linux-kernel - Re: [PATCH v1] lib/sort: Add 64 bit swap function

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Date:	Fri, 15 May 2015 09:22:12 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Daniel Wagner <daniel.wagner@...-carit.de>
Cc:	akpm@...ux-foundation.org, mm-commits@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] lib/sort: Add 64 bit swap function

On Wed, May 13 2015, Daniel Wagner <daniel.wagner@...-carit.de> wrote:

>  
> -	if (!swap_func)
> -		swap_func = (size == 4 ? u32_swap : generic_swap);
> +	if (!swap_func) {
> +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
> +		switch (size) {
> +		case 4:
> +			swap_func = u32_swap;
> +			break;
> +		case 8:
> +			swap_func = u64_swap;
> +			break;
> +		}
> +#else
> +		switch (size) {
> +		case 4:
> +			if (((unsigned long)base & 3) == 0)
> +				swap_func = u32_swap;
> +			break;
> +		case 8:
> +			if (((unsigned long)base & 7) == 0)
> +				swap_func = u64_swap;
> +			break;
> +		}
> +#endif /* CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS */
> +
> +		if (!swap_func)
> +			swap_func = generic_swap;
> +	}

I was more thinking of something like

static int alignment_ok(const void *base, int align)
{
	return IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) ||
		((unsigned long)base & (align - 1)) == 0;
}

...

	if (!swap_func) {
		if (size == 4 && alignment_ok(base, 4))
			swap_func = u32_swap;
		else if (size == 8 && alignment_ok(base, 8))
			swap_func = u64_swap;
		else
			swap_func = generic_swap;
	}

It seems to generate the same code (I usually worry about how gcc messes
up switches), so this is just a readability thing.

Rasmus
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