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 Lesser 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 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__) || 4 <= __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 #endif
219 
220 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
221 #ifdef _GL_STATIC_ASSERT_H
222 # if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
223 #  define _Static_assert(...) \
224      _GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
225 # endif
226 # if __cpp_static_assert < 201411 && !defined static_assert
227 #  define static_assert _Static_assert /* C11 requires this #define.  */
228 # endif
229 #endif
230 
231 /* @assert.h omit start@  */
232 
233 #if 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
234 # define _GL_HAS_BUILTIN_TRAP 1
235 #elif defined __has_builtin
236 # define _GL_HAS_BUILTIN_TRAP __has_builtin (__builtin_trap)
237 #else
238 # define _GL_HAS_BUILTIN_TRAP 0
239 #endif
240 
241 #if 4 < __GNUC__ + (5 <= __GNUC_MINOR__)
242 # define _GL_HAS_BUILTIN_UNREACHABLE 1
243 #elif defined __has_builtin
244 # define _GL_HAS_BUILTIN_UNREACHABLE __has_builtin (__builtin_unreachable)
245 #else
246 # define _GL_HAS_BUILTIN_UNREACHABLE 0
247 #endif
248 
249 /* Each of these macros verifies that its argument R is nonzero.  To
250    be portable, R should be an integer constant expression.  Unlike
251    assert (R), there is no run-time overhead.
252 
253    There are two macros, since no single macro can be used in all
254    contexts in C.  verify_expr (R, E) is for scalar contexts, including
255    integer constant expression contexts.  verify (R) is for declaration
256    contexts, e.g., the top level.  */
257 
258 /* Verify requirement R at compile-time.  Return the value of the
259    expression E.  */
260 
261 #define verify_expr(R, E) \
262    (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
263 
264 /* Verify requirement R at compile-time, as a declaration without a
265    trailing ';'.  verify (R) acts like static_assert (R) except that
266    it is portable to C11/C++14 and earlier, it can issue better
267    diagnostics, and its name is shorter and may be more convenient.  */
268 
269 #ifdef __PGI
270 /* PGI barfs if R is long.  */
271 # define verify(R) _GL_VERIFY (R, "verify (...)", -)
272 #else
273 # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
274 #endif
275 
276 /* Assume that R always holds.  Behavior is undefined if R is false,
277    fails to evaluate, or has side effects.
278 
279    'assume (R)' is a directive from the programmer telling the
280    compiler that R is true so the compiler needn't generate code to
281    test R.  This is why 'assume' is in verify.h: it's related to
282    static checking (in this case, static checking done by the
283    programmer), not dynamic checking.
284 
285    'assume (R)' can affect compilation of all the code, not just code
286    that happens to be executed after the assume (R) is "executed".
287    For example, if the code mistakenly does 'assert (R); assume (R);'
288    the compiler is entitled to optimize away the 'assert (R)'.
289 
290    Although assuming R can help a compiler generate better code or
291    diagnostics, performance can suffer if R uses hard-to-optimize
292    features such as function calls not inlined by the compiler.
293 
294    Avoid Clang's __builtin_assume, as it breaks GNU Emacs master
295    as of 2020-08-23T21:09:49Z!eggert@cs.ucla.edu; see
296    <https://bugs.gnu.org/43152#71>.  It's not known whether this breakage
297    is a Clang bug or an Emacs bug; play it safe for now.  */
298 
299 #if _GL_HAS_BUILTIN_UNREACHABLE
300 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
301 #elif 1200 <= _MSC_VER
302 # define assume(R) __assume (R)
303 #elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP
304   /* Doing it this way helps various packages when configured with
305      --enable-gcc-warnings, which compiles with -Dlint.  It's nicer
306      when 'assume' silences warnings even with older GCCs.  */
307 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
308 #else
309   /* Some tools grok NOTREACHED, e.g., Oracle Studio 12.6.  */
310 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
311 #endif
312 
313 /* @assert.h omit end@  */
314 
315 #endif
316