do this, then use memmove() which actually
guarantees to work with overlapping ranges.  Don't rely on non-guaranteed
behaviour of current implementations of memcpy().  Did you really check
the two dozen assembly implementations of memcpy() in the kernel?

> 2) digits can be extracted in a very simple loop which costs only
> 	1 multiplication and shift (division by constant is not division)

> +noinline
> +char *_print_integer_u32(char *p, u32 x)
> +{
> +	do {
> +		*--p = '0' + (x % 10);
> +	} while (x /= 10);
> +	return p;
> +}

Why not do two digits at a time like put_dec() does?