lib/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2024 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 C23.  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 (201112 <= __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 (202311 <= __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 C23 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 202311 <= __STDC_VERSION__ || 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 /* Define _Static_assert if needed.  */
 226 /* With clang ≥ 3.8.0 in C++ mode, _Static_assert already works and accepts
 227    1 or 2 arguments.  We better don't override it, because clang's standard
 228    C++ library uses static_assert inside classes in several places, and our
 229    replacement via _GL_VERIFY does not work in these contexts.  */
 230 # if (defined __cplusplus && defined __clang__ \
 231       && (4 <= __clang_major__ + (8 <= __clang_minor__)))
 232 #  if 5 <= __clang_major__
 233 /* Avoid "warning: 'static_assert' with no message is a C++17 extension".  */
 234 #   pragma clang diagnostic ignored "-Wc++17-extensions"
 235 #  else
 236 /* Avoid "warning: static_assert with no message is a C++1z extension".  */
 237 #   pragma clang diagnostic ignored "-Wc++1z-extensions"
 238 #  endif
 239 # elif !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
 240 #  if !defined _MSC_VER || defined __clang__
 241 #   define _Static_assert(...) \
 242       _GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
 243 #  else
 244 #   if defined __cplusplus && _MSC_VER >= 1910
 245      /* In MSVC 14.1 or newer, static_assert accepts one or two arguments,
 246         but _Static_assert is not defined.  */
 247 #    define _Static_assert static_assert
 248 #   else
 249      /* Work around MSVC preprocessor incompatibility with ISO C; see
 250         <https://stackoverflow.com/questions/5134523/>.  */
 251 #    define _Static_assert(R, ...) \
 252        _GL_VERIFY ((R), "static assertion failed", -)
 253 #   endif
 254 #  endif
 255 # endif
 256 /* Define static_assert if needed.  */
 257 # if (!defined static_assert \
 258       && __STDC_VERSION__ < 202311 \
 259       && (!defined __cplusplus \
 260           || (__cpp_static_assert < 201411 \
 261               && __GNUG__ < 6 && __clang_major__ < 6 && _MSC_VER < 1910)))
 262 #  if defined __cplusplus && _MSC_VER >= 1900 && !defined __clang__
 263 /* MSVC 14 in C++ mode supports the two-arguments static_assert but not
 264    the one-argument static_assert, and it does not support _Static_assert.
 265    We have to play preprocessor tricks to distinguish the two cases.
 266    Since the MSVC preprocessor is not ISO C compliant (see above),.
 267    the solution is specific to MSVC.  */
 268 #   define _GL_EXPAND(x) x
 269 #   define _GL_SA1(a1) static_assert ((a1), "static assertion failed")
 270 #   define _GL_SA2 static_assert
 271 #   define _GL_SA3 static_assert
 272 #   define _GL_SA_PICK(x1,x2,x3,x4,...) x4
 273 #   define static_assert(...) _GL_EXPAND(_GL_SA_PICK(__VA_ARGS__,_GL_SA3,_GL_SA2,_GL_SA1)) (__VA_ARGS__)
 274 /* Avoid "fatal error C1189: #error:  The C++ Standard Library forbids macroizing keywords."  */
 275 #   define _ALLOW_KEYWORD_MACROS 1
 276 #  else
 277 #   define static_assert _Static_assert /* C11 requires this #define. */
 278 #  endif
 279 # endif
 280 #endif
 281
 282 /* @assert.h omit start@  */
 283
 284 #if defined __clang_major__ && __clang_major__ < 5
 285 # define _GL_HAS_BUILTIN_TRAP 0
 286 #elif 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
 287 # define _GL_HAS_BUILTIN_TRAP 1
 288 #elif defined __has_builtin
 289 # define _GL_HAS_BUILTIN_TRAP __has_builtin (__builtin_trap)
 290 #else
 291 # define _GL_HAS_BUILTIN_TRAP 0
 292 #endif
 293
 294 #ifndef _GL_HAS_BUILTIN_UNREACHABLE
 295 # if defined __clang_major__ && __clang_major__ < 5
 296 #  define _GL_HAS_BUILTIN_UNREACHABLE 0
 297 # elif 4 < __GNUC__ + (5 <= __GNUC_MINOR__)
 298 #  define _GL_HAS_BUILTIN_UNREACHABLE 1
 299 # elif defined __has_builtin
 300 #  define _GL_HAS_BUILTIN_UNREACHABLE __has_builtin (__builtin_unreachable)
 301 # else
 302 #  define _GL_HAS_BUILTIN_UNREACHABLE 0
 303 # endif
 304 #endif
 305
 306 /* Each of these macros verifies that its argument R is nonzero.  To
 307    be portable, R should be an integer constant expression.  Unlike
 308    assert (R), there is no run-time overhead.
 309
 310    There are two macros, since no single macro can be used in all
 311    contexts in C.  verify_expr (R, E) is for scalar contexts, including
 312    integer constant expression contexts.  verify (R) is for declaration
 313    contexts, e.g., the top level.  */
 314
 315 /* Verify requirement R at compile-time.  Return the value of the
 316    expression E.  */
 317
 318 #define verify_expr(R, E) \
 319    (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
 320
 321 /* Verify requirement R at compile-time, as a declaration without a
 322    trailing ';'.  verify (R) acts like static_assert (R) except that
 323    it is portable to C11/C++14 and earlier, it can issue better
 324    diagnostics, and its name is shorter and may be more convenient.  */
 325
 326 #ifdef __PGI
 327 /* PGI barfs if R is long.  */
 328 # define verify(R) _GL_VERIFY (R, "verify (...)", -)
 329 #else
 330 # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
 331 #endif
 332
 333 /* Assume that R always holds.  Behavior is undefined if R is false,
 334    fails to evaluate, or has side effects.
 335
 336    'assume (R)' is a directive from the programmer telling the
 337    compiler that R is true so the compiler needn't generate code to
 338    test R.  This is why 'assume' is in verify.h: it's related to
 339    static checking (in this case, static checking done by the
 340    programmer), not dynamic checking.
 341
 342    'assume (R)' can affect compilation of all the code, not just code
 343    that happens to be executed after the assume (R) is "executed".
 344    For example, if the code mistakenly does 'assert (R); assume (R);'
 345    the compiler is entitled to optimize away the 'assert (R)'.
 346
 347    Although assuming R can help a compiler generate better code or
 348    diagnostics, performance can suffer if R uses hard-to-optimize
 349    features such as function calls not inlined by the compiler.
 350
 351    Avoid Clang's __builtin_assume, as it breaks GNU Emacs master
 352    as of 2020-08-23T21:09:49Z!eggert@cs.ucla.edu; see
 353    <https://bugs.gnu.org/43152#71>.  It's not known whether this breakage
 354    is a Clang bug or an Emacs bug; play it safe for now.  */
 355
 356 #if _GL_HAS_BUILTIN_UNREACHABLE
 357 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
 358 #elif 1200 <= _MSC_VER
 359 # define assume(R) __assume (R)
 360 #elif 202311 <= __STDC_VERSION__
 361 # include <stddef.h>
 362 # define assume(R) ((R) ? (void) 0 : unreachable ())
 363 #elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP
 364   /* Doing it this way helps various packages when configured with
 365      --enable-gcc-warnings, which compiles with -Dlint.  It's nicer
 366      if 'assume' silences warnings with GCC 3.4 through GCC 4.4.7 (2012).  */
 367 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
 368 #else
 369   /* Some older tools grok NOTREACHED, e.g., Oracle Studio 12.6 (2017).  */
 370 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
 371 #endif
 372
 373 /* @assert.h omit end@  */
 374
 375 #endif