[PATCH] [media] zl10353: use div_u64 instead of do_div

On Friday 12 February 2016 13:21:33 Nicolas Pitre wrote:

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On Fri, 12 Feb 2016, Arnd Bergmann wrote:

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On Friday 12 February 2016 14:32:20 Mauro Carvalho Chehab wrote:

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Em Fri, 12 Feb 2016 15:27:18 +0100
Arnd Bergmann [off-list ref] escreveu:

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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.

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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.

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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                        \