From: Explorer09 <explorer09@gmail.com>
Date: Sun, 12 Feb 2017 11:59:52 +0000 (+0800)
Subject: scanner: compute powers of two faster.
X-Git-Tag: v2.6.4~27
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=c7a545ae0907c81cf52139dd391659586570f60c;p=flex

scanner: compute powers of two faster.

Replace the naive "for" loop in determining power of two with a clever
bitwise solution. This code is around the Internet already and is in
Public Domain.
---

diff --git a/src/dfa.c b/src/dfa.c
index 0fcc5b3..ab10314 100644
--- a/src/dfa.c
+++ b/src/dfa.c
@@ -463,14 +463,9 @@ void ntod (void)
 			/* We still may want to use the table if numecs
 			 * is a power of 2.
 			 */
-			int     power_of_two;
-
-			for (power_of_two = 1; power_of_two <= csize;
-			     power_of_two *= 2)
-				if (numecs == power_of_two) {
-					use_NUL_table = true;
-					break;
-				}
+			if (numecs <= csize && is_power_of_2(numecs)) {
+				use_NUL_table = true;
+			}
 		}
 
 		if (use_NUL_table)
diff --git a/src/flexdef.h b/src/flexdef.h
index 97aec14..e5ab8e0 100644
--- a/src/flexdef.h
+++ b/src/flexdef.h
@@ -109,6 +109,8 @@
 #define ABS(x) ((x) < 0 ? -(x) : (x))
 #endif
 
+/* Whether an integer is a power of two */
+#define is_power_of_2(n) ((n) > 0 && ((n) & ((n) - 1)) == 0)
 
 #define unspecified -1