On 12 February 2016 at 22:01, Arnd Bergmann <arnd@xxxxxxxx> wrote: > 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@xxxxxxxx> 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 \ > I remember seeing this patch, but I don't remember the exact context. But when you think about it, !!cond can be a build time constant even if cond is not, as long as you can prove statically that cond != 0. So I think this change is obviously correct, and an improvement since it will remove the profiling overhead of branches that are not true branches in the first place. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
