On Wed, 12 Aug 2015 15:20:54 -0700 Andy Lutomirski <l...@kernel.org> wrote:
> On 08/12/2015 08:49 AM, Linus Torvalds wrote: > > On Tue, Aug 11, 2015 at 11:24 PM, Christoph Hellwig <h...@infradead.org> > > wrote: > >> > >> Maybe it's time to rely on gcc to handle 64 bit divisions now? > > > > Ugh. gcc still does a pretty horrible job at it. While gcc knows that > > a widening 32x32->64 multiplication can be simplified, it doesn't do > > the same thing for a 64/32->64 division, and always calls __udivdi3 > > for it. > > > > Now, __udivdi3 does avoid the general nasty case by then testing the > > upper 32 bits of the divisor against zero, so it's not entirely > > disastrous. It's just ugly. > > > > But perhaps more importantly, I'm not at all sure libgcc is > > kernel-safe. In particular, I'm not at all sure it *remains* > > kernel-safe. Just as an example: can you guarantee that libgcc doesn't > > implement integer division on some architecture by using the FP > > hardware? > > > > There's been a few cases where not having libgcc saved us headaches. I > > forget the exact details, but it was something like several years ago > > that we had gcc start to generate some insane crap exception handling > > for C code generation, and the fact that we didn't include libgcc was > > what made us catch it because of the resulting link error. > > > > libgcc just isn't reliable in kernel space. I'm not opposed to some > > random architecture using it (arch/tile does include "-lgcc" for > > example), but I _do_ object to the notion that we say "let's use > > libgcc in general". > > > > So no. I do not believe that the occasional pain of a few people who > > do 64-bit divides incorrectly is a good enough argument to start using > > libgcc. > > > > Does your objection still apply if we supplied our own implementations > of a handful of libgcc helpers? It's not just a matter of "how fast is the divide". The 32-bit build error is supposed to prompt people to ask "did I really need to use 64 bits". That *used* to work. A bit. But nowadays the errors are detected so late that the fix (often by someone other than the original developer) is to just slap a do_div() in there. And as the build error no longer appears to be having the desired effect, I too have been wondering if it's time to just give up and implement __udivdi and friends. Or maybe there's a way of breaking 64-bit builds instead ;) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
