kernel.h: add BUILD_BUG() macro (1399ff86) · Commits · 戴 / test

We can place this in definitions that we expect the compiler to remove by dead code elimination. If this assertion fails, we get a nice error message at build time. The GCC function attribute error("message") was added in version 4.3, so we define a new macro __linktime_error(message) to expand to this for GCC-4.3 and later. This will give us an error diagnostic from the compiler on the line that fails. For other compilers __linktime_error(message) expands to nothing, and we have to be content with a link time error, but at least we will still get a build error. BUILD_BUG() expands to the undefined function __build_bug_failed() and will fail at link time if the compiler ever emits code for it. On GCC-4.3 and later, attribute((error())) is used so that the failure will be noted at compile time instead. Signed-off-by:

David Daney <david.daney@cavium.com> Acked-by:

David Rientjes <rientjes@google.com> Cc: DM <dm.n9107@gmail.com> Cc: Ralf Baechle <ralf@linux-mips.org> Acked-by:

David Howells <dhowells@redhat.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler-gcc4.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -29,6 +29,7 @@
		the kernel context */
		#define __cold __attribute__((__cold__))

		#define __linktime_error(message) __attribute__((__error__(message)))

		#if __GNUC_MINOR__ >= 5
		/*

include/linux/compiler.h

+3 −1

Original line number	Diff line number	Diff line
		@@ -293,7 +293,9 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
		#ifndef __compiletime_error
		# define __compiletime_error(message)
		#endif

		#ifndef __linktime_error
		# define __linktime_error(message)
		#endif
		/*
		* Prevent the compiler from merging or refetching accesses. The compiler
		* is also forbidden from reordering successive instances of ACCESS_ONCE(),

include/linux/kernel.h

+16 −0

Original line number	Diff line number	Diff line
		@@ -665,6 +665,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
		#define BUILD_BUG_ON_ZERO(e) (0)
		#define BUILD_BUG_ON_NULL(e) ((void*)0)
		#define BUILD_BUG_ON(condition)
		#define BUILD_BUG() (0)
		#else /* __CHECKER__ */

		/* Force a compilation error if a constant expression is not a power of 2 */
		@@ -703,6 +704,21 @@ extern int __build_bug_on_failed;
		if (condition) __build_bug_on_failed = 1; \
		} while(0)
		#endif

		/**
		* BUILD_BUG - break compile if used.
		*
		* If you have some code that you expect the compiler to eliminate at
		* build time, you should use BUILD_BUG to detect if it is
		* unexpectedly used.
		*/
		#define BUILD_BUG() \
		do { \
		extern void __build_bug_failed(void) \
		__linktime_error("BUILD_BUG failed"); \
		__build_bug_failed(); \
		} while (0)

		#endif /* __CHECKER__ */

		/* Trap pasters of __FUNCTION__ at compile-time */

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