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. 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. Nico, any other thoughts on this? Arnd -- 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
