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 \ -- 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
