1 /* Compile-time assert-like macros.
2
3 Copyright (C) 2005-2006, 2009-2023 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 (202000 <= __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 # if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
226 # define _Static_assert(R, ...) \
227 _GL_VERIFY ((R), "static assertion failed", -)
228 # endif
229 # if (!defined static_assert \
230 && __STDC_VERSION__ < 202311 \
231 && (!defined __cplusplus \
232 || (__cpp_static_assert < 201411 \
233 && __GNUG__ < 6 && __clang_major__ < 6)))
234 # if defined __cplusplus && _MSC_VER >= 1900 && !defined __clang__
235 /* MSVC 14 in C++ mode supports the two-arguments static_assert but not
236 the one-argument static_assert, and it does not support _Static_assert.
237 We have to play preprocessor tricks to distinguish the two cases.
238 Since the MSVC preprocessor is not ISO C compliant (cf.
239 <https://stackoverflow.com/questions/5134523/>), the solution is specific
240 to MSVC. */
241 # define _GL_EXPAND(x) x
242 # define _GL_SA1(a1) static_assert ((a1), "static assertion failed")
243 # define _GL_SA2 static_assert
244 # define _GL_SA3 static_assert
245 # define _GL_SA_PICK(x1,x2,x3,x4,...) x4
246 # define static_assert(...) _GL_EXPAND(_GL_SA_PICK(__VA_ARGS__,_GL_SA3,_GL_SA2,_GL_SA1)) (__VA_ARGS__)
247 # else
248 # define static_assert _Static_assert /* C11 requires this #define. */
249 # endif
250 # endif
251 #endif
252
253 /* @assert.h omit start@ */
254
255 #if 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
256 # define _GL_HAS_BUILTIN_TRAP 1
257 #elif defined __has_builtin
258 # define _GL_HAS_BUILTIN_TRAP __has_builtin (__builtin_trap)
259 #else
260 # define _GL_HAS_BUILTIN_TRAP 0
261 #endif
262
263 #if 4 < __GNUC__ + (5 <= __GNUC_MINOR__)
264 # define _GL_HAS_BUILTIN_UNREACHABLE 1
265 #elif defined __has_builtin
266 # define _GL_HAS_BUILTIN_UNREACHABLE __has_builtin (__builtin_unreachable)
267 #else
268 # define _GL_HAS_BUILTIN_UNREACHABLE 0
269 #endif
270
271 /* Each of these macros verifies that its argument R is nonzero. To
272 be portable, R should be an integer constant expression. Unlike
273 assert (R), there is no run-time overhead.
274
275 There are two macros, since no single macro can be used in all
276 contexts in C. verify_expr (R, E) is for scalar contexts, including
277 integer constant expression contexts. verify (R) is for declaration
278 contexts, e.g., the top level. */
279
280 /* Verify requirement R at compile-time. Return the value of the
281 expression E. */
282
283 #define verify_expr(R, E) \
284 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
285
286 /* Verify requirement R at compile-time, as a declaration without a
287 trailing ';'. verify (R) acts like static_assert (R) except that
288 it is portable to C11/C++14 and earlier, it can issue better
289 diagnostics, and its name is shorter and may be more convenient. */
290
291 #ifdef __PGI
292 /* PGI barfs if R is long. */
293 # define verify(R) _GL_VERIFY (R, "verify (...)", -)
294 #else
295 # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
296 #endif
297
298 /* Assume that R always holds. Behavior is undefined if R is false,
299 fails to evaluate, or has side effects.
300
301 'assume (R)' is a directive from the programmer telling the
302 compiler that R is true so the compiler needn't generate code to
303 test R. This is why 'assume' is in verify.h: it's related to
304 static checking (in this case, static checking done by the
305 programmer), not dynamic checking.
306
307 'assume (R)' can affect compilation of all the code, not just code
308 that happens to be executed after the assume (R) is "executed".
309 For example, if the code mistakenly does 'assert (R); assume (R);'
310 the compiler is entitled to optimize away the 'assert (R)'.
311
312 Although assuming R can help a compiler generate better code or
313 diagnostics, performance can suffer if R uses hard-to-optimize
314 features such as function calls not inlined by the compiler.
315
316 Avoid Clang's __builtin_assume, as it breaks GNU Emacs master
317 as of 2020-08-23T21:09:49Z!eggert@cs.ucla.edu; see
318 <https://bugs.gnu.org/43152#71>. It's not known whether this breakage
319 is a Clang bug or an Emacs bug; play it safe for now. */
320
321 #if _GL_HAS_BUILTIN_UNREACHABLE
322 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
323 #elif 1200 <= _MSC_VER
324 # define assume(R) __assume (R)
325 #elif 202311 <= __STDC_VERSION__
326 # include <stddef.h>
327 # define assume(R) ((R) ? (void) 0 : unreachable ())
328 #elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP
329 /* Doing it this way helps various packages when configured with
330 --enable-gcc-warnings, which compiles with -Dlint. It's nicer
331 if 'assume' silences warnings with GCC 3.4 through GCC 4.4.7 (2012). */
332 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
333 #else
334 /* Some older tools grok NOTREACHED, e.g., Oracle Studio 12.6 (2017). */
335 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
336 #endif
337
338 /* @assert.h omit end@ */
339
340 #endif