Re: [power-ieee128] Which options for libquadmath / native ieee128

[Michael Meissner <meissner@linux.ibm.com>]

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: meissner@linux.ibm.com