gnulib/import/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2022 Free Software Foundation, Inc.
   4
   5    This file is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU Lesser General Public License as
   7    published by the Free Software Foundation; either version 2.1 of the
   8    License, or (at your option) any later version.
   9
  10    This file is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */
  19
  20 #ifndef _GL_VERIFY_H
  21 #define _GL_VERIFY_H
  22
  23
  24 /* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert (R, DIAGNOSTIC)
  25    works as per C11.  This is supported by GCC 4.6.0+ and by clang 4+.
  26
  27    Define _GL_HAVE__STATIC_ASSERT1 to 1 if _Static_assert (R) works as
  28    per C2x.  This is supported by GCC 9.1+.
  29
  30    Support compilers claiming conformance to the relevant standard,
  31    and also support GCC when not pedantic.  If we were willing to slow
  32    'configure' down we could also use it with other compilers, but
  33    since this affects only the quality of diagnostics, why bother?  */
  34 #ifndef __cplusplus
  35 # if (201112L <= __STDC_VERSION__ \
  36       || (!defined __STRICT_ANSI__ \
  37           && (4 < __GNUC__ + (6 <= __GNUC_MINOR__) || 5 <= __clang_major__)))
  38 #  define _GL_HAVE__STATIC_ASSERT 1
  39 # endif
  40 # if (202000L <= __STDC_VERSION__ \
  41       || (!defined __STRICT_ANSI__ && 9 <= __GNUC__))
  42 #  define _GL_HAVE__STATIC_ASSERT1 1
  43 # endif
  44 #endif
  45
  46 /* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
  47    system headers, defines a conflicting _Static_assert that is no
  48    better than ours; override it.  */
  49 #ifndef _GL_HAVE__STATIC_ASSERT
  50 # include <stddef.h>
  51 # undef _Static_assert
  52 #endif
  53
  54 /* Each of these macros verifies that its argument R is nonzero.  To
  55    be portable, R should be an integer constant expression.  Unlike
  56    assert (R), there is no run-time overhead.
  57
  58    If _Static_assert works, verify (R) uses it directly.  Similarly,
  59    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  60    that is an operand of sizeof.
  61
  62    The code below uses several ideas for C++ compilers, and for C
  63    compilers that do not support _Static_assert:
  64
  65    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  66      integral or boolean or floating-point type, this yields an
  67      expression of integral type, whose value is later verified to be
  68      constant and nonnegative.
  69
  70    * Next this expression W is wrapped in a type
  71      struct _gl_verify_type {
  72        unsigned int _gl_verify_error_if_negative: W;
  73      }.
  74      If W is negative, this yields a compile-time error.  No compiler can
  75      deal with a bit-field of negative size.
  76
  77      One might think that an array size check would have the same
  78      effect, that is, that the type struct { unsigned int dummy[W]; }
  79      would work as well.  However, inside a function, some compilers
  80      (such as C++ compilers and GNU C) allow local parameters and
  81      variables inside array size expressions.  With these compilers,
  82      an array size check would not properly diagnose this misuse of
  83      the verify macro:
  84
  85        void function (int n) { verify (n < 0); }
  86
  87    * For the verify macro, the struct _gl_verify_type will need to
  88      somehow be embedded into a declaration.  To be portable, this
  89      declaration must declare an object, a constant, a function, or a
  90      typedef name.  If the declared entity uses the type directly,
  91      such as in
  92
  93        struct dummy {...};
  94        typedef struct {...} dummy;
  95        extern struct {...} *dummy;
  96        extern void dummy (struct {...} *);
  97        extern struct {...} *dummy (void);
  98
  99      two uses of the verify macro would yield colliding declarations
 100      if the entity names are not disambiguated.  A workaround is to
 101      attach the current line number to the entity name:
 102
 103        #define _GL_CONCAT0(x, y) x##y
 104        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 105        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
 106
 107      But this has the problem that two invocations of verify from
 108      within the same macro would collide, since the __LINE__ value
 109      would be the same for both invocations.  (The GCC __COUNTER__
 110      macro solves this problem, but is not portable.)
 111
 112      A solution is to use the sizeof operator.  It yields a number,
 113      getting rid of the identity of the type.  Declarations like
 114
 115        extern int dummy [sizeof (struct {...})];
 116        extern void dummy (int [sizeof (struct {...})]);
 117        extern int (*dummy (void)) [sizeof (struct {...})];
 118
 119      can be repeated.
 120
 121    * Should the implementation use a named struct or an unnamed struct?
 122      Which of the following alternatives can be used?
 123
 124        extern int dummy [sizeof (struct {...})];
 125        extern int dummy [sizeof (struct _gl_verify_type {...})];
 126        extern void dummy (int [sizeof (struct {...})]);
 127        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 128        extern int (*dummy (void)) [sizeof (struct {...})];
 129        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 130
 131      In the second and sixth case, the struct type is exported to the
 132      outer scope; two such declarations therefore collide.  GCC warns
 133      about the first, third, and fourth cases.  So the only remaining
 134      possibility is the fifth case:
 135
 136        extern int (*dummy (void)) [sizeof (struct {...})];
 137
 138    * GCC warns about duplicate declarations of the dummy function if
 139      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
 140      __COUNTER__ macro that can let us generate unique identifiers for
 141      each dummy function, to suppress this warning.
 142
 143    * This implementation exploits the fact that older versions of GCC,
 144      which do not support _Static_assert, also do not warn about the
 145      last declaration mentioned above.
 146
 147    * GCC warns if -Wnested-externs is enabled and 'verify' is used
 148      within a function body; but inside a function, you can always
 149      arrange to use verify_expr instead.
 150
 151    * In C++, any struct definition inside sizeof is invalid.
 152      Use a template type to work around the problem.  */
 153
 154 /* Concatenate two preprocessor tokens.  */
 155 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 156 #define _GL_CONCAT0(x, y) x##y
 157
 158 /* _GL_COUNTER is an integer, preferably one that changes each time we
 159    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 160    otherwise.  __LINE__ isn't perfect, but it's better than a
 161    constant.  */
 162 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 163 # define _GL_COUNTER __COUNTER__
 164 #else
 165 # define _GL_COUNTER __LINE__
 166 #endif
 167
 168 /* Generate a symbol with the given prefix, making it unique if
 169    possible.  */
 170 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 171
 172 /* Verify requirement R at compile-time, as an integer constant expression
 173    that returns 1.  If R is false, fail at compile-time, preferably
 174    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 175
 176 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 177    (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 178
 179 #ifdef __cplusplus
 180 # if !GNULIB_defined_struct__gl_verify_type
 181 template <int w>
 182   struct _gl_verify_type {
 183     unsigned int _gl_verify_error_if_negative: w;
 184   };
 185 #  define GNULIB_defined_struct__gl_verify_type 1
 186 # endif
 187 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 188     _gl_verify_type<(R) ? 1 : -1>
 189 #elif defined _GL_HAVE__STATIC_ASSERT
 190 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 191     struct {                                   \
 192       _Static_assert (R, DIAGNOSTIC);          \
 193       int _gl_dummy;                          \
 194     }
 195 #else
 196 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 197     struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 198 #endif
 199
 200 /* Verify requirement R at compile-time, as a declaration without a
 201    trailing ';'.  If R is false, fail at compile-time.
 202
 203    This macro requires three or more arguments but uses at most the first
 204    two, so that the _Static_assert macro optionally defined below supports
 205    both the C11 two-argument syntax and the C2x one-argument syntax.
 206
 207    Unfortunately, unlike C11, this implementation must appear as an
 208    ordinary declaration, and cannot appear inside struct { ... }.  */
 209
 210 #if 200410 <= __cpp_static_assert
 211 # define _GL_VERIFY(R, DIAGNOSTIC, ...) static_assert (R, DIAGNOSTIC)
 212 #elif defined _GL_HAVE__STATIC_ASSERT
 213 # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
 214 #else
 215 # define _GL_VERIFY(R, DIAGNOSTIC, ...)                                \
 216     extern int (*_GL_GENSYM (_gl_verify_function) (void))              \
 217       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 218 # if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
 219 #  pragma GCC diagnostic ignored "-Wnested-externs"
 220 # endif
 221 #endif
 222
 223 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 224 #ifdef _GL_STATIC_ASSERT_H
 225 # if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
 226 #  define _Static_assert(...) \
 227      _GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
 228 # endif
 229 # if __cpp_static_assert < 201411 && !defined static_assert
 230 #  define static_assert _Static_assert /* C11 requires this #define.  */
 231 # endif
 232 #endif
 233
 234 /* @assert.h omit start@  */
 235
 236 #if 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
 237 # define _GL_HAS_BUILTIN_TRAP 1
 238 #elif defined __has_builtin
 239 # define _GL_HAS_BUILTIN_TRAP __has_builtin (__builtin_trap)
 240 #else
 241 # define _GL_HAS_BUILTIN_TRAP 0
 242 #endif
 243
 244 #if 4 < __GNUC__ + (5 <= __GNUC_MINOR__)
 245 # define _GL_HAS_BUILTIN_UNREACHABLE 1
 246 #elif defined __has_builtin
 247 # define _GL_HAS_BUILTIN_UNREACHABLE __has_builtin (__builtin_unreachable)
 248 #else
 249 # define _GL_HAS_BUILTIN_UNREACHABLE 0
 250 #endif
 251
 252 /* Each of these macros verifies that its argument R is nonzero.  To
 253    be portable, R should be an integer constant expression.  Unlike
 254    assert (R), there is no run-time overhead.
 255
 256    There are two macros, since no single macro can be used in all
 257    contexts in C.  verify_expr (R, E) is for scalar contexts, including
 258    integer constant expression contexts.  verify (R) is for declaration
 259    contexts, e.g., the top level.  */
 260
 261 /* Verify requirement R at compile-time.  Return the value of the
 262    expression E.  */
 263
 264 #define verify_expr(R, E) \
 265    (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
 266
 267 /* Verify requirement R at compile-time, as a declaration without a
 268    trailing ';'.  verify (R) acts like static_assert (R) except that
 269    it is portable to C11/C++14 and earlier, it can issue better
 270    diagnostics, and its name is shorter and may be more convenient.  */
 271
 272 #ifdef __PGI
 273 /* PGI barfs if R is long.  */
 274 # define verify(R) _GL_VERIFY (R, "verify (...)", -)
 275 #else
 276 # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
 277 #endif
 278
 279 /* Assume that R always holds.  Behavior is undefined if R is false,
 280    fails to evaluate, or has side effects.
 281
 282    'assume (R)' is a directive from the programmer telling the
 283    compiler that R is true so the compiler needn't generate code to
 284    test R.  This is why 'assume' is in verify.h: it's related to
 285    static checking (in this case, static checking done by the
 286    programmer), not dynamic checking.
 287
 288    'assume (R)' can affect compilation of all the code, not just code
 289    that happens to be executed after the assume (R) is "executed".
 290    For example, if the code mistakenly does 'assert (R); assume (R);'
 291    the compiler is entitled to optimize away the 'assert (R)'.
 292
 293    Although assuming R can help a compiler generate better code or
 294    diagnostics, performance can suffer if R uses hard-to-optimize
 295    features such as function calls not inlined by the compiler.
 296
 297    Avoid Clang's __builtin_assume, as it breaks GNU Emacs master
 298    as of 2020-08-23T21:09:49Z!eggert@cs.ucla.edu; see
 299    <https://bugs.gnu.org/43152#71>.  It's not known whether this breakage
 300    is a Clang bug or an Emacs bug; play it safe for now.  */
 301
 302 #if _GL_HAS_BUILTIN_UNREACHABLE
 303 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
 304 #elif 1200 <= _MSC_VER
 305 # define assume(R) __assume (R)
 306 #elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP
 307   /* Doing it this way helps various packages when configured with
 308      --enable-gcc-warnings, which compiles with -Dlint.  It's nicer
 309      when 'assume' silences warnings even with older GCCs.  */
 310 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
 311 #else
 312   /* Some tools grok NOTREACHED, e.g., Oracle Studio 12.6.  */
 313 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
 314 #endif
 315
 316 /* @assert.h omit end@  */
 317
 318 #endif