Michael Meissner <meiss...@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 <meiss...@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: meiss...@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
