Re: [PATCH v2 04/19] host-utils: add 128-bit quotient support to divu128/divs128

[Richard Henderson]

On 9/2/21 11:07 PM, Luis Fernando Fujita Pires wrote:
> From: Richard Henderson <richard.henderson@linaro.org>
> > Hmm.  I'll note that we have a better divmod primitive in tree, but we aren't
> > using it
> > here: udiv_qrnnd in include/fpu/softfloat-macros.h.
>
> Good to know! I'll change to a (much simpler) implementation using udiv_qrnnd.
> Any pointers on what would be a good place to put udiv_qrnnd, so it can be used 
> by softloat.c and host-utils.c? Would host-utils.h be ok?

Yeah, that should be fine.

Note that udiv_qrnnd requires that the divisor be normalization, i.e. the msb of the 
denominator must be set.  So:

    int sh = clz64(den);
    uint64_t q, r;

    q = udiv_qrnnd(&r, num_hi, num_lo, den << sh);
    q <<= sh;
    r >>= sh;

IIRC.  You'll probably want to have a look at gmp source, from which this comes for best 
practice in implementing a larger division.


r~