On Mon, Dec 13, 2021 at 09:29:16PM +0100, Thomas Koenig wrote:
>
> Hi,
>
> looking at what the REAL(KIND=17) numbers should be compiled for, I see
> the following options that should be considered:
>
> a) xsaddqp and friends are not supported by the CPU; libquadmath should
> be called for all operations, including simple arithmetic.
Note, we do not use the emulator in libquadmath. Libgcc has support for doing
software emulation of the IEEE 128-bit basic support.
> b) xsaddqp and friends are supported, but glibc is too old and lacks the
> *ieee128 functions. libquadmath should be called for these
> functions.
Yes, this would be the place to call libquadmath. Or possibly don't use
libquadmath at all and don't allow KIND=17. It is probably better for the
users if we use libquadmath instead of disabling it all together.
> c) xsaddqp and friends are supported, and glibc is new enough. Call
> the *ieee128 functions.
The necessary support is in the little endian GLIBC 2.32 or newer. I don't
recall if you also need Elf abi V2 (which is default on little endian).
You could check via:
#if (((__GLIBC__ * 1000) + __GLIBC_MINOR__) >= 2032 && \
(__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__) \
(_CALL_ELF == 2))
If you are writing C code for the library, and using _Float128 for the type
instead of using long double and using the -mabi=ieeelongdouble switch, you
want to define the following macros before math.h or other system include files
are included:
#define __STDC_WANT_IEC_60559_TYPES_EXT__ 1
#define __STDC_WANT_IEC_60559_FUNCS_EXT__ 1
> What is the best way to check in the library config files that the quad
> precision instructions are supported (to differentiate between a) on the
> one hand and b) and c) on the other?
You can check whether long double is IEEE 128-bit via:
#if defined(__LONG_DOUBLE_IEEE128__) /* or */
#if __LONG_DOUBLE_IEEE128__
and similarly to check for IBM 128-bit long double:
#if defined(__LONG_DOUBLE_IBM128__) /* or */
#if __LONG_DOUBLE_IBM128__
To check whether IEEE 128-bit instructions are enabled:
#if defined(__FLOAT128_HARDWARE__) /* or */
#if __FLOAT128_HARDWARE__
To check whether the _Float128 or __float128 keywords are available (whether or
not the hardware supports the instructions):
#if defined(__FLOAT128_TYPE__) /* or */
#if __FLOAT128_TYPE__
Note, that C++ does not support the _Float128 type (which is in one of the IEC
papers), but it does support the non-standard __float128 keyword.
Unfortunately, __float128 _Complex does not work. There is a GLIBC macro that
gives the appropriate _Complex type for __float128 use in C++, but I don't
remember what it is.
Alternatively, it might be simpler to build the library parts using long double
and build those modules with the appropriate options.
>
> And which options to the compiler make sure the libquadmath library
> is called?
The libqaudmath library should always be linked in if it is built. I had
actually meant to turn off building it once all of the 2.32 support went in,
but I never did. Note, I haven't tested libquadmath in a long time.
To compile C/C++ code where long double is IEEE 128-bit use:
-mabi=ieeelongdouble -Wno-psabi -mno-gnu-attribute
For Fortran code, I think you have to remove the -Wno-psabi. But it may be
buggy.
Similarly to force long double to be IBM 128-bit, no matter what the defaults
are use:
-mabi=iibmlongdouble -Wno-psabi -mno-gnu-attribute
The no-gnu-attribute says to disable setting the GNU attribute that says what
the default long double type is. It is necessary when building libraries with
both 128-bit types.
--
Michael Meissner, IBM
PO Box 98, Ayer, Massachusetts, USA, 01432
email: meiss...@linux.ibm.com
