add generic-ilog2.h (for reference)

include/bitops-emulated/generic-ilog2.h

@@ -0,0 +1,60 @@
+/* generic-ilog2.h - generic ilog2 implementations.
+ *
+ * Copyright (C) 2024 Bruno Raoult ("br")
+ * Licensed under the GNU General Public License v3.0 or later.
+ * Some rights reserved. See COPYING.
+ *
+ * You should have received a copy of the GNU General Public License along with this
+ * program. If not, see <https://www.gnu.org/licenses/gpl-3.0-standalone.html>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later <https://spdx.org/licenses/GPL-3.0-or-later.html>
+ *
+ */
+#ifndef _GENERIC_ILOG2_H_
+#define _GENERIC_ILOG2_H_
+
+#include "br.h"
+
+/*
+ * See https://stackoverflow.com/a/11398748/3079831
+ */
+static const int __ilog2_32_table[32] = {
+    0,  9,  1, 10, 13, 21,  2, 29,
+    11, 14, 16, 18, 22, 25,  3, 30,
+    8, 12, 20, 28, 15, 17, 24,  7,
+    19, 27, 23,  6, 26,  5,  4, 31
+};
+
+static const int __ilog2_64_table[64] = {
+    63,  0, 58,  1, 59, 47, 53,  2,
+    60, 39, 48, 27, 54, 33, 42,  3,
+    61, 51, 37, 40, 49, 18, 28, 20,
+    55, 30, 34, 11, 43, 14, 22,  4,
+    62, 57, 46, 52, 38, 26, 32, 41,
+    50, 36, 17, 19, 29, 10, 13, 21,
+    56, 45, 25, 31, 35, 16,  9, 12,
+    44, 24, 15,  8, 23,  7,  6,  5
+};
+
+static __always_inline __const int __ilog2_32_emulated (u32 n)
+{
+    n |= n >> 1;
+    n |= n >> 2;
+    n |= n >> 4;
+    n |= n >> 8;
+    n |= n >> 16;
+    return __ilog2_32_table[(n*0x07C4ACDD) >> 27];
+}
+
+static __always_inline __const int __ilog2_64_emulated (u64 n)
+{
+    n |= n >> 1;
+    n |= n >> 2;
+    n |= n >> 4;
+    n |= n >> 8;
+    n |= n >> 16;
+    n |= n >> 32;
+    return __ilog2_64_table[((n - (n >> 1))*0x07EDD5E59A4E28C2LL) >> 58];
+}
+
+#endif /* _GENERIC_ILOG2_H_ */