linux-kernel - Re: [PATCH] BUILD_BUG_ON: make it handle more cases

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Message-Id: <1256245477.7495.30.camel@slab.austin.ibm.com>
Date:	Thu, 22 Oct 2009 14:04:37 -0700
From:	Hollis Blanchard <hollisb@...ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Jan Beulich <JBeulich@...ell.com>, sfr@...b.auug.org.au,
	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
	kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-next@...r.kernel.org
Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases

On Tue, 2009-10-20 at 14:15 +1030, Rusty Russell wrote:
> 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.
> 
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

Thanks Rusty, this indeed fixes the problem.

Acked-by: Hollis Blanchard <hollisb@...ibm.com>

-- 
Hollis Blanchard
IBM Linux Technology Center

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