[u/mdw/catacomb] / m4 / mdw-uint-bits.m4

dnl -*-autoconf-*-

### SYNOPSIS
###
###   dnl mdw_UINT_BITS(TYPE, ABBREV)
###
### DESCRIPTION
###
###   Defines ABBREV_BITS to be the number of bits representable by the
###   unsigned type named TYPE.  This works even if the compiler in question
###   is a cross-compiler, and correctly handles systems without 8-bit bytes,
###   or which have hidden bits in their integer representations.
###
### LICENSE
###
###   Copyright (c) 2013 Mark Wooding <mdw@distorted.org.uk>
###
###   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 2 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/>.
###
###   In particular, no exception to the GPL is granted regarding generated
###   `configure' scripts which are the output of Autoconf.

dnl Algorithm:
dnl
dnl We first find an upper bound on the number of bits in the type as
dnl sizeof(TYPE) * CHAR_BIT.  This is usually a good guess at the actual
dnl value, since most modern systems don't have hidden bits, so we test to
dnl see if it's correct.  If not, we start a binary search to find the true
dnl value.
dnl
dnl To test a guess n, we form X = (TYPE)~(TYPE)0, which is the largest value
dnl representable as a TYPE, and attempt to shift it right by n places.  This
dnl is a little subtle, since shifting by more than the word length is
dnl undefined behaviour.  The implementation isn't completely perfect.  It
dnl assumes that a shift of 15 bits must be allowed (which is reasonable,
dnl since an int must be at least 16 bits, and the standard integer
dnl promotions will map shorter integer types to an int).  If n > 15, we
dnl shift down in two stages, once by dividing by 2^15 = 32768, and once by
dnl shifting by n - 15; otherwise we just shift.  (This strange chicanery is
dnl to deal with Microsoft compilers which incorrectly `optimize' adjacent
dnl correct shifts into a single invalid one.)  If X >> n is nonzero then n
dnl is too small; if X >> (n - 1) is zero then n is too large; if neither
dnl condition holds then n is correct.
dnl
dnl This is, unfortunately, logarithmic if the initial guess is wrong, but
dnl that will happen rarely on interesting platforms.  Sites wanting to speed
dnl up the configuration process can pre-seed the configuration cache.  If
dnl anyone really cares, we can detect a native compiler (as opposed to a
dnl cross-compiler) and do the binary-search in C.

# Serial 1
AC_DEFUN([mdw_UINT_BITS],
  [AC_CACHE_CHECK([size of type `$1' in bits], [mdw_cv_$2_bits],
    [mdw_PROBE_CONSTANT([guess], [sizeof($1) * CHAR_BIT], [
#include <limits.h>
#ifdef HAVE_STDINT_H
#  include <stdint.h>
#endif])
     down=0
     while :; do
       mdw_PROBE_CONSTANT([answer],
			  [low($guess) ? -1 : low($guess - 1) ? 0 : +1], [
#ifdef HAVE_STDINT_H
#  include <stdint.h>
#endif
#define srs(x, n) ((n) <= 15 ? (x) >> (n) : ((x)/32768) >> ((n) - 15))
#define max (($1)~($1)0 | 0u)
#define low(b) (srs(max, b) != 0)
])
       case "$answer" in
	 0) break;;
	 1) up=$guess;;
	 -1) down=$guess;;
       esac
       guess=$(expr \( $up + $down \) / 2)
     done
     mdw_cv_$2_bits=$guess])
   AS_TR_SH($2_bits)=$mdw_cv_$2_bits
   AC_DEFINE_UNQUOTED(AS_TR_CPP([$2_BITS]), [$mdw_cv_$2_bits],
		      [number of bits in an object of type `$1'])])
Commit	Line	Data
1c3d4cf5 MW	1	dnl --autoconf--
	2
	3	### SYNOPSIS
	4	###
	5	### dnl mdw_UINT_BITS(TYPE, ABBREV)
	6	###
	7	### DESCRIPTION
	8	###
	9	### Defines ABBREV_BITS to be the number of bits representable by the
	10	### unsigned type named TYPE. This works even if the compiler in question
	11	### is a cross-compiler, and correctly handles systems without 8-bit bytes,
	12	### or which have hidden bits in their integer representations.
	13	###
	14	### LICENSE
	15	###
	16	### Copyright (c) 2013 Mark Wooding <mdw@distorted.org.uk>
	17	###
	18	### This program is free software: you can redistribute it and/or modify it
	19	### under the terms of the GNU General Public License as published by the
	20	### Free Software Foundation, either version 2 of the License, or (at your
	21	### option) any later version.
	22	###
	23	### This program is distributed in the hope that it will be useful, but
	24	### WITHOUT ANY WARRANTY; without even the implied warranty of
	25	### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
	26	### General Public License for more details.
	27	###
	28	### You should have received a copy of the GNU General Public License along
	29	### with this program. If not, see <http://www.gnu.org/licenses/>.
	30	###
	31	### In particular, no exception to the GPL is granted regarding generated
	32	### `configure' scripts which are the output of Autoconf.
	33
	34	dnl Algorithm:
	35	dnl
	36	dnl We first find an upper bound on the number of bits in the type as
	37	dnl sizeof(TYPE) * CHAR_BIT. This is usually a good guess at the actual
	38	dnl value, since most modern systems don't have hidden bits, so we test to
	39	dnl see if it's correct. If not, we start a binary search to find the true
	40	dnl value.
	41	dnl
	42	dnl To test a guess n, we form X = (TYPE)~(TYPE)0, which is the largest value
	43	dnl representable as a TYPE, and attempt to shift it right by n places. This
	44	dnl is a little subtle, since shifting by more than the word length is
	45	dnl undefined behaviour. The implementation isn't completely perfect. It
	46	dnl assumes that a shift of 15 bits must be allowed (which is reasonable,
	47	dnl since an int must be at least 16 bits, and the standard integer
	48	dnl promotions will map shorter integer types to an int). If n > 15, we
	49	dnl shift down in two stages, once by dividing by 2^15 = 32768, and once by
	50	dnl shifting by n - 15; otherwise we just shift. (This strange chicanery is
	51	dnl to deal with Microsoft compilers which incorrectly `optimize' adjacent
	52	dnl correct shifts into a single invalid one.) If X >> n is nonzero then n
	53	dnl is too small; if X >> (n - 1) is zero then n is too large; if neither
	54	dnl condition holds then n is correct.
	55	dnl
	56	dnl This is, unfortunately, logarithmic if the initial guess is wrong, but
	57	dnl that will happen rarely on interesting platforms. Sites wanting to speed
	58	dnl up the configuration process can pre-seed the configuration cache. If
	59	dnl anyone really cares, we can detect a native compiler (as opposed to a
	60	dnl cross-compiler) and do the binary-search in C.
	61
	62	# Serial 1
	63	AC_DEFUN([mdw_UINT_BITS],
	64	[AC_CACHE_CHECK([size of type `$1' in bits], [mdw_cv_$2_bits],
65	[mdw_PROBE_CONSTANT([guess], [sizeof($1) * CHAR_BIT], [
66	#include <limits.h>
67	#ifdef HAVE_STDINT_H
68	# include <stdint.h>
69	#endif])
70	down=0
71	while :; do
72	mdw_PROBE_CONSTANT([answer],
73	[low($guess) ? -1 : low($guess - 1) ? 0 : +1], [
74	#ifdef HAVE_STDINT_H
75	# include <stdint.h>
76	#endif
77	#define srs(x, n) ((n) <= 15 ? (x) >> (n) : ((x)/32768) >> ((n) - 15))
78	#define max (($1)~($1)0 \| 0u)
79	#define low(b) (srs(max, b) != 0)
80	])
81	case "$answer" in
82	0) break;;
83	1) up=$guess;;
84	-1) down=$guess;;
85	esac
86	guess=$(expr \( $up + $down \) / 2)
87	done
88	mdw_cv_$2_bits=$guess])
89	AS_TR_SH($2_bits)=$mdw_cv_$2_bits
90	AC_DEFINE_UNQUOTED(AS_TR_CPP([$2_BITS]), [$mdw_cv_$2_bits],
91	[number of bits in an object of type `$1'])])