From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH] fs: O_* bit numbers uniqueness check Date: Sat, 9 Jan 2010 21:44:56 +0800 Message-ID: <20100109134456.GC20015@localhost> References: <20100106065526.GB11368@localhost> <20100106162004.GA11773@shareable.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Roland Dreier , Andrew Morton , David Miller , Stephen Rothwell , Al Viro , Christoph Hellwig , Eric Paris , LKML , "linux-fsdevel@vger.kernel.org" To: Jamie Lokier Return-path: Received: from mga02.intel.com ([134.134.136.20]:65455 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751254Ab0AINpT (ORCPT ); Sat, 9 Jan 2010 08:45:19 -0500 Content-Disposition: inline In-Reply-To: <20100106162004.GA11773@shareable.org> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Jan 07, 2010 at 12:20:04AM +0800, Jamie Lokier wrote: > 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, Thanks for the hint! Would you sign this patch? --- bitops: compile time optimization for hweight_long(CONSTANT) This allows use of hweight_long() in BUILD_BUG_ON(). Suggested by Jamie. CC: Jamie Lokier CC: Roland Dreier Signed-off-by: Wu Fengguang --- include/linux/bitops.h | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) --- linux.orig/include/linux/bitops.h 2010-01-09 12:13:00.000000000 +0800 +++ linux/include/linux/bitops.h 2010-01-09 12:21:50.000000000 +0800 @@ -40,10 +40,14 @@ static __inline__ int get_count_order(un return order; } -static inline unsigned long hweight_long(unsigned long w) -{ - return sizeof(w) == 4 ? hweight32(w) : hweight64(w); -} +#define hweight_long(x) \ +( \ + __builtin_constant_p(x) ? \ + __builtin_popcountl(x) : \ + (sizeof(x) <= 4 ? \ + hweight32(x) : \ + hweight64(x)) \ +) /** * rol32 - rotate a 32-bit value left