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Date:   Tue, 22 Jun 2021 03:46:16 +0300
From:   Nick Kossifidis <mick@ics.forth.gr>
To:     Matteo Croce <mcroce@linux.microsoft.com>
Cc:     linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Atish Patra <atish.patra@wdc.com>,
        Emil Renner Berthing <kernel@esmil.dk>,
        Akira Tsukamoto <akira.tsukamoto@gmail.com>,
        Drew Fustini <drew@beagleboard.org>,
        Bin Meng <bmeng.cn@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Guo Ren <guoren@kernel.org>
Subject: Re: [PATCH v3 2/3] riscv: optimized memmove
Organization: FORTH
In-Reply-To: <20210617152754.17960-3-mcroce@linux.microsoft.com>
References: <20210617152754.17960-1-mcroce@linux.microsoft.com>
 <20210617152754.17960-3-mcroce@linux.microsoft.com>
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Στις 2021-06-17 18:27, Matteo Croce έγραψε:
> +
> +/*
> + * Simply check if the buffer overlaps an call memcpy() in case,
> + * otherwise do a simple one byte at time backward copy.
> + */
> +void *__memmove(void *dest, const void *src, size_t count)
> +{
> +	if (dest < src || src + count <= dest)
> +		return memcpy(dest, src, count);
> +
> +	if (dest > src) {
> +		const char *s = src + count;
> +		char *tmp = dest + count;
> +
> +		while (count--)
> +			*--tmp = *--s;
> +	}
> +	return dest;
> +}
> +EXPORT_SYMBOL(__memmove);
> +

Copying backwards byte-per-byte is suboptimal, I understand this is not 
a very common scenario but you could at least check if they are both 
word-aligned e.g. (((src + len) | (dst + len)) & mask), or missaligned 
by the same offset e.g. (((src + len) ^ (dst + len)) & mask) and still 
end up doing word-by-word copying. Ideally it would be great if you 
re-used the same technique you used for forwards copying on your memcpy.

> +void *memmove(void *dest, const void *src, size_t count) __weak
> __alias(__memmove);
> +EXPORT_SYMBOL(memmove);

As I mentioned on your memcpy patch, if you implement memmove, you can 
just alias memcpy to memmove and we won't have to worry about memcpy 
being used on overlapping regions.