On October 6, 2018 13:19:15 Marek Olšák <mar...@gmail.com> wrote:
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.
Correct. I'll fix that.
--Jason
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