Provide support for ctz

Author: BenBE

XUtils.c

@@ -371,3 +371,17 @@ double sumPositiveValues(const double* array, size_t count) {
    }
    return sum;
 }
+
+#if !defined(HAVE_BUILTIN_CTZ)
+// map a bit value mod 37 to its position
+static const uint8_t mod37BitPosition[] = {
+  32, 0, 1, 26, 2, 23, 27, 0, 3, 16, 24, 30, 28, 11, 0, 13, 4,
+  7, 17, 0, 25, 22, 31, 15, 29, 10, 12, 6, 0, 21, 14, 9, 5,
+  20, 8, 19, 18
+};
+
+/* Returns the number of trailing zero bits */
+unsigned int countTrailingZeros(unsigned int x) {
+   return mod37BitPosition[(-x & x) % 37];
+}
+#endif

XUtils.h

@@ -131,6 +131,15 @@ int compareRealNumbers(double a, double b);
    nonnegative. */
 double sumPositiveValues(const double* array, size_t count);
 
+/* Returns the number of trailing zero bits */
+#if defined(HAVE_BUILTIN_CTZ)
+static inline unsigned int countTrailingZeros(unsigned int x) {
+   return !x ? 32 : __builtin_ctz(x);
+}
+#else
+unsigned int countTrailingZeros(unsigned int x);
+#endif
+
 /* IEC unit prefixes */
 static const char unitPrefixes[] = { 'K', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y', 'R', 'Q' };
 

configure.ac

@@ -273,6 +273,13 @@ AC_LINK_IFELSE([
    [AC_MSG_RESULT(no)
    AC_MSG_ERROR([can not find required macros: NAN, isgreater() and isgreaterequal()])])
 
+AC_MSG_CHECKING(for __builtin_ctz)
+AC_COMPILE_IFELSE([
+   AC_LANG_PROGRAM([], [[__builtin_ctz(1); /* Supported in GCC 3.4 or later */]])],
+   [AC_DEFINE([HAVE_BUILTIN_CTZ], 1, [Define to 1 if the compiler supports '__builtin_ctz' function.])
+   AC_MSG_RESULT(yes)],
+   AC_MSG_RESULT(no))
+
 # ----------------------------------------------------------------------