Commit 7ef88ad5 authored by Rusty Russell's avatar Rusty Russell
Browse files

BUILD_BUG_ON: make it handle more cases 



BUILD_BUG_ON used to use the optimizer to do code elimination or fail
at link time; it was changed to first the size of a negative array (a
nicer compile time error), then (in
8c87df45) to a bitfield.

This forced us to change some non-constant cases to MAYBE_BUILD_BUG_ON();
as Jan points out in that commit, it didn't work as intended anyway.

bitfields: needs a literal constant at parse time, and can't be put under
	"if (__builtin_constant_p(x))" for example.
negative array: can handle anything, but if the compiler can't tell it's
	a constant, silently has no effect.
link time: breaks link if the compiler can't determine the value, but the
	linker output is not usually as informative as a compiler error.

If we use the negative-array-size method *and* the link time trick,
we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
branches, and maximal ability for the compiler to detect errors at
build time.

We also document it thoroughly.

Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
Cc: Jan Beulich <JBeulich@novell.com>
Acked-by: default avatarHollis Blanchard <hollisb@us.ibm.com>
parent 1bae4ce2

include/linux/kernel.h

+27 −6
Original line number Diff line number Diff line
@@ -575,12 +575,6 @@ struct sysinfo { 
	char _f[20-2*sizeof(long)-sizeof(int)];	/* Padding: libc5 uses this.. */

};



/* Force a compilation error if condition is true */

#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))



/* Force a compilation error if condition is constant and true */

#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))



/* Force a compilation error if a constant expression is not a power of 2 */

#define BUILD_BUG_ON_NOT_POWER_OF_2(n)			\

	BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))
@@ -592,6 +586,33 @@ struct sysinfo { 
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))

#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))



/**

 * BUILD_BUG_ON - break compile if a condition is true.

 * @cond: the condition which the compiler should know is false.

 *

 * If you have some code which relies on certain constants being equal, or

 * other compile-time-evaluated condition, you should use BUILD_BUG_ON to

 * detect if someone changes it.

 *

 * The implementation uses gcc's reluctance to create a negative array, but

 * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments

 * to inline functions).  So as a fallback we use the optimizer; if it can't

 * prove the condition is false, it will cause a link error on the undefined

 * "__build_bug_on_failed".  This error message can be harder to track down

 * though, hence the two different methods.

 */

#ifndef __OPTIMIZE__

#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))

#else

extern int __build_bug_on_failed;

#define BUILD_BUG_ON(condition)					\

	do {							\

		((void)sizeof(char[1 - 2*!!(condition)]));	\

		if (condition) __build_bug_on_failed = 1;	\

	} while(0)

#endif

#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)



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

#define __FUNCTION__ (__func__)

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