linux-kernel - Re: Unsigned widening casts of binary "not" operations..

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Message-ID: <20130423155212.GI31170@thunk.org>
Date:	Tue, 23 Apr 2013 11:52:12 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Laight <David.Laight@...lab.com>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Unsigned widening casts of binary "not" operations..

On Tue, Apr 23, 2013 at 08:42:49AM -0700, Linus Torvalds wrote:
> The best case is for code that does bitmasking ops like this to avoid
> any casts (implicit or explicit) by just avoiding mixed types. That
> was, to a large degree, my hope for the sparse patch, but it's
> nontrivial. Many types are rather "natural" (ie constants have a
> natural "int" type, sizeof() is size_t, etc) and in other cases you
> want to do the same ops for two different types (with the case that
> caused me to start to look at it being the "align to page boundary"
> for a virtual address vs a PAE phys_addr_t) so you can't really avoid
> mixing things in some circumstances.

Maybe it's worth creating a magic helper function, called something
like mask_out() that handles the casting automatically, and it makes
it clear to a reader what you're trying to do?

   	      	     	  	 	   - Ted
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