350 lines
14 KiB
C++
350 lines
14 KiB
C++
/* Compile-time assert-like macros.
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Copyright (C) 2005-2006, 2009-2023 Free Software Foundation, Inc.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation; either version 2.1 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
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#ifndef _GL_VERIFY_H
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#define _GL_VERIFY_H
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/* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert (R, DIAGNOSTIC)
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works as per C11. This is supported by GCC 4.6.0+ and by clang 4+.
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Define _GL_HAVE__STATIC_ASSERT1 to 1 if _Static_assert (R) works as
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per C23. This is supported by GCC 9.1+.
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Support compilers claiming conformance to the relevant standard,
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and also support GCC when not pedantic. If we were willing to slow
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'configure' down we could also use it with other compilers, but
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since this affects only the quality of diagnostics, why bother? */
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#ifndef __cplusplus
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# if (201112 <= __STDC_VERSION__ \
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|| (!defined __STRICT_ANSI__ \
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&& (4 < __GNUC__ + (6 <= __GNUC_MINOR__) || 5 <= __clang_major__)))
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# define _GL_HAVE__STATIC_ASSERT 1
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# endif
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# if (202311 <= __STDC_VERSION__ \
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|| (!defined __STRICT_ANSI__ && 9 <= __GNUC__))
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# define _GL_HAVE__STATIC_ASSERT1 1
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# endif
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#endif
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/* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
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system headers, defines a conflicting _Static_assert that is no
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better than ours; override it. */
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#ifndef _GL_HAVE__STATIC_ASSERT
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# include <stddef.h>
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# undef _Static_assert
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#endif
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/* Each of these macros verifies that its argument R is nonzero. To
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be portable, R should be an integer constant expression. Unlike
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assert (R), there is no run-time overhead.
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If _Static_assert works, verify (R) uses it directly. Similarly,
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_GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
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that is an operand of sizeof.
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The code below uses several ideas for C++ compilers, and for C
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compilers that do not support _Static_assert:
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* The first step is ((R) ? 1 : -1). Given an expression R, of
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integral or boolean or floating-point type, this yields an
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expression of integral type, whose value is later verified to be
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constant and nonnegative.
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* Next this expression W is wrapped in a type
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struct _gl_verify_type {
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unsigned int _gl_verify_error_if_negative: W;
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}.
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If W is negative, this yields a compile-time error. No compiler can
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deal with a bit-field of negative size.
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One might think that an array size check would have the same
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effect, that is, that the type struct { unsigned int dummy[W]; }
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would work as well. However, inside a function, some compilers
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(such as C++ compilers and GNU C) allow local parameters and
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variables inside array size expressions. With these compilers,
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an array size check would not properly diagnose this misuse of
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the verify macro:
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void function (int n) { verify (n < 0); }
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* For the verify macro, the struct _gl_verify_type will need to
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somehow be embedded into a declaration. To be portable, this
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declaration must declare an object, a constant, a function, or a
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typedef name. If the declared entity uses the type directly,
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such as in
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struct dummy {...};
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typedef struct {...} dummy;
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extern struct {...} *dummy;
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extern void dummy (struct {...} *);
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extern struct {...} *dummy (void);
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two uses of the verify macro would yield colliding declarations
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if the entity names are not disambiguated. A workaround is to
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attach the current line number to the entity name:
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#define _GL_CONCAT0(x, y) x##y
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#define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
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extern struct {...} * _GL_CONCAT (dummy, __LINE__);
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But this has the problem that two invocations of verify from
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within the same macro would collide, since the __LINE__ value
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would be the same for both invocations. (The GCC __COUNTER__
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macro solves this problem, but is not portable.)
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A solution is to use the sizeof operator. It yields a number,
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getting rid of the identity of the type. Declarations like
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extern int dummy [sizeof (struct {...})];
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extern void dummy (int [sizeof (struct {...})]);
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extern int (*dummy (void)) [sizeof (struct {...})];
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can be repeated.
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* Should the implementation use a named struct or an unnamed struct?
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Which of the following alternatives can be used?
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extern int dummy [sizeof (struct {...})];
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extern int dummy [sizeof (struct _gl_verify_type {...})];
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extern void dummy (int [sizeof (struct {...})]);
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extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
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extern int (*dummy (void)) [sizeof (struct {...})];
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extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
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In the second and sixth case, the struct type is exported to the
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outer scope; two such declarations therefore collide. GCC warns
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about the first, third, and fourth cases. So the only remaining
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possibility is the fifth case:
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extern int (*dummy (void)) [sizeof (struct {...})];
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* GCC warns about duplicate declarations of the dummy function if
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-Wredundant-decls is used. GCC 4.3 and later have a builtin
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__COUNTER__ macro that can let us generate unique identifiers for
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each dummy function, to suppress this warning.
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* This implementation exploits the fact that older versions of GCC,
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which do not support _Static_assert, also do not warn about the
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last declaration mentioned above.
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* GCC warns if -Wnested-externs is enabled and 'verify' is used
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within a function body; but inside a function, you can always
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arrange to use verify_expr instead.
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* In C++, any struct definition inside sizeof is invalid.
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Use a template type to work around the problem. */
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/* Concatenate two preprocessor tokens. */
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#define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
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#define _GL_CONCAT0(x, y) x##y
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/* _GL_COUNTER is an integer, preferably one that changes each time we
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use it. Use __COUNTER__ if it works, falling back on __LINE__
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otherwise. __LINE__ isn't perfect, but it's better than a
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constant. */
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#if defined __COUNTER__ && __COUNTER__ != __COUNTER__
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# define _GL_COUNTER __COUNTER__
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#else
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# define _GL_COUNTER __LINE__
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#endif
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/* Generate a symbol with the given prefix, making it unique if
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possible. */
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#define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
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/* Verify requirement R at compile-time, as an integer constant expression
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that returns 1. If R is false, fail at compile-time, preferably
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with a diagnostic that includes the string-literal DIAGNOSTIC. */
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#define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
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(!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
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#ifdef __cplusplus
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# if !GNULIB_defined_struct__gl_verify_type
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template <int w>
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struct _gl_verify_type {
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unsigned int _gl_verify_error_if_negative: w;
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};
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# define GNULIB_defined_struct__gl_verify_type 1
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# endif
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# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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_gl_verify_type<(R) ? 1 : -1>
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#elif defined _GL_HAVE__STATIC_ASSERT
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# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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struct { \
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_Static_assert (R, DIAGNOSTIC); \
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int _gl_dummy; \
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}
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#else
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# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
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#endif
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/* Verify requirement R at compile-time, as a declaration without a
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trailing ';'. If R is false, fail at compile-time.
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This macro requires three or more arguments but uses at most the first
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two, so that the _Static_assert macro optionally defined below supports
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both the C11 two-argument syntax and the C23 one-argument syntax.
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Unfortunately, unlike C11, this implementation must appear as an
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ordinary declaration, and cannot appear inside struct { ... }. */
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#if 202311 <= __STDC_VERSION__ || 200410 <= __cpp_static_assert
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# define _GL_VERIFY(R, DIAGNOSTIC, ...) static_assert (R, DIAGNOSTIC)
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#elif defined _GL_HAVE__STATIC_ASSERT
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# define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
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#else
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# define _GL_VERIFY(R, DIAGNOSTIC, ...) \
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extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
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[_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
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# if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
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# pragma GCC diagnostic ignored "-Wnested-externs"
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# endif
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#endif
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/* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */
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#ifdef _GL_STATIC_ASSERT_H
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# if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
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# if !defined _MSC_VER || defined __clang__
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# define _Static_assert(...) \
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_GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
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# else
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/* Work around MSVC preprocessor incompatibility with ISO C; see
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<https://stackoverflow.com/questions/5134523/>. */
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# define _Static_assert(R, ...) \
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_GL_VERIFY ((R), "static assertion failed", -)
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# endif
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# endif
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# if (!defined static_assert \
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&& __STDC_VERSION__ < 202311 \
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&& (!defined __cplusplus \
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|| (__cpp_static_assert < 201411 \
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&& __GNUG__ < 6 && __clang_major__ < 6)))
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# if defined __cplusplus && _MSC_VER >= 1900 && !defined __clang__
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/* MSVC 14 in C++ mode supports the two-arguments static_assert but not
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the one-argument static_assert, and it does not support _Static_assert.
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We have to play preprocessor tricks to distinguish the two cases.
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Since the MSVC preprocessor is not ISO C compliant (see above),.
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the solution is specific to MSVC. */
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# define _GL_EXPAND(x) x
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# define _GL_SA1(a1) static_assert ((a1), "static assertion failed")
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# define _GL_SA2 static_assert
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# define _GL_SA3 static_assert
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# define _GL_SA_PICK(x1,x2,x3,x4,...) x4
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# define static_assert(...) _GL_EXPAND(_GL_SA_PICK(__VA_ARGS__,_GL_SA3,_GL_SA2,_GL_SA1)) (__VA_ARGS__)
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# else
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# define static_assert _Static_assert /* C11 requires this #define. */
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# endif
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# endif
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#endif
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/* @assert.h omit start@ */
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#if defined __clang_major__ && __clang_major__ < 5
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# define _GL_HAS_BUILTIN_TRAP 0
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#elif 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
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# define _GL_HAS_BUILTIN_TRAP 1
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#elif defined __has_builtin
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# define _GL_HAS_BUILTIN_TRAP __has_builtin (__builtin_trap)
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#else
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# define _GL_HAS_BUILTIN_TRAP 0
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#endif
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#if defined __clang_major__ && __clang_major__ < 5
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# define _GL_HAS_BUILTIN_UNREACHABLE 0
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#elif 4 < __GNUC__ + (5 <= __GNUC_MINOR__)
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# define _GL_HAS_BUILTIN_UNREACHABLE 1
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#elif defined __has_builtin
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# define _GL_HAS_BUILTIN_UNREACHABLE __has_builtin (__builtin_unreachable)
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#else
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# define _GL_HAS_BUILTIN_UNREACHABLE 0
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#endif
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/* Each of these macros verifies that its argument R is nonzero. To
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be portable, R should be an integer constant expression. Unlike
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assert (R), there is no run-time overhead.
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There are two macros, since no single macro can be used in all
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contexts in C. verify_expr (R, E) is for scalar contexts, including
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integer constant expression contexts. verify (R) is for declaration
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contexts, e.g., the top level. */
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/* Verify requirement R at compile-time. Return the value of the
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expression E. */
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#define verify_expr(R, E) \
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(_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
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/* Verify requirement R at compile-time, as a declaration without a
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trailing ';'. verify (R) acts like static_assert (R) except that
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it is portable to C11/C++14 and earlier, it can issue better
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diagnostics, and its name is shorter and may be more convenient. */
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#ifdef __PGI
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/* PGI barfs if R is long. */
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# define verify(R) _GL_VERIFY (R, "verify (...)", -)
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#else
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# define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
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#endif
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/* Assume that R always holds. Behavior is undefined if R is false,
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fails to evaluate, or has side effects.
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'assume (R)' is a directive from the programmer telling the
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compiler that R is true so the compiler needn't generate code to
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test R. This is why 'assume' is in verify.h: it's related to
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static checking (in this case, static checking done by the
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programmer), not dynamic checking.
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'assume (R)' can affect compilation of all the code, not just code
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that happens to be executed after the assume (R) is "executed".
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For example, if the code mistakenly does 'assert (R); assume (R);'
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the compiler is entitled to optimize away the 'assert (R)'.
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Although assuming R can help a compiler generate better code or
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diagnostics, performance can suffer if R uses hard-to-optimize
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features such as function calls not inlined by the compiler.
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Avoid Clang's __builtin_assume, as it breaks GNU Emacs master
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as of 2020-08-23T21:09:49Z!eggert@cs.ucla.edu; see
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<https://bugs.gnu.org/43152#71>. It's not known whether this breakage
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is a Clang bug or an Emacs bug; play it safe for now. */
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#if _GL_HAS_BUILTIN_UNREACHABLE
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# define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
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#elif 1200 <= _MSC_VER
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# define assume(R) __assume (R)
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#elif 202311 <= __STDC_VERSION__
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# include <stddef.h>
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# define assume(R) ((R) ? (void) 0 : unreachable ())
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#elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP
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/* Doing it this way helps various packages when configured with
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--enable-gcc-warnings, which compiles with -Dlint. It's nicer
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if 'assume' silences warnings with GCC 3.4 through GCC 4.4.7 (2012). */
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# define assume(R) ((R) ? (void) 0 : __builtin_trap ())
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#else
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/* Some older tools grok NOTREACHED, e.g., Oracle Studio 12.6 (2017). */
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# define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
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#endif
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/* @assert.h omit end@ */
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#endif
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