From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1Lx4o9-0001N9-6J for qemu-devel@nongnu.org; Thu, 23 Apr 2009 15:42:05 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1Lx4o4-0001MM-00 for qemu-devel@nongnu.org; Thu, 23 Apr 2009 15:42:04 -0400 Received: from [199.232.76.173] (port=56938 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Lx4o3-0001MJ-Rm for qemu-devel@nongnu.org; Thu, 23 Apr 2009 15:41:59 -0400 Received: from bsdimp.com ([199.45.160.85]:53082 helo=harmony.bsdimp.com) by monty-python.gnu.org with esmtps (TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1Lx4o3-00027J-7u for qemu-devel@nongnu.org; Thu, 23 Apr 2009 15:41:59 -0400 Date: Thu, 23 Apr 2009 13:41:36 -0600 (MDT) Message-Id: <20090423.134136.-135509976.imp@bsdimp.com> Subject: Re: [Qemu-devel] [7234] Use a more natural order From: "M. Warner Losh" In-Reply-To: <20090423192844.GJ3795@csclub.uwaterloo.ca> References: <20090423185308.GH3795@csclub.uwaterloo.ca> <20090423.131250.756905613.imp@bsdimp.com> <20090423192844.GJ3795@csclub.uwaterloo.ca> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: lsorense@csclub.uwaterloo.ca Cc: blauwirbel@gmail.com, qemu-devel@nongnu.org In message: <20090423192844.GJ3795@csclub.uwaterloo.ca> lsorense@csclub.uwaterloo.ca (Lennart Sorensen) writes: : On Thu, Apr 23, 2009 at 01:12:50PM -0600, M. Warner Losh wrote: : > In message: <20090423185308.GH3795@csclub.uwaterloo.ca> : > lsorense@csclub.uwaterloo.ca (Lennart Sorensen) writes: : > : On Thu, Apr 23, 2009 at 06:29:47PM +0000, Blue Swirl wrote: : > : > Revision: 7234 : > : > http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=7234 : > : > Author: blueswir1 : > : > Date: 2009-04-23 18:29:47 +0000 (Thu, 23 Apr 2009) : > : > Log Message: : > : > ----------- : > : > Use a more natural order : > : : > : It may be more natural, but it is also less safe. : > : : > : After all : > : : > : if (0 = x) { : > : : > : fails compile, while : > : : > : if (x = 0) { : > : : > : compiles silently even when you didn't mean that. : > : > This style is evil and must die. I don't know any nice way to put : > it. It encourages sloppiness. Also, it breaks down when you add : > inequality: : > : > if (x < 1) : > : > becomes : > : > if (1 >= x) : : No it doesn't. It becomes: : : if (1 > x) : : Why would it be anything else? Hmmm, see how tricky this style is? It is confusing computing the contrapositive to the expression you want to express. Or rather you aren't computing the contrapositive here, which is what got me into trouble. I usually don't make mistakes like this, and I made it in coming up with the example. : > which is also error prone. : > : > The compiler will warn about your example, but won't warn if I : > transcribe things wrongly as : > : > if (1 < x) : : Nothing wrong with that. That's perfectly valid, if you want to check : that x is greater than 1. Correct. The compiler doesn't warn you that you've gotten your backwards flipping around wrong. Which is the argument for this style when it comes to equality. So you've traded one class of problems for another. And this class of problem is just as hard to find. I've fixed several bugs like this over the years from coders that thought this was a good way to program. if (1 < x) rather than if (x < 1) is the most common pattern I've had to fix. : Putting constants first means that you can't accidentally use assignment : when you meant equality. You can't fix all the stupidies possible in C, : but you can at least try to avoid some of them when possible. I find this argument unpersuasive when the compiler will already warn me about if (x = 0). Warner