On Sat, Oct 6, 2018 at 12:11 AM Jason Ekstrand <ja...@jlekstrand.net> wrote:
>
> From: Marek Olšák <marek.ol...@amd.com>
>
> ---
> src/util/fast_idiv_by_const.c | 21 +++++++++++++++++++++
> src/util/fast_idiv_by_const.h | 5 +++--
> 2 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/src/util/fast_idiv_by_const.c b/src/util/fast_idiv_by_const.c
> index 65a9e640789..7b93316268c 100644
> --- a/src/util/fast_idiv_by_const.c
> +++ b/src/util/fast_idiv_by_const.c
> @@ -52,6 +52,27 @@ util_compute_fast_udiv_info(uint64_t D, unsigned num_bits,
> unsigned UINT_BITS)
> /* The eventual result */
> struct util_fast_udiv_info result;
>
> + if (util_is_power_of_two_or_zero64(D)) {
> + unsigned div_shift = util_logbase2_64(D);
> +
> + if (div_shift) {
> + /* Dividing by a power of two. */
> + result.multiplier = 1ull << (UINT_BITS - div_shift);
> + result.pre_shift = 0;
> + result.post_shift = 0;
> + result.increment = 0;
> + return result;
> + } else {
> + /* Dividing by 1. */
> + /* Assuming: floor((num + 1) * (2^32 - 1) / 2^32) = num */
> + result.multiplier = UINT_BITS == 64 ? UINT64_MAX :
> + (1ull << UINT_BITS) - 1;
> + result.pre_shift = 0;
> + result.post_shift = 0;
> + result.increment = 1;
> + return result;
> + }
> + }
>
> /* The extra shift implicit in the difference between UINT_BITS and
> num_bits
> */
> diff --git a/src/util/fast_idiv_by_const.h b/src/util/fast_idiv_by_const.h
> index 231311f84be..3363fb9ee71 100644
> --- a/src/util/fast_idiv_by_const.h
> +++ b/src/util/fast_idiv_by_const.h
> @@ -98,8 +98,9 @@ util_compute_fast_sdiv_info(int64_t D, unsigned SINT_BITS);
> * emit("result >>>= UINT_BITS")
> * if m.post_shift > 0: emit("result >>>= m.post_shift")
> *
> - * The shifts by UINT_BITS may be "free" if the high half of the full
> multiply
> - * is put in a separate register.
> + * This second version works even if D is a power of two. The shifts by
I think you meant to say that the second version works even if D is 1.
Marek
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