1 /* Compile-time assert-like macros.
2
3 Copyright (C) 2005-2006, 2009-2021 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 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