On Thu, Aug 05, 2021 at 12:19:37PM -0600, Sandra Loosemore wrote:
> 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.
All PowerPC systems that I'm aware of that use 128-bit floating point use the
IBM format. It is anticipated that one or more Linux distributions in the
future may move to IEEE 128-bit format, but right now, I'm not aware that any
have moved.
> >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?
__float128 (and _Float128 in C) is always IEEE 128-bit, if IEEE 128-bit is
supported. By default, IEEE 128-bit is only supported on little endian PowerPC
64-bit systems.
For C (but not C++), you can declare constants with the f128 suffix so that
they would be compatible with the _Float128 type.
>
> (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?
You can look at the constants in float.h:
For a system with IBM long double:
FLT_RADIX = 2
LDBL_MANT_DIG = 106
LDBL_DIG = 31
LDBL_MIN_EXP = -968
LDBL_MIN_10_EXP = -291
LDBL_MAX_EXP = 1024
LDBL_MAX_10_EXP = 308
For a system with IEEE long double:
FLT_RADIX = 2
LDBL_MANT_DIG = 113
LDBL_DIG = 33
LDBL_MIN_EXP = -16381
LDBL_MIN_10_EXP = -4931
LDBL_MAX_EXP = 16384
LDBL_MAX_10_EXP = 4932
For a system that uses 64-bit numbers for long double:
FLT_RADIX = 2
LDBL_MANT_DIG = 53
LDBL_DIG = 15
LDBL_MIN_EXP = -1021
LDBL_MIN_10_EXP = -307
LDBL_MAX_EXP = 1024
LDBL_MAX_10_EXP = 308
In addition, the PowerPC GCC defines __LONG_DOUBLE_IEEE128__ if long double is
IEEE 128-bit, and __LONG_DOUBLE_IBM128__ if long double IBM 128-bit. If long
double is 64-bit, the macro __LONG_DOUBLE_128__ is not defined.
> (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.
--
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meiss...@linux.ibm.com, phone: +1 (978) 899-4797
