On 8/5/21 11:33 AM, Michael Meissner wrote:
At the moment, we only fully support C and C++ when changing the long double
format (between IEEE 128-bit, IBM 128-bit, and 64-bit) when the compiler is
invoked (and assuming you are using GLIBC 2.32 or newer).
For Fortran and the other languages, the only way to change the floating point
format is to configure the compiler with the '--with-long-double-format=ieee'
configuration option. This makes TFmode use IEEE 128-bit floating point
instead of IBM 128-bit floating point.
My understanding from reading the code is is that for GNU/Linux targets,
PowerPC already defaults to the IEEE format for TFmode? I'm not sure
what targets the IBM format might be the default on.
It would take somebody familar with the Fortran front end and libraries to make
the same sort of modifications that were made for the C and C++ languages.
Basically you need build libgfortran so that it has support for both floating
point formats, using different names. You would need to modify the fortran
front end to call the alternate functions when the switch is used to change the
floating point format. It might be nice to have a Fortran specific way to
specify which of the two floating point formats are used for REAL*16 (similar
to the use of __float128 and __ibm128 in C/C++, and also _Float128 in just C).
If you are going to do it, good luck.
Well, I am actually not at all interested in doing that. My questions
for the PowerPC experts are:
(1) When is the __float128 type defined, and which format does it
specify? Is it always the IEEE format, or does it specify the same
format as TFmode/long double?
(2) If __float128 is not always the same 128-bit format as TFmode/long
double, how can I detect that in the Fortran front end in a
target-independent way? Is it possible without adding a new target hook?
(3) Can we do anything about the "Additional Floating Types" section in
extend.texi? It's not clear on the answer to (1), and I think the stuff
about "future versions of GCC" is bit-rotten as it goes back to at least
GCC 6. (Either it's been implemented by now, or the idea was discarded.)
Basically, I want the Fortran front end to define c_float128 to 16 if C
supports __float128 and it corresponds to TFmode, otherwise it ought to
continue to define c_float128 to -4. I do not want to make the Fortran
front end support multiple 128-bit encodings at the same time, just
accurately define c_float128.
-Sandra
