Re: [PATCH] common implementation of iterative div/mod

[Jeremy Fitzhardinge]

Andrew Morton wrote:
> On Thu, 08 May 2008 16:16:41 +0100
> Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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.
> >     
If you get a chance, could you fix the tpyo.

> > 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.
>   

OK.

   J

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