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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'])]) |