count-leading-zeros: new module

I needed gcc's clz to determine the most significant bit of a
number (useful for things like truncating to a power of 2),
and was surprised it is not a standardized function (the
opposite direction of finding the least significant bit is
given by ffs).  This borrows heavily from the design of the
count-one-bits module.

* modules/count-leading-zeros: New module.
* m4/count-leading-zeros.m4: New file.
* lib/count-leading-zeros.h: Likewise.
* modules/count-leading-zeros-tests: New test.
* tests/test-count-leading-zeros.c: New file.
* MODULES.html.sh (Integer arithmetic functions): Document it.

Signed-off-by: Eric Blake <eblake@redhat.com>

7 files changed, 231 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 4c318b912f..80d1b61045 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,13 @@
+2012-08-10  Eric Blake  <eblake@redhat.com>
+
+	count-leading-zeros: new module
+	* modules/count-leading-zeros: New module.
+	* m4/count-leading-zeros.m4: New file.
+	* lib/count-leading-zeros.h: Likewise.
+	* modules/count-leading-zeros-tests: New test.
+	* tests/test-count-leading-zeros.c: New file.
+	* MODULES.html.sh (Integer arithmetic functions): Document it.
+
 2012-08-07  Simon Josefsson  <simon@josefsson.org>
 	    Jim Meyering  <meyering@redhat.com>
 
diff --git a/MODULES.html.sh b/MODULES.html.sh
index f4c823f317..d67c41ddc6 100755
--- a/MODULES.html.sh
+++ b/MODULES.html.sh
@@ -1755,6 +1755,7 @@ func_all_modules ()
   func_echo "$element"
 
   func_begin_table
+  func_module count-leading-zeros
   func_module count-one-bits
   func_module ffs
   func_module ffsl
diff --git a/lib/count-leading-zeros.h b/lib/count-leading-zeros.h
new file mode 100644
index 0000000000..4d7c291cf0
--- /dev/null
+++ b/lib/count-leading-zeros.h
@@ -0,0 +1,100 @@
+/* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
+   Copyright (C) 2012 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 Eric Blake.  */
+
+#ifndef COUNT_LEADING_ZEROS_H
+# define COUNT_LEADING_ZEROS_H 1
+
+#include <limits.h>
+#include <stdlib.h>
+#include "verify.h"
+
+/* Expand the code which computes the number of leading zeros of the local
+   variable 'x' of type TYPE (an unsigned integer type) and returns it
+   from the current function.  */
+#if 0 && __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
+# define COUNT_LEADING_ZEROS(BUILTIN, TYPE)     \
+  return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
+#else
+# define COUNT_LEADING_ZEROS(BUILTIN, TYPE)                             \
+  /* This condition is written so as to avoid shifting by more than     \
+     31 bits at once, and also avoids a random HP-UX cc bug.  */        \
+  verify (((TYPE) -1 >> 31 >> 31 >> 2) == 0); /* TYPE has at most 64 bits */ \
+  int count = 0;                                                        \
+  if (1 < (TYPE) -1 >> 31) { /* TYPE has more than 32 bits? */          \
+    count = count_leading_zeros_32 (x >> 31 >> 1);                      \
+    if (count < 32)                                                     \
+      return count;                                                     \
+  }                                                                     \
+  return count + count_leading_zeros_32 (x);
+
+/* Compute and return the number of leading zeros in the least
+   significant 32 bits of X. */
+static inline int
+count_leading_zeros_32 (unsigned int x)
+{
+  int count = 0;
+  if (x & 0xffff0000U)
+    x >>= 16;
+  else
+    count += 16;
+  if (x & 0xff00)
+    x >>= 8;
+  else
+    count += 8;
+  if (x & 0xf0)
+    x >>= 4;
+  else
+    count += 4;
+  if (x & 0xc)
+    x >>= 2;
+  else
+    count += 2;
+  if (x & 2)
+    x >>= 1;
+  else
+    count++;
+  if (!(x & 1))
+    count++;
+  return count;
+}
+#endif
+
+/* Compute and return the number of leading zeros in X. */
+static inline int
+count_leading_zeros (unsigned int x)
+{
+  COUNT_LEADING_ZEROS (__builtin_clz, unsigned int);
+}
+
+/* Compute and return the number of leading zeros in X. */
+static inline int
+count_leading_zeros_l (unsigned long int x)
+{
+  COUNT_LEADING_ZEROS (__builtin_clzl, unsigned long int);
+}
+
+#if HAVE_UNSIGNED_LONG_LONG_INT
+/* Compute and return the number of leading zeros in X. */
+static inline int
+count_leading_zeros_ll (unsigned long long int x)
+{
+  COUNT_LEADING_ZEROS (__builtin_clzll, unsigned long long int);
+}
+#endif
+
+#endif /* COUNT_LEADING_ZEROS_H */
diff --git a/m4/count-leading-zeros.m4 b/m4/count-leading-zeros.m4
new file mode 100644
index 0000000000..1efb9414e9
--- /dev/null
+++ b/m4/count-leading-zeros.m4
@@ -0,0 +1,15 @@
+# count-leading-zeros.m4 serial 1
+dnl Copyright (C) 2012 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_LEADING_ZEROS],
+[
+  dnl We don't need (and can't compile) count_leading_zeros_ll
+  dnl unless the type 'unsigned long long int' exists.
+  AC_REQUIRE([AC_TYPE_UNSIGNED_LONG_LONG_INT])
+
+  dnl Prerequisites of lib/count-leading-zeros.h.
+  AC_REQUIRE([AC_C_INLINE])
+])
diff --git a/modules/count-leading-zeros b/modules/count-leading-zeros
new file mode 100644
index 0000000000..60ec930a30
--- /dev/null
+++ b/modules/count-leading-zeros
@@ -0,0 +1,23 @@
+Description:
+Counts the number of leading 0-bits in a word.
+
+Files:
+lib/count-leading-zeros.h
+m4/count-leading-zeros.m4
+
+Depends-on:
+verify
+
+configure.ac:
+gl_COUNT_LEADING_ZEROS
+
+Makefile.am:
+
+Include:
+"count-leading-zeros.h"
+
+License:
+LGPLv2+
+
+Maintainer:
+Eric Blake
diff --git a/modules/count-leading-zeros-tests b/modules/count-leading-zeros-tests
new file mode 100644
index 0000000000..1aff87beda
--- /dev/null
+++ b/modules/count-leading-zeros-tests
@@ -0,0 +1,11 @@
+Files:
+tests/test-count-leading-zeros.c
+tests/macros.h
+
+Depends-on:
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-count-leading-zeros
+check_PROGRAMS += test-count-leading-zeros
diff --git a/tests/test-count-leading-zeros.c b/tests/test-count-leading-zeros.c
new file mode 100644
index 0000000000..8dadd2ce33
--- /dev/null
+++ b/tests/test-count-leading-zeros.c
@@ -0,0 +1,71 @@
+/*
+ * Copyright (C) 2012 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 Eric Blake.  */
+
+#include <config.h>
+
+#include "count-leading-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_LEADING_ZEROS(FUNC, TYPE, BITS, MAX, ONE)    \
+  ASSERT (FUNC (0) == BITS);                                    \
+  for (i = 0; i < BITS; i++)                                    \
+    {                                                           \
+      ASSERT (FUNC (ONE << i) == BITS - i - 1);                 \
+      for (j = 0; j < i; j++)                                   \
+        ASSERT (FUNC ((ONE << i) | (ONE << j)) == BITS - i - 1);\
+    }                                                           \
+  for (i = 0; i < 1000; i++)                                    \
+    {                                                           \
+      TYPE value = rand () ^ (rand () << 31 << 1);              \
+      int count = 0;                                            \
+      for (j = 0; j < BITS; j++)                                \
+        if (value & (ONE << j))                                 \
+          count = BITS - j - 1;                                 \
+      ASSERT (count == FUNC (value));                           \
+    }                                                           \
+  ASSERT (FUNC (MAX) == 0);
+
+  TEST_COUNT_LEADING_ZEROS (count_leading_zeros, unsigned int,
+                            UINT_BIT, UINT_MAX, 1U);
+  TEST_COUNT_LEADING_ZEROS (count_leading_zeros_l, unsigned long int,
+                            ULONG_BIT, ULONG_MAX, 1UL);
+#ifdef HAVE_UNSIGNED_LONG_LONG_INT
+  TEST_COUNT_LEADING_ZEROS (count_leading_zeros_ll, unsigned long long int,
+                            ULLONG_BIT, ULLONG_MAX, 1ULL);
+#endif
+
+  return 0;
+}