lib/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2020 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program 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 General Public License for more details.
  14
  15    You should have received a copy of the GNU 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 later, in C
  26    mode.
  27
  28    Define _GL_HAVE__STATIC_ASSERT1 to 1 if _Static_assert (R) works as
  29    per C2X, and define _GL_HAVE_STATIC_ASSERT1 if static_assert (R)
  30    works as per C++17.  This is supported by GCC 9.1 and later.
  31
  32    Support compilers claiming conformance to the relevant standard,
  33    and also support GCC when not pedantic.  If we were willing to slow
  34    'configure' down we could also use it with other compilers, but
  35    since this affects only the quality of diagnostics, why bother?  */
  36 #ifndef __cplusplus
  37 # if (201112L <= __STDC_VERSION__ \
  38       || (!defined __STRICT_ANSI__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)))
  39 #  define _GL_HAVE__STATIC_ASSERT 1
  40 # endif
  41 # if (202000L <= __STDC_VERSION__ \
  42       || (!defined __STRICT_ANSI__ && 9 <= __GNUC__))
  43 #  define _GL_HAVE__STATIC_ASSERT1 1
  44 # endif
  45 #else
  46 # if 201703L <= __cplusplus || 9 <= __GNUC__
  47 #  define _GL_HAVE_STATIC_ASSERT1 1
  48 # endif
  49 #endif
  50
  51 /* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
  52    system headers, defines a conflicting _Static_assert that is no
  53    better than ours; override it.  */
  54 #ifndef _GL_HAVE__STATIC_ASSERT
  55 # include <stddef.h>
  56 # undef _Static_assert
  57 #endif
  58
  59 /* Each of these macros verifies that its argument R is nonzero.  To
  60    be portable, R should be an integer constant expression.  Unlike
  61    assert (R), there is no run-time overhead.
  62
  63    If _Static_assert works, verify (R) uses it directly.  Similarly,
  64    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  65    that is an operand of sizeof.
  66
  67    The code below uses several ideas for C++ compilers, and for C
  68    compilers that do not support _Static_assert:
  69
  70    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  71      integral or boolean or floating-point type, this yields an
  72      expression of integral type, whose value is later verified to be
  73      constant and nonnegative.
  74
  75    * Next this expression W is wrapped in a type
  76      struct _gl_verify_type {
  77        unsigned int _gl_verify_error_if_negative: W;
  78      }.
  79      If W is negative, this yields a compile-time error.  No compiler can
  80      deal with a bit-field of negative size.
  81
  82      One might think that an array size check would have the same
  83      effect, that is, that the type struct { unsigned int dummy[W]; }
  84      would work as well.  However, inside a function, some compilers
  85      (such as C++ compilers and GNU C) allow local parameters and
  86      variables inside array size expressions.  With these compilers,
  87      an array size check would not properly diagnose this misuse of
  88      the verify macro:
  89
  90        void function (int n) { verify (n < 0); }
  91
  92    * For the verify macro, the struct _gl_verify_type will need to
  93      somehow be embedded into a declaration.  To be portable, this
  94      declaration must declare an object, a constant, a function, or a
  95      typedef name.  If the declared entity uses the type directly,
  96      such as in
  97
  98        struct dummy {...};
  99        typedef struct {...} dummy;
 100        extern struct {...} *dummy;
 101        extern void dummy (struct {...} *);
 102        extern struct {...} *dummy (void);
 103
 104      two uses of the verify macro would yield colliding declarations
 105      if the entity names are not disambiguated.  A workaround is to
 106      attach the current line number to the entity name:
 107
 108        #define _GL_CONCAT0(x, y) x##y
 109        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 110        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
 111
 112      But this has the problem that two invocations of verify from
 113      within the same macro would collide, since the __LINE__ value
 114      would be the same for both invocations.  (The GCC __COUNTER__
 115      macro solves this problem, but is not portable.)
 116
 117      A solution is to use the sizeof operator.  It yields a number,
 118      getting rid of the identity of the type.  Declarations like
 119
 120        extern int dummy [sizeof (struct {...})];
 121        extern void dummy (int [sizeof (struct {...})]);
 122        extern int (*dummy (void)) [sizeof (struct {...})];
 123
 124      can be repeated.
 125
 126    * Should the implementation use a named struct or an unnamed struct?
 127      Which of the following alternatives can be used?
 128
 129        extern int dummy [sizeof (struct {...})];
 130        extern int dummy [sizeof (struct _gl_verify_type {...})];
 131        extern void dummy (int [sizeof (struct {...})]);
 132        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 133        extern int (*dummy (void)) [sizeof (struct {...})];
 134        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 135
 136      In the second and sixth case, the struct type is exported to the
 137      outer scope; two such declarations therefore collide.  GCC warns
 138      about the first, third, and fourth cases.  So the only remaining
 139      possibility is the fifth case:
 140
 141        extern int (*dummy (void)) [sizeof (struct {...})];
 142
 143    * GCC warns about duplicate declarations of the dummy function if
 144      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
 145      __COUNTER__ macro that can let us generate unique identifiers for
 146      each dummy function, to suppress this warning.
 147
 148    * This implementation exploits the fact that older versions of GCC,
 149      which do not support _Static_assert, also do not warn about the
 150      last declaration mentioned above.
 151
 152    * GCC warns if -Wnested-externs is enabled and 'verify' is used
 153      within a function body; but inside a function, you can always
 154      arrange to use verify_expr instead.
 155
 156    * In C++, any struct definition inside sizeof is invalid.
 157      Use a template type to work around the problem.  */
 158
 159 /* Concatenate two preprocessor tokens.  */
 160 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 161 #define _GL_CONCAT0(x, y) x##y
 162
 163 /* _GL_COUNTER is an integer, preferably one that changes each time we
 164    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 165    otherwise.  __LINE__ isn't perfect, but it's better than a
 166    constant.  */
 167 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 168 # define _GL_COUNTER __COUNTER__
 169 #else
 170 # define _GL_COUNTER __LINE__
 171 #endif
 172
 173 /* Generate a symbol with the given prefix, making it unique if
 174    possible.  */
 175 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 176
 177 /* Verify requirement R at compile-time, as an integer constant expression
 178    that returns 1.  If R is false, fail at compile-time, preferably
 179    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 180
 181 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 182    (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 183
 184 #ifdef __cplusplus
 185 # if !GNULIB_defined_struct__gl_verify_type
 186 template <int w>
 187   struct _gl_verify_type {
 188     unsigned int _gl_verify_error_if_negative: w;
 189   };
 190 #  define GNULIB_defined_struct__gl_verify_type 1
 191 # endif
 192 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 193     _gl_verify_type<(R) ? 1 : -1>
 194 #elif defined _GL_HAVE__STATIC_ASSERT
 195 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 196     struct {                                   \
 197       _Static_assert (R, DIAGNOSTIC);          \
 198       int _gl_dummy;                          \
 199     }
 200 #else
 201 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 202     struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 203 #endif
 204
 205 /* Verify requirement R at compile-time, as a declaration without a
 206    trailing ';'.  If R is false, fail at compile-time.
 207
 208    This macro requires three or more arguments but uses at most the first
 209    two, so that the _Static_assert macro optionally defined below supports
 210    both the C11 two-argument syntax and the C2X one-argument syntax.
 211
 212    Unfortunately, unlike C11, this implementation must appear as an
 213    ordinary declaration, and cannot appear inside struct { ... }.  */
 214
 215 #if defined _GL_HAVE__STATIC_ASSERT
 216 # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
 217 #else
 218 # define _GL_VERIFY(R, DIAGNOSTIC, ...)                                \
 219     extern int (*_GL_GENSYM (_gl_verify_function) (void))              \
 220       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 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 !defined _GL_HAVE_STATIC_ASSERT1 && !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 #if _GL_HAS_BUILTIN_UNREACHABLE
 298 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
 299 #elif 1200 <= _MSC_VER
 300 # define assume(R) __assume (R)
 301 #elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP
 302   /* Doing it this way helps various packages when configured with
 303      --enable-gcc-warnings, which compiles with -Dlint.  It's nicer
 304      when 'assume' silences warnings even with older GCCs.  */
 305 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
 306 #else
 307   /* Some tools grok NOTREACHED, e.g., Oracle Studio 12.6.  */
 308 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
 309 #endif
 310
 311 /* @assert.h omit end@  */
 312
 313 #endif