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
186template <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
314