linux-kernel - Re: [PATCH 2/5] do_div(): generic optimization for constant divisor on 32-bit machines

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Date:	Wed, 04 Nov 2015 21:42:58 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Arnd Bergmann <arnd@...db.de>, kbuild test robot <lkp@...el.com>,
	kbuild-all@...org, Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	rmk+kernel@....linux.org.uk, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] do_div(): generic optimization for constant divisor on 32-bit machines

Nicolas Pitre <nicolas.pitre@...aro.org> writes:

> On Tue, 3 Nov 2015, Arnd Bergmann wrote:
>
>> On Tuesday 03 November 2015 13:32:17 kbuild test robot wrote:
>> > 
>> >    net/can/bcm.c: In function 'bcm_proc_show':
>> > >> net/can/bcm.c:223:1: warning: the frame size of 1156 bytes is larger than 1024 bytes [-Wframe-larger-than=]
>> >     }
>> 
>> Interesting, that is a lot of stack for a function that only has a couple
>> of local variables:
>> 
>> #define IFNAMSIZ        16
>>         char ifname[IFNAMSIZ];
>>         struct sock *sk = (struct sock *)m->private;
>>         struct bcm_sock *bo = bcm_sk(sk);
>>         struct bcm_op *op;
>> 
>> 
>> This is a parisc-allyesconfig kernel, so I assume that CONFIG_PROFILE_ALL_BRANCHES
>> is on, which instruments every 'if' in the kernel. If that causes problems,
>> we could decide to disable the do_div optimization whenever CONFIG_PROFILE_ALL_BRANCHES
>> is enabled.
>
> I have an ARM allyesconfig build here where that function needs a frame 
> of 88 bytes only. And that is with my do_div optimization applied.
>
> With the do_div optimization turned off, the stack frame is still 88 
> bytes.
>
> Turning on CONFIG_PROFILE_ALL_BRANCHES makes the frame size to grow to 
> 96 bytes.
>
> Keeping CONFIG_PROFILE_ALL_BRANCHES=y and activating the do_div 
> optimization again, and the function frame size goes back to 88 bytes.
>
> So I wonder what parisc gcc could be doing with this code.

I've seen parisc gcc do many strange things.

-- 
Måns Rullgård
mans@...sr.com
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