Re: [PATCH v5 03/18] qemu/int128: addition of div/rem 128-bit operations

[Alistair Francis]

On Sat, Nov 13, 2021 at 1:06 AM Frédéric Pétrot
<frederic.petrot@univ-grenoble-alpes.fr> wrote:
>
> Addition of div and rem on 128-bit integers, using the 128/64->128 divu and
> 64x64->128 mulu in host-utils.
> These operations will be used within div/rem helpers in the 128-bit riscv
> target.
>
> Signed-off-by: Frédéric Pétrot <frederic.petrot@univ-grenoble-alpes.fr>
> Co-authored-by: Fabien Portas <fabien.portas@grenoble-inp.org>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>  include/qemu/int128.h |   6 ++
>  util/int128.c         | 145 ++++++++++++++++++++++++++++++++++++++++++
>  util/meson.build      |   1 +
>  3 files changed, 152 insertions(+)
>  create mode 100644 util/int128.c
>
> diff --git a/include/qemu/int128.h b/include/qemu/int128.h
> index b6d517aea4..ef41892dac 100644
> --- a/include/qemu/int128.h
> +++ b/include/qemu/int128.h
> @@ -386,4 +386,10 @@ static inline void bswap128s(Int128 *s)
>      *s = bswap128(*s);
>  }
>
> +#define UINT128_MAX int128_make128(~0LL, ~0LL)
> +Int128 int128_divu(Int128, Int128);
> +Int128 int128_remu(Int128, Int128);
> +Int128 int128_divs(Int128, Int128);
> +Int128 int128_rems(Int128, Int128);
> +
>  #endif /* INT128_H */
> diff --git a/util/int128.c b/util/int128.c
> new file mode 100644
> index 0000000000..c2ddf197e1
> --- /dev/null
> +++ b/util/int128.c
> @@ -0,0 +1,145 @@
> +#include "qemu/osdep.h"
> +#include "qemu/host-utils.h"
> +#include "qemu/int128.h"
> +
> +#ifdef CONFIG_INT128
> +
> +Int128 int128_divu(Int128 a, Int128 b)
> +{
> +    return (__uint128_t)a / (__uint128_t)b;
> +}
> +
> +Int128 int128_remu(Int128 a, Int128 b)
> +{
> +    return (__uint128_t)a % (__uint128_t)b;
> +}
> +
> +Int128 int128_divs(Int128 a, Int128 b)
> +{
> +    return a / b;
> +}
> +
> +Int128 int128_rems(Int128 a, Int128 b)
> +{
> +    return a % b;
> +}
> +
> +#else
> +
> +/*
> + * Division and remainder algorithms for 128-bit due to Stefan Kanthak,
> + * https://skanthak.homepage.t-online.de/integer.html#udivmodti4
> + * Preconditions:
> + *     - function should never be called with v equals to 0, it has to
> + *       be dealt with beforehand
> + *     - quotien pointer must be valid
> + */
> +static Int128 divrem128(Int128 u, Int128 v, Int128 *q)
> +{
> +    Int128 qq;
> +    uint64_t hi, lo, tmp;
> +    int s;
> +
> +    if ((s = clz64(v.hi)) == 64) {
> +        /* we have uu÷0v => let's use divu128 */
> +        hi = u.hi;
> +        lo = u.lo;
> +        tmp = divu128(&lo, &hi, v.lo);
> +        *q = int128_make128(lo, hi);
> +        return int128_make128(tmp, 0);
> +    } else {
> +        hi = int128_gethi(int128_lshift(v, s));
> +
> +        if (hi > u.hi) {
> +            lo = u.lo;
> +            tmp = u.hi;
> +            divu128(&lo, &tmp, hi);
> +            lo = int128_gethi(int128_lshift(int128_make128(lo, 0), s));
> +        } else { /* prevent overflow */
> +            lo = u.lo;
> +            tmp = u.hi - hi;
> +            divu128(&lo, &tmp, hi);
> +            lo = int128_gethi(int128_lshift(int128_make128(lo, 1), s));
> +        }
> +
> +        qq = int128_make64(lo);
> +
> +        tmp = lo * v.hi;
> +        mulu64(&lo, &hi, lo, v.lo);
> +        hi += tmp;
> +
> +        if (hi < tmp     /* quotient * divisor >= 2**128 > dividend */
> +            || hi > u.hi /* quotient * divisor > dividend */
> +            || (hi == u.hi && lo > u.lo)) {
> +            qq.lo -= 1;
> +            mulu64(&lo, &hi, qq.lo, v.lo);
> +            hi += qq.lo * v.hi;
> +        }
> +
> +        *q = qq;
> +        u.hi -= hi + (u.lo < lo);
> +        u.lo -= lo;
> +        return u;
> +    }
> +}
> +
> +Int128 int128_divu(Int128 a, Int128 b)
> +{
> +    Int128 q;
> +    divrem128(a, b, &q);
> +    return q;
> +}
> +
> +Int128 int128_remu(Int128 a, Int128 b)
> +{
> +    Int128 q;
> +    return divrem128(a, b, &q);
> +}
> +
> +Int128 int128_divs(Int128 a, Int128 b)
> +{
> +    Int128 q;
> +    bool sgna = !int128_nonneg(a);
> +    bool sgnb = !int128_nonneg(b);
> +
> +    if (sgna) {
> +        a = int128_neg(a);
> +    }
> +
> +    if (sgnb) {
> +        b = int128_neg(b);
> +    }
> +
> +    divrem128(a, b, &q);
> +
> +    if (sgna != sgnb) {
> +        q = int128_neg(q);
> +    }
> +
> +    return q;
> +}
> +
> +Int128 int128_rems(Int128 a, Int128 b)
> +{
> +    Int128 q, r;
> +    bool sgna = !int128_nonneg(a);
> +    bool sgnb = !int128_nonneg(b);
> +
> +    if (sgna) {
> +        a = int128_neg(a);
> +    }
> +
> +    if (sgnb) {
> +        b = int128_neg(b);
> +    }
> +
> +    r = divrem128(a, b, &q);
> +
> +    if (sgna) {
> +        r = int128_neg(r);
> +    }
> +
> +    return r;
> +}
> +
> +#endif
> diff --git a/util/meson.build b/util/meson.build
> index 05b593055a..e676b2f6c6 100644
> --- a/util/meson.build
> +++ b/util/meson.build
> @@ -48,6 +48,7 @@ util_ss.add(files('transactions.c'))
>  util_ss.add(when: 'CONFIG_POSIX', if_true: files('drm.c'))
>  util_ss.add(files('guest-random.c'))
>  util_ss.add(files('yank.c'))
> +util_ss.add(files('int128.c'))
>
>  if have_user
>    util_ss.add(files('selfmap.c'))
> --
> 2.33.1
>
>