Re: [PATCH v2 07/10] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

Daniel Santos
2012-10-06T04:28:57Z

On 10/05/2012 03:59 PM, Josh Triplett wrote:
> On Fri, Oct 05, 2012 at 02:42:46PM -0500, danielfsantos@att.net wrote:
>> --- a/include/linux/compiler.h
>> +++ b/include/linux/compiler.h
>> @@ -296,6 +296,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>> #endif
>> #ifndef __compiletime_error
>> # define __compiletime_error(message)
>> +# define __compiletime_error_fallback(condition) \
>> + ((void)sizeof(char[1 - 2*!!(condition)]))
>> +#endif
>> +#ifndef __compiletime_error_fallback
>> +# define __compiletime_error_fallback(condition) (void)(0)
>
> Might want to use do { } while (0) here, to force the use of a
> semicolon and avoid the use of __compiletime_error_fallback in an
> expression.
Sure! But while we're here, we may want to consider a few other macros
in bug.h. These two are intended to be used as an expression:

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

They are using a different technique to generate the compile-time error,
perhaps because the negative sized array wasn't always working past gcc
4.4? Either way, perhaps these can become

#define BUILD_BUG_ON_ZERO(e) ({BUILD_BUG_ON(e); 0;})
#define BUILD_BUG_ON_NULL(e) ({BUILD_BUG_ON(e); (void*)0;})

This would again give us our cute error message. However, I don't know
when this style of expression began to be supported (I know it's a gcc
extension), but I'm guessing it's pre gcc 3.2 because it's used in
kernel.h. Also:

#define BUILD_BUG_ON_NOT_POWER_OF_2(n) \
BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))

can become:

#define BUILD_BUG_ON_NOT_POWER_OF_2(n) \
BUILD_BUG_ON_MSG((n) == 0 || (((n) & ((n) - 1)) != 0), \
#n " not a power of two")

I think the only thing that would leave unfinished is the __OPTIMIZE__ check
in the BUILD_BUG_ON definition. This is a throw-back to the days before
BUILD_BUG_ON_NON_CONST (oops, that's still in another patch set). Well, if
you look at version 1 of this patch set, you'll see that it has that check,
since __builtin_constant_p never returns one in an unoptimized build.
However, that's a bit more work because we will need to examine every use of
BUILD_BUG_ON and __builtin_constant_p. I only found 2-3 last time I looked,
one of which was commented outwith the remark that it "breaks in funny
ways",
which we certainly already know about __builtin_constant_p. Another was a
pretty complicated expression, but I'll have to look them up again.

Please let me know what you think.

Daniel