New module 'count-trailing-zeros'.

* MODULES.html.sh: Mention it.
* lib/count-trailing-zeros.c, lib/count-trailing-zeros.h:
* m4/count-trailing-zeros.m4, modules/count-trailing-zeros:
* modules/count-trailing-zeros-tests:
* tests/test-count-trailing-zeros.c:
New files.

ChangeLog

@@ -1,5 +1,13 @@
 2013-10-06  Paul Eggert  <eggert@cs.ucla.edu>
 
+	New module 'count-trailing-zeros'.
+	* MODULES.html.sh: Mention it.
+	* lib/count-trailing-zeros.c, lib/count-trailing-zeros.h:
+	* m4/count-trailing-zeros.m4, modules/count-trailing-zeros:
+	* modules/count-trailing-zeros-tests:
+	* tests/test-count-trailing-zeros.c:
+	New files.
+
 	count-leading-zeros: port to MSC; support types wider than 64 bits
 	The ideas behind the MSC port are stolen from Emacs.
 	* lib/count-leading-zeros.h:

MODULES.html.sh

@@ -1757,6 +1757,7 @@ func_all_modules ()
   func_begin_table
   func_module count-leading-zeros
   func_module count-one-bits
+  func_module count-trailing-zeros
   func_module ffs
   func_module ffsl
   func_module ffsll

lib/count-trailing-zeros.c

@@ -0,0 +1,3 @@
+#include <config.h>
+#define COUNT_TRAILING_ZEROS_INLINE _GL_EXTERN_INLINE
+#include "count-trailing-zeros.h"

lib/count-trailing-zeros.h

@@ -0,0 +1,106 @@
+/* count-trailing-zeros.h -- counts the number of trailing 0 bits in a word.
+   Copyright 2013 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Paul Eggert.  */
+
+#ifndef COUNT_TRAILING_ZEROS_H
+#define COUNT_TRAILING_ZEROS_H 1
+
+#include <limits.h>
+#include <stdlib.h>
+
+#ifndef _GL_INLINE_HEADER_BEGIN
+ #error "Please include config.h first."
+#endif
+_GL_INLINE_HEADER_BEGIN
+#ifndef COUNT_TRAILING_ZEROS_INLINE
+# define COUNT_TRAILING_ZEROS_INLINE _GL_INLINE
+#endif
+
+/* Assuming the GCC builtin is BUILTIN and the MSC builtin is MSC_BUILTIN,
+   expand to code that computes the number of trailing zeros of the local
+   variable 'x' of type TYPE (an unsigned integer type) and return it
+   from the current function.  */
+#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
+# define COUNT_TRAILING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)               \
+  return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
+#elif _MSC_VER
+# pragma intrinsic _BitScanForward
+# pragma intrinsic _BitScanForward64
+# define COUNT_TRAILING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)               \
+    do                                                                  \
+      {                                                                 \
+        unsigned long result;                                           \
+        return MSC_BUILTIN (&result, x) ? result : CHAR_BIT * sizeof x; \
+      }                                                                 \
+    while (0)
+#else
+# define COUNT_TRAILING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)               \
+    do                                                                  \
+      {                                                                 \
+        int count = 0;                                                  \
+        if (! x)                                                        \
+          return CHAR_BIT * sizeof x;                                   \
+        for (count = 0;                                                 \
+             (count < CHAR_BIT * sizeof x - 32                          \
+              && ! (x & 0xffffffffU));                                  \
+             count += 32)                                               \
+          x = x >> 31 >> 1;                                             \
+        return count + count_trailing_zeros_32 (x);                     \
+      }                                                                 \
+    while (0)
+
+/* Compute and return the number of trailing zeros in the least
+   significant 32 bits of X.  One of these bits must be nonzero.  */
+COUNT_TRAILING_ZEROS_INLINE int
+count_trailing_zeros_32 (unsigned int x)
+{
+  /* http://graphics.stanford.edu/~seander/bithacks.html */
+  static const char de_Bruijn_lookup[32] = {
+    0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
+    31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
+  };
+  return de_Bruijn_lookup[(((x & -x) * 0x077cb531U) & 0xffffffffU) >> 27];
+}
+#endif
+
+/* Compute and return the number of trailing zeros in X. */
+COUNT_TRAILING_ZEROS_INLINE int
+count_trailing_zeros (unsigned int x)
+{
+  COUNT_TRAILING_ZEROS (__builtin_ctz, _BitScanForward, unsigned int);
+}
+
+/* Compute and return the number of trailing zeros in X. */
+COUNT_TRAILING_ZEROS_INLINE int
+count_trailing_zeros_l (unsigned long int x)
+{
+  COUNT_TRAILING_ZEROS (__builtin_ctzl, _BitScanForward, unsigned long int);
+}
+
+#if HAVE_UNSIGNED_LONG_LONG_INT
+/* Compute and return the number of trailing zeros in X. */
+COUNT_TRAILING_ZEROS_INLINE int
+count_trailing_zeros_ll (unsigned long long int x)
+{
+  COUNT_TRAILING_ZEROS (__builtin_ctzll, _BitScanForward64,
+                        unsigned long long int);
+}
+#endif
+
+_GL_INLINE_HEADER_END
+
+#endif

m4/count-trailing-zeros.m4

@@ -0,0 +1,12 @@
+# count-trailing-zeros.m4
+dnl Copyright (C) 2013 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_COUNT_TRAILING_ZEROS],
+[
+  dnl We don't need (and can't compile) count_trailing_zeros_ll
+  dnl unless the type 'unsigned long long int' exists.
+  AC_REQUIRE([AC_TYPE_UNSIGNED_LONG_LONG_INT])
+])

modules/count-trailing-zeros

@@ -0,0 +1,25 @@
+Description:
+Counts the number of trailing 0-bits in a word.
+
+Files:
+lib/count-trailing-zeros.c
+lib/count-trailing-zeros.h
+m4/count-trailing-zeros.m4
+
+Depends-on:
+extern-inline
+
+configure.ac:
+gl_COUNT_TRAILING_ZEROS
+
+Makefile.am:
+lib_SOURCES += count-trailing-zeros.c
+
+Include:
+"count-trailing-zeros.h"
+
+License:
+LGPLv2+
+
+Maintainer:
+all

modules/count-trailing-zeros-tests

@@ -0,0 +1,11 @@
+Files:
+tests/test-count-trailing-zeros.c
+tests/macros.h
+
+Depends-on:
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-count-trailing-zeros
+check_PROGRAMS += test-count-trailing-zeros

tests/test-count-trailing-zeros.c

@@ -0,0 +1,71 @@
+/* Test counting of trailing zeros.
+   Copyright (C) 2013 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Paul Eggert.  */
+
+#include <config.h>
+
+#include "count-trailing-zeros.h"
+
+#include <limits.h>
+#include <stdio.h>
+
+#include "macros.h"
+
+#define UINT_BIT (sizeof (unsigned int) * CHAR_BIT)
+#define ULONG_BIT (sizeof (unsigned long int) * CHAR_BIT)
+#define ULLONG_BIT (sizeof (unsigned long long int) * CHAR_BIT)
+
+#ifndef ULLONG_MAX
+# define HALF (1ULL << (sizeof (unsigned long long int) * CHAR_BIT - 1))
+# define ULLONG_MAX (HALF - 1 + HALF)
+#endif
+
+int
+main (int argc, char *argv[])
+{
+  int i, j;
+
+#define TEST_COUNT_TRAILING_ZEROS(FUNC, TYPE, BITS, MAX, ONE)   \
+  ASSERT (FUNC (0) == BITS);                                    \
+  for (i = 0; i < BITS; i++)                                    \
+    {                                                           \
+      ASSERT (FUNC (ONE << i) == i);                            \
+      for (j = 0; j < i; j++)                                   \
+        ASSERT (FUNC ((ONE << i) | (ONE << j)) == j);           \
+    }                                                           \
+  for (i = 0; i < 1000; i++)                                    \
+    {                                                           \
+      TYPE value = rand () ^ (rand () << 31 << 1);              \
+      int count = 0;                                            \
+      for (j = BITS - 1; 0 <= j; j--)                           \
+        if (value & (ONE << j))                                 \
+          count = j;                                            \
+      ASSERT (count == FUNC (value));                           \
+    }                                                           \
+  ASSERT (FUNC (MAX) == 0);
+
+  TEST_COUNT_TRAILING_ZEROS (count_trailing_zeros, unsigned int,
+                             UINT_BIT, UINT_MAX, 1U);
+  TEST_COUNT_TRAILING_ZEROS (count_trailing_zeros_l, unsigned long int,
+                             ULONG_BIT, ULONG_MAX, 1UL);
+#ifdef HAVE_UNSIGNED_LONG_LONG_INT
+  TEST_COUNT_TRAILING_ZEROS (count_trailing_zeros_ll, unsigned long long int,
+                             ULLONG_BIT, ULLONG_MAX, 1ULL);
+#endif
+
+  return 0;
+}