[RFC/PATCH] Remove the workaround for _Float128 precision [PR107299]

[Michael Meissner]

On Fri, Jan 06, 2023 at 07:41:07PM -0500, Michael Meissner wrote:
> On Wed, Dec 21, 2022 at 09:40:24PM +0000, Joseph Myers wrote:
> > On Wed, 21 Dec 2022, Segher Boessenkool wrote:
> > 
> > > > --- a/gcc/tree.cc
> > > > +++ b/gcc/tree.cc
> > > > @@ -9442,15 +9442,6 @@ build_common_tree_nodes (bool signed_char)
> > > >        if (!targetm.floatn_mode (n, extended).exists (&mode))
> > > >  	continue;
> > > >        int precision = GET_MODE_PRECISION (mode);
> > > > -      /* Work around the rs6000 KFmode having precision 113 not
> > > > -	 128.  */
> > > 
> > > It has precision 126 now fwiw.
> > > 
> > > Joseph: what do you think about this patch?  Is the workaround it
> > > removes still useful in any way, do we need to do that some other way if
> > > we remove this?
> > 
> > I think it's best for the TYPE_PRECISION, for any type with the binary128 
> > format, to be 128 (not 126).
> > 
> > It's necessary that _Float128, _Float64x and long double all have the same 
> > TYPE_PRECISION when they have the same (binary128) format, or at least 
> > that TYPE_PRECISION for _Float128 >= that for long double >= that for 
> > _Float64x, so that the rules in c_common_type apply properly.
> > 
> > How the TYPE_PRECISION compares to that of __ibm128, or of long double 
> > when that's double-double, is less important.
> 
> I spent a few days on working on this.  I have patches to make the 3 128-bit
> types to all have TYPE_PRECISION of 128.  To do this, I added a new mode macro
> (FRACTIONAL_FLOAT_MODE_NO_WIDEN) that takes the same arguments as
> FRACTIONAL_FLOAT_MODE.

...

I had the patches to change the precision to 128, and I just ran them.  C and
C++ do not seem to be bothered by changing the precision to 128 (once I got it
to build, etc.).  But Fortran on the other hand does actually use the precision
to differentiate between IBM extended double and IEEE 128-bit.  In particular,
the following 3 tests fail when long double is IBM extended double:

	gfortran.dg/PR100914.f90
	gfortran.dg/c-interop/typecodes-array-float128.f90
	gfortran.dg/c-interop/typecodes-scalar-float128.f90

I tried adding code to use the old precisions for Fortran, but not for C/C++,
but it didn't seem to work.

So while it might be possible to use a single 128 for the precision, it needs
more work and attention, particularly on the Fortran side.

I'm not sure it is worth it to try and change things.

-- 
Michael Meissner, IBM
PO Box 98, Ayer, Massachusetts, USA, 01432
email: meissner@linux.ibm.com