Message-ID: <6737272.LXr2g355Yt@wuerfel>
Date:	Fri, 12 Feb 2016 22:01:01 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
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 Friday 12 February 2016 13:21:33 Nicolas Pitre wrote:
> 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.

Right, this goes back to the dawn of time.

> > 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?

I have a fairly recent gcc, but I also never got around to submit
it properly.

However, I did stumble over an older patch I did now, which I could
not remember what it was good for. It does fix the problem, and
it seems to be a better solution.

	Arnd

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index b5acbb404854..b5ff9881bef8 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -148,7 +148,7 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
  */
 #define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
 #define __trace_if(cond) \
-	if (__builtin_constant_p((cond)) ? !!(cond) :			\
+	if (__builtin_constant_p(!!(cond)) ? !!(cond) :			\
 	({								\
 		int ______r;						\
 		static struct ftrace_branch_data			\

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