From: Matthew Wilcox <willy@infradead.org>
To: kernel test robot <lkp@intel.com>
Cc: Mel Gorman <mgorman@suse.de>,
kbuild-all@lists.01.org, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: mm/page_alloc.c:984:9: warning: iteration 119304648 invokes undefined behavior
Date: Thu, 19 May 2022 22:28:24 +0100 [thread overview]
Message-ID: <Yoa2eJ6+nyvK1EL+@casper.infradead.org> (raw)
In-Reply-To: <202205200521.65W2rpLt-lkp@intel.com>
On Fri, May 20, 2022 at 05:14:52AM +0800, kernel test robot wrote:
> All warnings (new ones prefixed by >>):
>
> mm/page_alloc.c: In function 'rmqueue.isra':
> >> mm/page_alloc.c:984:9: warning: iteration 119304648 invokes undefined behavior [-Waggressive-loop-optimizations]
> 984 | list_add(&page->lru, &area->free_list[migratetype]);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/page_alloc.c:2305:21: note: within this loop
> 2305 | while (high > low) {
> | ~~~~~^~~~~
That's OK. 'high' and 'low' are page orders. They both lie in the
range [0,63], so this will not iterate 119304648 times (what a funny
number; I wonder why GCC chose it).
Can we make GCC happier by telling it that low and high are unsigned?
It should have figured out that 'high' is <= 63 already, as that's a
consequence of:
unsigned long size = 1 << high;
WARNING: multiple messages have this Message-ID (diff)
From: Matthew Wilcox <willy@infradead.org>
To: kbuild-all@lists.01.org
Subject: Re: mm/page_alloc.c:984:9: warning: iteration 119304648 invokes undefined behavior
Date: Thu, 19 May 2022 22:28:24 +0100 [thread overview]
Message-ID: <Yoa2eJ6+nyvK1EL+@casper.infradead.org> (raw)
In-Reply-To: <202205200521.65W2rpLt-lkp@intel.com>
[-- Attachment #1: Type: text/plain, Size: 952 bytes --]
On Fri, May 20, 2022 at 05:14:52AM +0800, kernel test robot wrote:
> All warnings (new ones prefixed by >>):
>
> mm/page_alloc.c: In function 'rmqueue.isra':
> >> mm/page_alloc.c:984:9: warning: iteration 119304648 invokes undefined behavior [-Waggressive-loop-optimizations]
> 984 | list_add(&page->lru, &area->free_list[migratetype]);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/page_alloc.c:2305:21: note: within this loop
> 2305 | while (high > low) {
> | ~~~~~^~~~~
That's OK. 'high' and 'low' are page orders. They both lie in the
range [0,63], so this will not iterate 119304648 times (what a funny
number; I wonder why GCC chose it).
Can we make GCC happier by telling it that low and high are unsigned?
It should have figured out that 'high' is <= 63 already, as that's a
consequence of:
unsigned long size = 1 << high;
next prev parent reply other threads:[~2022-05-19 21:28 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-19 21:14 mm/page_alloc.c:984:9: warning: iteration 119304648 invokes undefined behavior kernel test robot
2022-05-19 21:28 ` Matthew Wilcox [this message]
2022-05-19 21:28 ` Matthew Wilcox
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