From http://www.jwz.org/xscreensaver/xscreensaver-5.37.tar.gz

[xscreensaver] / utils / pow2.c

diff --git a/utils/pow2.c b/utils/pow2.c
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+++ b/utils/pow2.c
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+/* pow2, Copyright (c) 2016 Dave Odell <dmo2118@gmail.com>
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation.  No representations are made about the suitability of this
+ * software for any purpose.  It is provided "as is" without express or
+ * implied warranty.
+ */
+
+#include "pow2.h"
+
+#include <limits.h>
+
+int
+i_log2 (size_t x)
+{
+  /* -1 works best for to_pow2. */
+  if (!x)
+    return -1;
+
+  /* GCC 3.4 also has this. */
+# if defined __GNUC__ && __GNUC__ >= 4 || defined __clang__
+  return sizeof(long) * CHAR_BIT - __builtin_clzl(x) - 1;
+# else
+  {
+    unsigned bits = sizeof(x) * CHAR_BIT;
+    size_t mask = (size_t)-1;
+    unsigned result = bits - 1;
+
+    while (bits) {
+      if (!(x & mask)) {
+        result -= bits;
+        x <<= bits;
+      }
+
+      bits >>= 1;
+      mask <<= bits;
+    }
+
+    return result;
+  }
+# endif
+}
+
+size_t
+to_pow2 (size_t x)
+{
+  return !x ? 1 : 1 << (i_log2(x - 1) + 1);
+}