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

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Date:	Wed, 4 Nov 2015 16:04:57 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
cc:	kbuild test robot <lkp@...el.com>, kbuild-all@...org,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Måns Rullgård <mans@...sr.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

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.


Nicolas
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