linux-kernel - Re: [PATCH] bitops: simplify generic bit finding functions

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Message-ID: <alpine.LFD.1.10.0804271335330.3119@woody.linux-foundation.org>
Date:	Sun, 27 Apr 2008 13:38:34 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Harvey Harrison <harvey.harrison@...il.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bitops: simplify generic bit finding functions

On Sun, 27 Apr 2008, Harvey Harrison wrote:
>
> Oh, I didn't realize, I only did this because sparse started spewing out
> lots of:
> include/linux/bitops.h:166:32: warning: shift too big (65536) for type unsigned long

Well, that's really just a sparse bug/misfeature that didn't matter 
before.

It happens because the warning is done as part of constant expression 
evaluation, but then the expression isn't actually *used*, so when we 
optimize it away - at a later date - it's too late to undo the warning.

I don't rightly know how to fix it. We do want to do the constant 
evaluation early (since all the optimizations that may then make it a 
non-issue depends on constants being constants!), but in order to not 
output the warning we'd have to turn the constant into a "constant with 
warning 'xyz' if used".

Which we have no support for in sparse yet.

		Linus
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