linux-kernel - Re: [PATCH] fs: O_* bit numbers uniqueness check

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Message-ID: <20100106162004.GA11773@shareable.org>
Date:	Wed, 6 Jan 2010 16:20:04 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Eric Paris <eparis@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: O_* bit numbers uniqueness check

Roland Dreier wrote:
> 
>  > +	/* please add new bits here to ensure allocation uniqueness */
>  > +	BUG_ON(20 != hweight32(
>  > +		O_RDONLY	| O_WRONLY	| O_RDWR	|
> 
> I wonder if there's a way to make this BUILD_BUG_ON(), so the problem is
> caught at compile time (a change like adding an O_ flag is often compile
> tested only for obscure archs).  One could create a compile-time
> macro-ized version of hweight32() and use that, I guess, although I'm
> not sure it's worth the ugliness.

Not ugly at all:

#define hweight32(x) __builtin_popcount(x)

Checked GCC 3.4.3 / 4.1 / 4.4: It expands as a compile-time constant
if the argument is a compile-time constant, so can be used in
BUILD_BUG_ON() and even for array sizes etc.

If GCC's __builtin_popcount() isn't good enough for non-constant
values, the macro would be more involved:

#define hweight32(x) (__builtin_constant_p(x) ? __builtin_popcount(x) \
                      : hweight32_nonconstant(x))

-- Jamie
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