On Mon, Jan 08, 2018 at 10:17:06AM -0600, Segher Boessenkool wrote: > On Thu, Jan 04, 2018 at 06:05:55PM -0500, Michael Meissner wrote: > > This patch is the beginning step to switching the PowerPC long double > > support > > from IBM extended double to IEEE 128-bit floating point on PowerPC servers. > > It > > will be necessary to have this patch or a similar patch to allow the GLIBC > > team > > to begin their modifications in GLIBC 2.28, so that by the time GCC 9 comes > > out, we can decide to switch the default. It is likely, the default will > > only > > be switched on the 64-bit little endian PowerPC systems, when a distribution > > goes through a major level, such that they can contemplate major changes. > > I would hope the default changes for BE systems at the same time (at > least those with VSX, but ideally *all*).
Note, the change has to be on a system by system basis. We will need to support distributions that use the IBM extended double for the long double format, and we will need to support distributions for the IEEE 128-bit format. It all depends on what the host system uses. While the work can be done, I don't know of any BE distribution that will be using GCC 8 as their main compiler. > > If you do not use the configuration option --with-long-double-format=ieee or > > --with-long-double-format=ibm, the system will not build multilibs, and just > > build normal libraries with the default set to IBM extended double. If you > > do > > use either of the switches, and allow multilibs, it will build two sets of > > multilibs, one for -mabi=ieeelongdouble and one for -mabi=ibmlongdouble. > > Huh. Why not always, then? There already is an option to turn off > multilibs, for people who really really want that. I'm trying not to surprise people building compilers for a setup that does not work. In the GCC 9 timeframe, when there is GLIBC support for it, we can make it default (assuming we keep the multilibs). It is a chicken and egg problem. Real users (as opposed to GCC and GLIBC developers) would need GLIBC 2.28 in order to use the IEEE multilib. But if we don't provide the switch or multilib as an option, it makes the GLIBC work harder. I suspect that libstc++-v3 may be more of an issue than GLIBC, since we have people starting to look at the GLIBC work, but we can't really do anything about libstdc++-v3 until we have a GLIBC. -- Michael Meissner, IBM IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA email: meiss...@linux.vnet.ibm.com, phone: +1 (978) 899-4797
