svn commit: r355344 - head/sys/sys

[Ryan Libby]

Author: rlibby
Date: Tue Dec  3 17:43:57 2019
New Revision: 355344
URL: https://svnweb.freebsd.org/changeset/base/355344

Log:
  bitset: avoid pessimized code when bitset size is not constant
  
  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

Modified:
  head/sys/sys/bitset.h

Modified: head/sys/sys/bitset.h
==============================================================================
--- head/sys/sys/bitset.h	Tue Dec  3 17:43:52 2019	(r355343)
+++ head/sys/sys/bitset.h	Tue Dec  3 17:43:57 2019	(r355344)
@@ -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)))