[newlib 03/22] bitset: avoid pessimized code when bitset size is not constant

[Sebastian Huber]

From: Ryan Libby <rlibby at FreeBSD.org>

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
---
 newlib/libc/sys/rtems/include/sys/bitset.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/newlib/libc/sys/rtems/include/sys/bitset.h b/newlib/libc/sys/rtems/include/sys/bitset.h
index d119c23ca..0334939c9 100644
--- a/newlib/libc/sys/rtems/include/sys/bitset.h
+++ b/newlib/libc/sys/rtems/include/sys/bitset.h
@@ -37,12 +37,19 @@
 #include <sys/_types.h>
 #include <machine/_bitcount.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)))
-- 
2.35.3