Mesa (master): util: Add power-of-two divisor support to compute_fast_udiv_info
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Wed Oct 10 18:13:55 UTC 2018
Module: Mesa
Branch: master
Commit: a9be8dddfedb1d19e43b900bdfd33731d3c390c4
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=a9be8dddfedb1d19e43b900bdfd33731d3c390c4
Author: Marek Olšák <marek.olsak at amd.com>
Date: Fri Oct 5 20:42:16 2018 -0500
util: Add power-of-two divisor support to compute_fast_udiv_info
Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
Reviewed-by: Marek Olšák <marek.olsak at amd.com>
---
src/util/fast_idiv_by_const.c | 21 +++++++++++++++++++++
src/util/fast_idiv_by_const.h | 4 ++--
2 files changed, 23 insertions(+), 2 deletions(-)
diff --git a/src/util/fast_idiv_by_const.c b/src/util/fast_idiv_by_const.c
index 65a9e64078..7b93316268 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 231311f84b..92a3ccdf22 100644
--- a/src/util/fast_idiv_by_const.h
+++ b/src/util/fast_idiv_by_const.h
@@ -98,8 +98,8 @@ 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 1. The shifts by UINT_BITS may be
+ * "free" if the high half of the full multiply is put in a separate register.
*
* saturated_increment(n) means "increment n unless it would wrap to 0," i.e.
* if n == (1 << UINT_BITS)-1: result = n
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