git.distorted.org.uk Git - u/mdw/catacomb/blob - m4/mdw-probe-constant.m4

   1 dnl -*-autoconf-*-
   2
   3 ### SYNOPSIS
   4 ###
   5 ###   mdw_PROBE_CONSTANT(VAR, EXPR, [PREAMBLE], [IF-FAILED])
   6 ###
   7 ### DESCRIPTION
   8 ###
   9 ###   Extracts the value of a a constant integer expression from the
  10 ###   compiler.  This works even if the compiler in question doesn't target
  11 ###   the current architecture.  The value must be in the range -10^244 < x <
  12 ###   10^244; this is probably fair enough.  In the extraordinarily unliklely
  13 ###   event that the constant value is outside these bounds, the macro will
  14 ###   fail rather than silently giving a wrong answer.
  15 ###
  16 ###   The result of the macro is that the shell variable VAR has the value of
  17 ###   the expression EXPR, in decimal.  The PREAMBLE, if given, is inserted
  18 ###   before EXPR is evaluated; it should contain #include and #define
  19 ###   directives which are used to compute the value of the expression.
  20 ###
  21 ###   The idea for this macro came from the AC_C_COMPILE_VALUE macro by
  22 ###   Ilguiz Latypov; this implementation has a number of advantages:
  23 ###
  24 ###     * it has an immense range of representable values, notably including
  25 ###       negative numbers; and
  26 ###
  27 ###     * it returns the value directly in a shell variable rather than
  28 ###       inventing an AC_DEFINE for it.
  29 ###
  30 ### LICENSE
  31 ###
  32 ###   Copyright (c) 2013 Mark Wooding <mdw@distorted.org.uk>
  33 ###
  34 ###   This program is free software: you can redistribute it and/or modify it
  35 ###   under the terms of the GNU General Public License as published by the
  36 ###   Free Software Foundation, either version 2 of the License, or (at your
  37 ###   option) any later version.
  38 ###
  39 ###   This program is distributed in the hope that it will be useful, but
  40 ###   WITHOUT ANY WARRANTY; without even the implied warranty of
  41 ###   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  42 ###   General Public License for more details.
  43 ###
  44 ###   You should have received a copy of the GNU General Public License along
  45 ###   with this program. If not, see <http://www.gnu.org/licenses/>.
  46 ###
  47 ###   In particular, no exception to the GPL is granted regarding generated
  48 ###   `configure' scripts which are the output of Autoconf.
  49
  50 # Serial 1
  51 AC_COPYRIGHT([
  52 Portions copyright (c) 2013 Mark Wooding.
  53
  54 This configure script is free software: you can redistribute it and/or
  55 modify it under he terms of the GNU General Public License as published
  56 by the Free Software Foundation, either version 2 of the License, or
  57 (at your option) any later version.])
  58
  59 AC_DEFUN([mdw__PROBE_CONSTANT_SETUP],
  60   [mdw__probe_constant_body="[
  61
  62 /* The following program is copyright (c) 2013 Mark Wooding.  It is free
  63  * software: you can redistribute it and/or modify it under the terms of the
  64  * GNU General Public License as published by the Free Software Foundation,
  65  * either version 2 of the License, or (at your option) any later version.
  66  *
  67  * This program is distributed in the hope that it will be useful, but
  68  * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  69  * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  70  * for more details.
  71  *
  72  * You should have received a copy of the GNU General Public License along
  73  * with this program. If not, see <http://www.gnu.org/licenses/>.
  74  */
  75
  76 /* The constant: 1 billion.  We'll pull digits out in groups of nine, since
  77  * we can work with constants of at least the size of a C \`long'.
  78  */
  79 #define MDW__G 1000000000
  80
  81 /* An empty macro, used as an argument sometimes. */
  82 #define MDW__E
  83
  84 /* A cheesy compile-time assertion.  If X is zero, then we try to declare
  85  * an array with a negative number of elements.  Wrap this up in an anonymous
  86  * struct so that we don't have to worry about naming things if we make
  87  * more than one assertion.
  88  */
  89 #define MDW__ASSERT(x) struct { int v[1 - 2*!(x)]; }
  90
  91 /* Return the value of X/DIV, with further divisions D applied, truncating
  92  * towards zero.  DIV must be greater than one.  This works even if X is
  93  * negative, never tries to divide negative numbers, and doesn't try to
  94  * negate the most-negative value.  There are three cases: if X <= -DIV then
  95  * X/DIV = -(X + DIV)/DIV - 1, and X + DIV is less negative than X so this is
  96  * a safe negation; if -DIV < X < 0 then the result is zero; otherwise, X
  97  * is nonnegative so the straightforward division is safe.  Because DIV > 1,
  98  * X/DIV is safe to negate, and we can apply the remaining divisions to it.
  99  */
 100 #define MDW__SHIFT(x, div, d) \\
 101   ((x) >= 0 ? ((x)/div d) : \\
 102    (x) <= -(div) ? -((-((x) + (div))/(div) + 1) d) : 0)
 103
 104 /* Extract the bottommost digit of X, as an integer: i.e., the value of
 105  * abs(X) mod 10.  This works even if X is negative, never tries to divide
 106  * negative numbers, and doesn't try to divide the most-negative value.
 107  */
 108 #define MDW__RAW_DIGIT(x) (((x) < 0 ? -((x) + 1) % 10 + 1 : (x)) % 10)
 109
 110 /* Extract the bottommost digit of X, as a character; if X is zero, then
 111  * produce a space instead.  This avoids leading zeroes which can be
 112  * misinterpreted by callers.
 113  */
 114 #define MDW__TEXT_DIGIT(x) ((x) ? '0' + MDW__RAW_DIGIT(x) : ' ')
 115
 116 /* Extract the bottommost digit of the probe value, after dividing by DIV
 117  * and applying the divisons D.
 118  */
 119 #define MDW__DIGIT(div, d) \\
 120   MDW__TEXT_DIGIT(MDW__SHIFT(MDW__PROBE_EXPR, div, d))
 121
 122 /* Extract the bottommost six digits of the probe value after dividing by 10
 123  * and then applying the divisions D.
 124  */
 125 #define MDW__9DIGITS(d) \\
 126   MDW__DIGIT(1000000000, d), \\
 127   MDW__DIGIT( 100000000, d), \\
 128   MDW__DIGIT(  10000000, d), \\
 129   MDW__DIGIT(   1000000, d), \\
 130   MDW__DIGIT(    100000, d), \\
 131   MDW__DIGIT(     10000, d), \\
 132   MDW__DIGIT(      1000, d), \\
 133   MDW__DIGIT(       100, d), \\
 134   MDW__DIGIT(        10, d)
 135
 136 /* Increasingly huge divisions.  PN divides by 10^(9*2^N). */
 137 #define MDW__P0 /MDW__G
 138 #define MDW__P1 MDW__P0 MDW__P0
 139 #define MDW__P2 MDW__P1 MDW__P1
 140 #define MDW__P3 MDW__P2 MDW__P2
 141 #define MDW__P4 MDW__P3 MDW__P3
 142 #define MDW__P5 MDW__P4 MDW__P4
 143
 144 /* Increasingly long sequences of digits.  DN(P) produces the 9 * 2^N
 145  * digits after applying divisions P.
 146  */
 147 #define MDW__D0(p) MDW__9DIGITS(p MDW__P0), MDW__9DIGITS(p MDW__E)
 148 #define MDW__D1(p) MDW__D0(p MDW__P1), MDW__D0(p)
 149 #define MDW__D2(p) MDW__D1(p MDW__P2), MDW__D1(p)
 150 #define MDW__D3(p) MDW__D2(p MDW__P3), MDW__D2(p)
 151 #define MDW__D4(p) MDW__D3(p MDW__P4), MDW__D3(p)
 152
 153 /* Ensure that our exponential cascade is sufficient to represent the
 154  * expression.
 155  */
 156 MDW__ASSERT(MDW__SHIFT(MDW__PROBE_EXPR, 10, MDW__P5) == 0);
 157
 158 /* Format the output.  Everything is taken care of except the bottommost
 159  * digit, which we handle seaprately because we actually want a \`leading'
 160  * zero here if the constant value is actually zero.  Format it so that
 161  * we can extract it from the object file.
 162  */
 163 const char mdw__probe_output[] = {
 164   '\\n',
 165   'm', 'd', 'w', '-',
 166   'p', 'r', 'o', 'b', 'e', '-',
 167   'v', 'a', 'l', 'u', 'e', '=', '\"',
 168   (MDW__PROBE_EXPR < 0 ? '-' : ' '),
 169   MDW__D4(MDW__E),
 170   '0' + MDW__RAW_DIGIT(MDW__PROBE_EXPR),
 171   '\"', '\\n'
 172 };]"])
 173
 174 AC_DEFUN([mdw_PROBE_CONSTANT],
 175   [AC_REQUIRE([mdw__PROBE_CONSTANT_SETUP])
 176    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[$3
 177 #define MDW__PROBE_EXPR ($2)
 178 $mdw__probe_constant_body]])],
 179      [$1=$(sed -n \
 180           's:^mdw-probe-value="\(-\|\) *\([[0-9]]*\)"$:\1\2:p' conftest.o)],
 181      [m4_if([$4], [],
 182             [AC_MSG_FAILURE([failed to evaluate expression])],
 183             [$4])])])