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bitset: avoid pessimized code when bitset size is not constant

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We have a couple optimizations for when the bitset is known to be just
one word.  But with dynamically sized bitsets, it was actually more work
to determine the size than just to do the necessary computation.  Now,
only use the optimization when the size is known to be constant.

Reviewed by:	markj
Discussed with:	jeff
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D22639
This commit is contained in:
Ryan Libby 2019-12-03 17:43:57 +00:00
parent aa4e741821
commit 1bd8c5e427
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=355344
1 changed files with 9 additions and 2 deletions
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11
sys/sys/bitset.h
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@ -34,12 +34,19 @@
#ifndef _SYS_BITSET_H_
#define _SYS_BITSET_H_
/*
* Whether expr is both constant and true. Result is itself constant.
* Used to enable optimizations for sets with a known small size.
*/
#define __constexpr_cond(expr) (__builtin_constant_p((expr)) && (expr))
#define __bitset_mask(_s, n) \
(1L << ((__bitset_words((_s)) == 1) ? \
(1L << (__constexpr_cond(__bitset_words((_s)) == 1) ? \
(__size_t)(n) : ((n) % _BITSET_BITS)))
#define __bitset_word(_s, n) \
((__bitset_words((_s)) == 1) ? 0 : ((n) / _BITSET_BITS))
(__constexpr_cond(__bitset_words((_s)) == 1) ? \
0 : ((n) / _BITSET_BITS))
#define BIT_CLR(_s, n, p) \
((p)->__bits[__bitset_word(_s, n)] &= ~__bitset_mask((_s), (n)))