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. > 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 -- 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
