Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1333123

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

Show all headers | View raw

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@arndb.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			\

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH] [media] zl10353: use div_u64 instead of do_div Arnd Bergmann <arnd@arndb.de> - 2016-02-12 15:30 +0100
  Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Mauro Carvalho Chehab <mchehab@osg.samsung.com> - 2016-02-12 17:40 +0100
    Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Arnd Bergmann <arnd@arndb.de> - 2016-02-12 18:10 +0100
      Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Nicolas Pitre <nicolas.pitre@linaro.org> - 2016-02-12 19:30 +0100
        Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Arnd Bergmann <arnd@arndb.de> - 2016-02-12 22:10 +0100
          Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Nicolas Pitre <nicolas.pitre@linaro.org> - 2016-02-12 22:40 +0100
            Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Arnd Bergmann <arnd@arndb.de> - 2016-02-12 22:50 +0100
          Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-02-13 09:50 +0100
            Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Nicolas Pitre <nicolas.pitre@linaro.org> - 2016-02-13 23:00 +0100
              Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-02-14 09:00 +0100
                Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Nicolas Pitre <nicolas.pitre@linaro.org> - 2016-02-14 18:00 +0100
                Re: [PATCH] [media] zl10353: use div_u64 instead of do_div Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-02-14 20:10 +0100

csiph-web