On Wed, Oct 06, 2021 at 11:42:11PM -0400, Michael Meissner wrote: > On Wed, Oct 06, 2021 at 10:17:44AM -0500, Segher Boessenkool wrote: > > On Wed, Oct 06, 2021 at 08:59:53AM +0200, Thomas Koenig wrote: > > > On 05.10.21 23:54, Segher Boessenkool wrote: > > > >>There is also the issue of binary data. If some user has written > > > >>out data in double double and wants to read it in as IEEE quad, > > > >>the results are going to be garbage. Another option for CONVERT > > > >>might be the solution to that, or, as you wrote, having a > > > >>REAL(KIND=15). It should be inaccessible via SELECTED_REAL_KIND, > > > >>though. > > > > > > > >That means flipping the default on all PowerPC to no longer be double- > > > >double. This means that you should have IEEE QP work everywhere, or the > > > >people who do need more than double precision will have no recourse. > > > > > > I think we can exclude big-endian POWER from this - they do not have > > > IEEE QP support, correct? So, exclude that from the SONAME change. > > > > Not correct, no. IEEE QP works fine in either endianness. > > > > I don't know what the libraries do, but in GCC it works just fine. > > It only has the support if you add the options to enable IEEE 128-bit support > when compiling programs. It is off by default.
You need -mvsx and -mfloat128, both on by default on power7 and later. Without hardware QP support it will call __mulkf3 and friends. But the parameter passing is identical: in the high VSRs (i.e. the VRs). There are no differences between BE and LE here. > > Converting double-double to IEEE QP should not be hard or slow? > > There are a lot of corner cases to get it right. IIRC, there are a few values > that double double can represent that are not expressable with exact precision > in IEEE 128-bit. Yes, but those are trivial to get right. The value of a double-double is the sum of both doubles. The sign is the sign of the first component though (which matters if adding -0. and +0.), that is all, all other cases magically work out as wanted afaics. Segher
