| Message ID | 1255976369.13995.98.camel@slab.beaverton.ibm.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
On Tue, 20 Oct 2009 04:49:29 am Hollis Blanchard wrote: > On Thu, 2009-10-15 at 08:27 +0100, Jan Beulich wrote: > > My perspective is that it just uncovered already existing brokenness. > > Sorry, I thought it was clear, but to be more explicit: I propose the > following patch, which replaces the current BUILD_BUG_ON implementation > with Rusty's version. OK, I switched my brain back on. Yeah, I agree: we may still want BUILD_OR_RUNTIME_BUG_ON one day, but I like this. It's just missing the giant comment that it needs :) /** * 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 is less neat, and can be harder to * track down. */ Thanks! Rusty.
On Tue, 2009-10-20 at 11:42 +1030, Rusty Russell wrote: > On Tue, 20 Oct 2009 04:49:29 am Hollis Blanchard wrote: > > On Thu, 2009-10-15 at 08:27 +0100, Jan Beulich wrote: > > > My perspective is that it just uncovered already existing brokenness. > > > > Sorry, I thought it was clear, but to be more explicit: I propose the > > following patch, which replaces the current BUILD_BUG_ON implementation > > with Rusty's version. > > OK, I switched my brain back on. Yeah, I agree: we may still want > BUILD_OR_RUNTIME_BUG_ON one day, but I like this. > > It's just missing the giant comment that it needs :) > > /** > * 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 is less neat, and can be harder to > * track down. > */ Do you want to put together a signed-off patch Rusty? It's your code, so I don't feel comfortable doing that. Once we have that, can we remove the mysterious MAYBE_BUILD_BUG_ON statements introduced in previous patches? (Does it BUG or doesn't it??)
diff --git a/include/linux/kernel.h b/include/linux/kernel.h --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -677,18 +677,19 @@ 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 condition is true, but also produce a - result (of value 0 and type size_t), so the expression can be used - e.g. in a structure initializer (or where-ever else comma expressions - aren't permitted). */ -#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); })) -#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); })) +#ifndef __OPTIMIZE__ +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) +#else +/* If it's a constant, catch it at compile time, otherwise at link time. */ +extern int __build_bug_on_failed; +#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1) +#define BUILD_BUG_ON(condition) \ + do { \ + ((void)sizeof(char[1 - 2*!!(condition)])); \ + if (condition) __build_bug_on_failed = 1; \ + } while(0) +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition) +#endif /* Trap pasters of __FUNCTION__ at compile-time */ #define __FUNCTION__ (__func__)