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Date: Thu, 8 May 2008 13:26:30 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: segher@kernel.crashing.org, hancockr@shaw.ca, lists@nerdbynature.de,
       linux-kernel@vger.kernel.org, mingo@elte.hu, tglx@linutronix.de,
       johnstul@us.ibm.com
Subject: Re: [PATCH] common implementation of iterative div/mod
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On Thu, 08 May 2008 16:16:41 +0100
Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> We have a few instances of the open-coded iterative div/mod loop, used
> when we don't expcet the dividend to be much bigger than the divisor.
> Unfortunately modern gcc's have the tendency to strength "reduce" this
> into a full mod operation, which isn't necessarily any faster, and
> even if it were, doesn't exist if gcc implements it in libgcc.
> 
> The workaround is to put a dummy asm statement in the loop to prevent
> gcc from performing the transformation.
> 
> This patch creates a single implementation of this loop, and uses it
> to replace the open-coded versions I know about.

Fair enough.  I'll plan on feeding this into 2.6.26 soon.

>  #endif /* BITS_PER_LONG == 32 */
> +
> +/*
> + * Iterative div/mod for use when dividend is not expected to be much
> + * bigger than divisor.
> + */
> +unsigned iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
> +{
> +	unsigned ret = 0;
> +
> +	while(dividend >= divisor) {
> +		/* The following asm() prevents the compiler from
> +		   optimising this loop into a modulo operation.  */
> +		asm("" : "+rm"(dividend));
> +
> +		dividend -= divisor;
> +		ret++;
> +	}
> +
> +	*remainder = dividend;
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(iter_div_u64_rem);
> 

I think it would be better to do s/unsigned/u32/ here.  It's cosmetic, but
all this sort of code is pretty formal about the sizes of the types which
it uses, and it sure needs to be.