logb now works correctly on large integers

* admin/merge-gnulib (GNULIB_MODULES): Add count-leading-zeros.
* etc/NEWS: Document the change.
* lib/count-leading-zeros.c, lib/count-leading-zeros.h:
* m4/count-leading-zeros.m4: New files, copied from Gnulib.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* src/floatfns.c: Include count-leading-zeros.h.
(Flogb): Do not convert fixnum to float before taking the log,
as the rounding error can cause the answer to be off by 1.
* src/lisp.h (EMACS_UINT_WIDTH): New constant.
* test/src/floatfns-tests.el (logb-extreme-fixnum): New test.

1 files changed, 114 insertions, 0 deletions

diff --git a/lib/count-leading-zeros.h b/lib/count-leading-zeros.h
new file mode 100644
index 00000000000..e197137e66e
--- /dev/null
+++ b/lib/count-leading-zeros.h
@@ -0,0 +1,114 @@
+/* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
+   Copyright (C) 2012-2017 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>
+
+#ifndef _GL_INLINE_HEADER_BEGIN
+ #error "Please include config.h first."
+#endif
+_GL_INLINE_HEADER_BEGIN
+#ifndef COUNT_LEADING_ZEROS_INLINE
+# define COUNT_LEADING_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 leading 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_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
+  return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
+#elif _MSC_VER
+# pragma intrinsic _BitScanReverse
+# pragma intrinsic _BitScanReverse64
+# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
+    do                                                                  \
+      {                                                                 \
+        unsigned long result;                                           \
+        return MSC_BUILTIN (&result, x) ? result : CHAR_BIT * sizeof x; \
+      }                                                                 \
+    while (0)
+#else
+# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
+    do                                                                  \
+      {                                                                 \
+        int count;                                                      \
+        unsigned int leading_32;                                        \
+        if (! x)                                                        \
+          return CHAR_BIT * sizeof x;                                   \
+        for (count = 0;                                                 \
+             (leading_32 = ((x >> (sizeof (TYPE) * CHAR_BIT - 32))      \
+                            & 0xffffffffU),                             \
+              count < CHAR_BIT * sizeof x - 32 && !leading_32);         \
+             count += 32)                                               \
+          x = x << 31 << 1;                                             \
+        return count + count_leading_zeros_32 (leading_32);             \
+      }                                                                 \
+    while (0)
+
+/* Compute and return the number of leading zeros in X,
+   where 0 < X < 2**32.  */
+COUNT_LEADING_ZEROS_INLINE int
+count_leading_zeros_32 (unsigned int x)
+{
+  /* http://graphics.stanford.edu/~seander/bithacks.html */
+  static const char de_Bruijn_lookup[32] = {
+    31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
+    23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
+  };
+
+  x |= x >> 1;
+  x |= x >> 2;
+  x |= x >> 4;
+  x |= x >> 8;
+  x |= x >> 16;
+  return de_Bruijn_lookup[((x * 0x07c4acddU) & 0xffffffffU) >> 27];
+}
+#endif
+
+/* Compute and return the number of leading zeros in X. */
+COUNT_LEADING_ZEROS_INLINE int
+count_leading_zeros (unsigned int x)
+{
+  COUNT_LEADING_ZEROS (__builtin_clz, _BitScanReverse, unsigned int);
+}
+
+/* Compute and return the number of leading zeros in X. */
+COUNT_LEADING_ZEROS_INLINE int
+count_leading_zeros_l (unsigned long int x)
+{
+  COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long int);
+}
+
+#if HAVE_UNSIGNED_LONG_LONG_INT
+/* Compute and return the number of leading zeros in X. */
+COUNT_LEADING_ZEROS_INLINE int
+count_leading_zeros_ll (unsigned long long int x)
+{
+  COUNT_LEADING_ZEROS (__builtin_clzll, _BitScanReverse64,
+                       unsigned long long int);
+}
+#endif
+
+_GL_INLINE_HEADER_END
+
+#endif /* COUNT_LEADING_ZEROS_H */