Re: [power-iee128] How to specify linker flags

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Jan 03, 2022 at 11:19:21AM +0100, Thomas Koenig wrote:
> > > If you are building libraries that contain modules with multiple
> > > long double
> > > types, you must use the '-mno-gnu-attribute'.  We also use the
> > > '-Wno-psabi'
> > > option, which silences the warning that you are switching long
> > > double types (if
> > > glibc is not 2.34 or newer).  We may need to tweak -Wno-psabi for
> > > use with
> > > Fortran.
> > 
> > I am now at the point where the object files are also compiled correctly
> > for the gfortran specifics:
> > 
> > 0000000000000000 <_gfortran_specific__abs_r17>:
> >     0:   09 00 43 f4     lxv     vs34,0(r3)
> >     4:   48 16 40 fc     xsabsqp v2,v2
> >     8:   20 00 80 4e     blr
> > 
> > However, the linker complains, as you said it would, about the different
> > formats:
> > 
> > /opt/at15.0/bin/ld: .libs/maxloc0_4_r16.o uses IBM long double,
> > .libs/_abs_r17.o uses IEEE long double
> > /opt/at15.0/bin/ld: failed to merge target specific data of file
> > .libs/_abs_r17.o
> > 
> > I know next to nothing about libtool, so I do not know how to
> > add the flags so the linker can find them.
> > 
> > Any pointers?
> 
> One additional point.  The linker does not understand
> -mno-gnu-attribute:
> 
> $ /opt/at15.0/bin/ld -mno-gnu-attribute
> /opt/at15.0/bin/ld: unrecognised emulation mode: no-gnu-attribute
> Supported emulations: elf64lppc elf32lppc elf32lppclinux elf32lppcsim
> elf64ppc elf32ppc elf32ppclinux elf32ppcsim
> 
> So, waiting for info to proceed.

-mno-gnu-attribute isn't a linker flag, but a compiler flag.
And e.g. libstdc++ configure uses it while compiling itself on
powerpc*linux:
        LONG_DOUBLE_COMPAT_FLAGS="$LONG_DOUBLE_COMPAT_FLAGS -mno-gnu-attribute"
        # Check for IEEE128 support in libm:
        AC_CHECK_LIB(m, __frexpieee128,
                     [ac_ldbl_ieee128_in_libc=yes],
                     [ac_ldbl_ieee128_in_libc=no])
        if test $ac_ldbl_ieee128_in_libc = yes; then
          # Determine which long double format is the compiler's default:
          AC_TRY_COMPILE(, [
            #ifndef __LONG_DOUBLE_IEEE128__
            #error compiler defaults to ibm128
            #endif
          ], [ac_ldbl_ieee128_default=yes], [ac_ldbl_ieee128_default=no])
          # Library objects should use default long double format.
          if test "$ac_ldbl_ieee128_default" = yes; then
            LONG_DOUBLE_128_FLAGS="-mno-gnu-attribute"
            # Except for the ones that explicitly use these flags:
            LONG_DOUBLE_ALT128_COMPAT_FLAGS="-mabi=ibmlongdouble -mno-gnu-attribute -Wno-psabi"
          else
            LONG_DOUBLE_128_FLAGS="-mno-gnu-attribute"
            LONG_DOUBLE_ALT128_COMPAT_FLAGS="-mabi=ieeelongdouble -mno-gnu-attribute -Wno-psabi"
          fi
          AC_DEFINE([_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT],1,
                [Define if compatibility should be provided for alternative 128-bit long double formats.])
          port_specific_symbol_files="$port_specific_symbol_files \$(top_srcdir)/config/os/gnu-linux/ldbl-ieee128-extra.ver"
          ac_ldbl_alt128_compat=yes
        else
          ac_ldbl_alt128_compat=no
        fi
        ;;
The idea behind this is that libstdc++ is written such that it can handle
both IBM extended and IEEE quad long double, so its object files are
compatible with both.

So I think we want:
2022-01-03  Jakub Jelinek  <jakub@redhat.com>

	* configure.ac (Use -mno-gnu-attribute together with
	-mabi=ibmlongdouble or -mabi=ieeelongdouble.

--- libgfortran/configure.ac	2021-12-31 11:08:19.032835533 +0000
+++ libgfortran/configure.ac	2022-01-03 10:32:16.927834682 +0000
@@ -163,9 +163,9 @@ if test "x$GCC" = "xyes"; then
 			   #error long double is double
 			   #endif]],
                          [[(void) 0;]])],
-        [AM_FCFLAGS="$AM_FCFLAGS -mabi=ibmlongdouble";
-	 AM_CFLAGS="$AM_CFLAGS -mabi=ibmlongdouble";
-	 CFLAGS="$CFLAGS -mabi=ibmlongdouble";
+        [AM_FCFLAGS="$AM_FCFLAGS -mabi=ibmlongdouble -mno-gnu-attribute";
+	 AM_CFLAGS="$AM_CFLAGS -mabi=ibmlongdouble -mno-gnu-attribute";
+	 CFLAGS="$CFLAGS -mabi=ibmlongdouble -mno-gnu-attribute";
 	 have_real_17=yes])
       ;;
     *)


	Jakub