linux-kernel - Re: [PATCH] Fix BUILD_BUG_ON in fs/compat_ioctl.c to build with gcc 4.5 snapshot

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Date:	Tue, 22 Dec 2009 01:13:57 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Andi Kleen <andi@...stfloor.org>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, jbeulich@...ell.com,
	rguenther@...e.de, arnd@...db.de
Subject: Re: [PATCH] Fix BUILD_BUG_ON in fs/compat_ioctl.c to build with
	gcc 4.5 snapshot

> No, it's solving the same problem, generically.  There are numerous places
> where we want to use BUILD_BUG_ON() but we don't have a constant expression.
> Look for MAYBE_BUILD_BUG_ON().

Ok so you're saying that should be just a MAYBE_BUILD_BUG_ON()? 

> 
> Subject: 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
> 8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
> 
> 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.

Ok maybe that works, but we need a fix for 2.6.33 too.

-Andi

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