lib/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2021 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, and by clang (also in C++ mode).
  27
  28    Define _GL_HAVE__STATIC_ASSERT1 to 1 if _Static_assert (R) works as
  29    per C2X.  This is supported by GCC 9.1 and later, and by clang in
  30    C++1z mode.
  31
  32    Define _GL_HAVE_STATIC_ASSERT1 if static_assert (R) works as per
  33    C++17.  This is supported by GCC 9.1 and later, and by clang in
  34    C++1z mode.
  35
  36    Support compilers claiming conformance to the relevant standard,
  37    and also support GCC when not pedantic.  If we were willing to slow
  38    'configure' down we could also use it with other compilers, but
  39    since this affects only the quality of diagnostics, why bother?  */
  40 #ifndef __cplusplus
  41 # if (201112L <= __STDC_VERSION__ \
  42       || (!defined __STRICT_ANSI__ \
  43           && (4 < __GNUC__ + (6 <= __GNUC_MINOR__) || 4 <= __clang_major__)))
  44 #  define _GL_HAVE__STATIC_ASSERT 1
  45 # endif
  46 # if (202000L <= __STDC_VERSION__ \
  47       || (!defined __STRICT_ANSI__ && 9 <= __GNUC__))
  48 #  define _GL_HAVE__STATIC_ASSERT1 1
  49 # endif
  50 #else
  51 # if 4 <= __clang_major__
  52 #  define _GL_HAVE__STATIC_ASSERT 1
  53 # endif
  54 # if 4 <= __clang_major__ && 201411 <= __cpp_static_assert
  55 #  define _GL_HAVE__STATIC_ASSERT1 1
  56 # endif
  57 # if 201703L <= __cplusplus \
  58      || 9 <= __GNUC__ \
  59      || (4 <= __clang_major__ && 201411 <= __cpp_static_assert)
  60 #  define _GL_HAVE_STATIC_ASSERT1 1
  61 # endif
  62 #endif
  63
  64 /* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
  65    system headers, defines a conflicting _Static_assert that is no
  66    better than ours; override it.  */
  67 #ifndef _GL_HAVE__STATIC_ASSERT
  68 # include <stddef.h>
  69 # undef _Static_assert
  70 #endif
  71
  72 /* Each of these macros verifies that its argument R is nonzero.  To
  73    be portable, R should be an integer constant expression.  Unlike
  74    assert (R), there is no run-time overhead.
  75
  76    If _Static_assert works, verify (R) uses it directly.  Similarly,
  77    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  78    that is an operand of sizeof.
  79
  80    The code below uses several ideas for C++ compilers, and for C
  81    compilers that do not support _Static_assert:
  82
  83    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  84      integral or boolean or floating-point type, this yields an
  85      expression of integral type, whose value is later verified to be
  86      constant and nonnegative.
  87
  88    * Next this expression W is wrapped in a type
  89      struct _gl_verify_type {
  90        unsigned int _gl_verify_error_if_negative: W;
  91      }.
  92      If W is negative, this yields a compile-time error.  No compiler can
  93      deal with a bit-field of negative size.
  94
  95      One might think that an array size check would have the same
  96      effect, that is, that the type struct { unsigned int dummy[W]; }
  97      would work as well.  However, inside a function, some compilers
  98      (such as C++ compilers and GNU C) allow local parameters and
  99      variables inside array size expressions.  With these compilers,
 100      an array size check would not properly diagnose this misuse of
 101      the verify macro:
 102
 103        void function (int n) { verify (n < 0); }
 104
 105    * For the verify macro, the struct _gl_verify_type will need to
 106      somehow be embedded into a declaration.  To be portable, this
 107      declaration must declare an object, a constant, a function, or a
 108      typedef name.  If the declared entity uses the type directly,
 109      such as in
 110
 111        struct dummy {...};
 112        typedef struct {...} dummy;
 113        extern struct {...} *dummy;
 114        extern void dummy (struct {...} *);
 115        extern struct {...} *dummy (void);
 116
 117      two uses of the verify macro would yield colliding declarations
 118      if the entity names are not disambiguated.  A workaround is to
 119      attach the current line number to the entity name:
 120
 121        #define _GL_CONCAT0(x, y) x##y
 122        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 123        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
 124
 125      But this has the problem that two invocations of verify from
 126      within the same macro would collide, since the __LINE__ value
 127      would be the same for both invocations.  (The GCC __COUNTER__
 128      macro solves this problem, but is not portable.)
 129
 130      A solution is to use the sizeof operator.  It yields a number,
 131      getting rid of the identity of the type.  Declarations like
 132
 133        extern int dummy [sizeof (struct {...})];
 134        extern void dummy (int [sizeof (struct {...})]);
 135        extern int (*dummy (void)) [sizeof (struct {...})];
 136
 137      can be repeated.
 138
 139    * Should the implementation use a named struct or an unnamed struct?
 140      Which of the following alternatives can be used?
 141
 142        extern int dummy [sizeof (struct {...})];
 143        extern int dummy [sizeof (struct _gl_verify_type {...})];
 144        extern void dummy (int [sizeof (struct {...})]);
 145        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 146        extern int (*dummy (void)) [sizeof (struct {...})];
 147        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 148
 149      In the second and sixth case, the struct type is exported to the
 150      outer scope; two such declarations therefore collide.  GCC warns
 151      about the first, third, and fourth cases.  So the only remaining
 152      possibility is the fifth case:
 153
 154        extern int (*dummy (void)) [sizeof (struct {...})];
 155
 156    * GCC warns about duplicate declarations of the dummy function if
 157      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
 158      __COUNTER__ macro that can let us generate unique identifiers for
 159      each dummy function, to suppress this warning.
 160
 161    * This implementation exploits the fact that older versions of GCC,
 162      which do not support _Static_assert, also do not warn about the
 163      last declaration mentioned above.
 164
 165    * GCC warns if -Wnested-externs is enabled and 'verify' is used
 166      within a function body; but inside a function, you can always
 167      arrange to use verify_expr instead.
 168
 169    * In C++, any struct definition inside sizeof is invalid.
 170      Use a template type to work around the problem.  */
 171
 172 /* Concatenate two preprocessor tokens.  */
 173 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 174 #define _GL_CONCAT0(x, y) x##y
 175
 176 /* _GL_COUNTER is an integer, preferably one that changes each time we
 177    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 178    otherwise.  __LINE__ isn't perfect, but it's better than a
 179    constant.  */
 180 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 181 # define _GL_COUNTER __COUNTER__
 182 #else
 183 # define _GL_COUNTER __LINE__
 184 #endif
 185
 186 /* Generate a symbol with the given prefix, making it unique if
 187    possible.  */
 188 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 189
 190 /* Verify requirement R at compile-time, as an integer constant expression
 191    that returns 1.  If R is false, fail at compile-time, preferably
 192    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 193
 194 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 195    (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 196
 197 #ifdef __cplusplus
 198 # if !GNULIB_defined_struct__gl_verify_type
 199 template <int w>
 200   struct _gl_verify_type {
 201     unsigned int _gl_verify_error_if_negative: w;
 202   };
 203 #  define GNULIB_defined_struct__gl_verify_type 1
 204 # endif
 205 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 206     _gl_verify_type<(R) ? 1 : -1>
 207 #elif defined _GL_HAVE__STATIC_ASSERT
 208 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 209     struct {                                   \
 210       _Static_assert (R, DIAGNOSTIC);          \
 211       int _gl_dummy;                          \
 212     }
 213 #else
 214 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 215     struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 216 #endif
 217
 218 /* Verify requirement R at compile-time, as a declaration without a
 219    trailing ';'.  If R is false, fail at compile-time.
 220
 221    This macro requires three or more arguments but uses at most the first
 222    two, so that the _Static_assert macro optionally defined below supports
 223    both the C11 two-argument syntax and the C2X one-argument syntax.
 224
 225    Unfortunately, unlike C11, this implementation must appear as an
 226    ordinary declaration, and cannot appear inside struct { ... }.  */
 227
 228 #if defined _GL_HAVE__STATIC_ASSERT
 229 # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
 230 #else
 231 # define _GL_VERIFY(R, DIAGNOSTIC, ...)                                \
 232     extern int (*_GL_GENSYM (_gl_verify_function) (void))              \
 233       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 234 #endif
 235
 236 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 237 #ifdef _GL_STATIC_ASSERT_H
 238 # if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
 239 #  define _Static_assert(...) \
 240      _GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
 241 # endif
 242 # if !defined _GL_HAVE_STATIC_ASSERT1 && !defined static_assert
 243 #  define static_assert _Static_assert /* C11 requires this #define.  */
 244 # endif
 245 #endif
 246
 247 /* @assert.h omit start@  */
 248
 249 #if 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
 250 # define _GL_HAS_BUILTIN_TRAP 1
 251 #elif defined __has_builtin
 252 # define _GL_HAS_BUILTIN_TRAP __has_builtin (__builtin_trap)
 253 #else
 254 # define _GL_HAS_BUILTIN_TRAP 0
 255 #endif
 256
 257 #if 4 < __GNUC__ + (5 <= __GNUC_MINOR__)
 258 # define _GL_HAS_BUILTIN_UNREACHABLE 1
 259 #elif defined __has_builtin
 260 # define _GL_HAS_BUILTIN_UNREACHABLE __has_builtin (__builtin_unreachable)
 261 #else
 262 # define _GL_HAS_BUILTIN_UNREACHABLE 0
 263 #endif
 264
 265 /* Each of these macros verifies that its argument R is nonzero.  To
 266    be portable, R should be an integer constant expression.  Unlike
 267    assert (R), there is no run-time overhead.
 268
 269    There are two macros, since no single macro can be used in all
 270    contexts in C.  verify_expr (R, E) is for scalar contexts, including
 271    integer constant expression contexts.  verify (R) is for declaration
 272    contexts, e.g., the top level.  */
 273
 274 /* Verify requirement R at compile-time.  Return the value of the
 275    expression E.  */
 276
 277 #define verify_expr(R, E) \
 278    (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
 279
 280 /* Verify requirement R at compile-time, as a declaration without a
 281    trailing ';'.  verify (R) acts like static_assert (R) except that
 282    it is portable to C11/C++14 and earlier, it can issue better
 283    diagnostics, and its name is shorter and may be more convenient.  */
 284
 285 #ifdef __PGI
 286 /* PGI barfs if R is long.  */
 287 # define verify(R) _GL_VERIFY (R, "verify (...)", -)
 288 #else
 289 # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
 290 #endif
 291
 292 /* Assume that R always holds.  Behavior is undefined if R is false,
 293    fails to evaluate, or has side effects.
 294
 295    'assume (R)' is a directive from the programmer telling the
 296    compiler that R is true so the compiler needn't generate code to
 297    test R.  This is why 'assume' is in verify.h: it's related to
 298    static checking (in this case, static checking done by the
 299    programmer), not dynamic checking.
 300
 301    'assume (R)' can affect compilation of all the code, not just code
 302    that happens to be executed after the assume (R) is "executed".
 303    For example, if the code mistakenly does 'assert (R); assume (R);'
 304    the compiler is entitled to optimize away the 'assert (R)'.
 305
 306    Although assuming R can help a compiler generate better code or
 307    diagnostics, performance can suffer if R uses hard-to-optimize
 308    features such as function calls not inlined by the compiler.
 309
 310    Avoid Clang's __builtin_assume, as it breaks GNU Emacs master
 311    as of 2020-08-23T21:09:49Z!eggert@cs.ucla.edu; see
 312    <https://bugs.gnu.org/43152#71>.  It's not known whether this breakage
 313    is a Clang bug or an Emacs bug; play it safe for now.  */
 314
 315 #if _GL_HAS_BUILTIN_UNREACHABLE
 316 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
 317 #elif 1200 <= _MSC_VER
 318 # define assume(R) __assume (R)
 319 #elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP
 320   /* Doing it this way helps various packages when configured with
 321      --enable-gcc-warnings, which compiles with -Dlint.  It's nicer
 322      when 'assume' silences warnings even with older GCCs.  */
 323 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
 324 #else
 325   /* Some tools grok NOTREACHED, e.g., Oracle Studio 12.6.  */
 326 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
 327 #endif
 328
 329 /* @assert.h omit end@  */
 330
 331 #endif