linux-kernel - Re: [PATCH] [media] zl10353: use div_u64 instead of do

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Message-ID: <alpine.LFD.2.20.1602121305180.13632@knanqh.ubzr>
Date:	Fri, 12 Feb 2016 13:21:33 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
cc:	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-arm-kernel@...ts.infradead.org,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [media] zl10353: use div_u64 instead of do_div

On Fri, 12 Feb 2016, Arnd Bergmann wrote:

> On Friday 12 February 2016 14:32:20 Mauro Carvalho Chehab wrote:
> > Em Fri, 12 Feb 2016 15:27:18 +0100
> > Arnd Bergmann <arnd@...db.de> escreveu:
> > 
> > > I noticed a build error in some randconfig builds in the zl10353 driver:
> > > 
> > > dvb-frontends/zl10353.c:138: undefined reference to `____ilog2_NaN'
> > > dvb-frontends/zl10353.c:138: undefined reference to `__aeabi_uldivmod'
> > > 
> > > The problem can be tracked down to the use of -fprofile-arcs (using
> > > CONFIG_GCOV_PROFILE_ALL) in combination with CONFIG_PROFILE_ALL_BRANCHES
> > > on gcc version 4.9 or higher, when it fails to reliably optimize
> > > constant expressions.
> > > 
> > > Using div_u64() instead of do_div() makes the code slightly more
> > > readable by both humans and by gcc, which gives the compiler enough
> > > of a break to figure it all out.
> > 
> > I'm not against this patch, but we have 94 occurrences of do_div() 
> > just at the media subsystem. If this is failing here, it would likely
> > fail with other drivers. So, I guess we should either fix do_div() or
> > convert all such occurrences to do_div64().
> 
> I agree that it's possible that the same problem exists elsewhere, but this is
> the only one that I ever saw (in five ranconfig builds out of 8035 last week).
> 
> I also tried changing do_div() to be an inline function with just a small
> macro wrapper around it for the odd calling conventions, which also made this
> error go away. I would assume that Nico had a good reason for doing do_div()
> the way he did.

The do_div() calling convention predates my work on it.  I assume it was 
originally done this way to better map onto the x86 instruction.

> In some other files, I saw the object code grow by a few
> instructions, but the examples I looked at were otherwise identical.
> 
> I can imagine that there might be cases where the constant-argument optimization
> of do_div fails when we go through an inline function in some combination
> of Kconfig options and compiler version, though I don't think that was
> the case here.

What could be tried is to turn __div64_const32() into a static inline 
and see if that makes a difference with those gcc versions we currently 
accept.

> Nico, any other thoughts on this?

This is all related to the gcc bug for which I produced a test case 
here:

http://article.gmane.org/gmane.linux.kernel.cross-arch/29801

Do you know if this is fixed in recent gcc?


Nicolas