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Re: [PATCH], Add configuration checks to PowerPC --with-long-double-format=ieee


Michael Meissner <meissner@linux.ibm.com> writes:

> This patch adds a simple check of whether the GLIBC should be capable of
> switching the long double format on the PowerPC to IEEE 128-bit floating point.
> At the moment, library work is not yet finished, but I'm assuming that the
> patches will be in place when GLIBC 2.28 is released.  If it turns out that the
> finished support does not make it until 2.29, we can adjust the patch later.
>
> Right now, if you use standard GLIBC 2.27 or earlier (ignoring the bits that
> actually use long double that will need to be handled), you will not be able to
> build libstdc++-v3 when long double is configured to be IEEE 128-bit due to
> errors with overloaded functions like issignalling (where both __float128 and
> long double versions are defined).  The GLIBC team has a fix for this, and it
> should appear in 2.28.
>
> This patch checks whether the GLIBC version is 2.28 before allowing you to
> switch the long double type.  Because the work to prepare GLIBC for the switch
> is being done using an Advance Toolchain framework, the patch allows an Advance
> Toolchain 2.27 with the --with-advance-toolchain configuration option (the
> official AT 11 release uses GLIBC 2.26 as a framework, and when completed the
> AT 12 release should use GLIBC 2.28).
>
> I have checked it on a little endian power8 system, building both toolchains
> using IBM long double and IEEE long double configurations.  The tests that
> depend on the library support for long double that failed before still fail.
>
> I also did IEEE long double builds using the host GLIBC and that AT 11, and
> verified that once GCC is configured it generates an error.  I built bootstrap
> compilers on a big endian system, and verified if I selected IEEE long double,
> it would fail, since I currently don't have a big endian GLIBC with the fixes
> installed.
>
> Can I check this in the trunk at on the GCC 8 branch?
>
> 2018-07-05  Michael Meissner  <meissner@linux.ibm.com>
>
> 	* configure.ac (powerpc64*-*-linux*): Combine big and little
> 	endian checks for the long double format.  Add checks to make sure
> 	the GLIBC can handle configuration of long double to be IEEE
> 	128-bit before building GCC.
> 	* configure: Regenerate.
>
> -- 
> Michael Meissner, IBM
> IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
> email: meissner@linux.ibm.com, phone: +1 (978) 899-4797
> [ ieee128-patch129b: text/plain ]
> Index: gcc/configure.ac
> ===================================================================
> --- gcc/configure.ac	(revision 262443)
> +++ gcc/configure.ac	(working copy)
> @@ -6031,23 +6031,48 @@ AC_ARG_WITH([long-double-format],
>    [AS_HELP_STRING([--with-long-double-format={ieee,ibm}]
>  		  [Specify whether PowerPC long double uses IEEE or IBM format])],[
>  case "$target:$with_long_double_format" in
> -  powerpc64le-*-linux*:ieee | powerpc64le-*-linux*:ibm)
> -    :
> -    ;;
> -  powerpc64-*-linux*:ieee | powerpc64-*-linux*:ibm)
> -    # IEEE 128-bit emulation is only built on 64-bit VSX Linux systems
> -    case "$with_cpu" in
> -      power7 | power8 | power9 | power1*)
> +  powerpc64le-*-linux*:ibm | powerpc64-*-linux*:ibm | \
> +  powerpc64le-*-linux*:ieee | powerpc64-*-linux*:ieee)
> +    # IEEE 128-bit emulation is only built on 64-bit VSX Linux systems.
> +    # Little endian 64-bit systems are always VSX, but big endian systems
> +    # might default to power4.
> +    case "$target:$with_cpu" in
> +      powerpc64le-* | *:power7 | *:power8 | *:power9 | *:power1*)
>  	:
>  	;;
>        *)
>  	AC_MSG_ERROR([Configuration option --with-long-double-format is only \
>  supported if the default cpu is power7 or newer])
>  	with_long_double_format=""
> -	;;
> -      esac
> -      ;;
> -  xpowerpc64*-*-linux*:*)
> +    esac
> +
> +    if test "x$with_long_double_format" = xieee; then
> +      # See if we have a new enough GLIBC to allow using IEEE 128-bit long
> +      # double.  We assume the public 2.28 GLIBC and the development version of
> +      # the Advance Toolchain (2.27) have all of the missing bits.
> +      ieee_minor="28"
> +      glibc_ieee="no"
> +      atoolchain=""
> +      if test "x$with_advance_toolchain" != x \
> +	 -a -d "/opt/$with_advance_toolchain/." \
> +	 -a -d "/opt/$with_advance_toolchain/bin/." \
> +	 -a -d "/opt/$with_advance_toolchain/include/."; then
> +
> +	ieee_minor="27"

These minor versions are confusing and shouldn't differ between the Advance
Toolchain and glibc.

I suggest to test with the following program:

#include <math.h>

int
main ()
{
  return !isinfl(__builtin_infl());
}

Build it with:
gcc -mabi=ieeelongdouble -fno-builtin -Wno-psabi -lm test-ldbl.c

If the execution of the program returns 0, your math library supports IEEE long
double.

-- 
Tulio Magno


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