Messages in this thread | | | Date | Thu, 8 May 2008 13:26:30 -0700 | From | Andrew Morton <> | 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.
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