| 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'])]) |