linux-kernel - Re: [PATCH 11/17] mm: page_alloc: Use unsigned int for order in more places

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Date:	Thu, 01 May 2014 08:38:24 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/17] mm: page_alloc: Use unsigned int for order in more
 places

On 05/01/2014 08:11 AM, Mel Gorman wrote:
> On Thu, May 01, 2014 at 07:35:47AM -0700, Dave Hansen wrote:
>> On 05/01/2014 01:44 AM, Mel Gorman wrote:
>>> X86 prefers the use of unsigned types for iterators and there is a
>>> tendency to mix whether a signed or unsigned type if used for page
>>> order. This converts a number of sites in mm/page_alloc.c to use
>>> unsigned int for order where possible.
>>
>> Does this actually generate any different code?  I'd actually expect
>> something like 'order' to be one of the easiest things for the compiler
>> to figure out an absolute range on.
> 
> Yeah, it generates different code. Considering that this patch affects an
> API that can be called external to the code block how would the compiler
> know what the range of order would be in all cases?

The compiler comprehends that if you do a check against a constant like
MAX_ORDER early in the function that the the variable now has a limited
range, like the check we do first-thing in __alloc_pages_slowpath().

The more I think about it, at least in page_alloc.c, I don't see any
checks for order<0, which means the compiler isn't free to do this
anyway.  Your move over to an unsigned type gives that check for free
essentially.

So this makes a lot of sense in any case.  I was just curious if it
affected the code.
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