[newlib-cygwin] bitset: avoid pessimized code when bitset size is not constant

[Sebastian Huber <sh@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=96c645a0b1e8c136f0d70106b8ab3d0acf621c12

commit 96c645a0b1e8c136f0d70106b8ab3d0acf621c12
Author: Ryan Libby <rlibby@FreeBSD.org>
Date:   Tue Dec 3 17:43:57 2019 +0000

    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

Diff:
---
 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)))