https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99708
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
__SIZEOF_FLOAT128__ is predefined on all targets that have the __float128 type,
except for rs6000:
grep '"__float128"' */*
i386/i386-builtins.c: lang_hooks.types.register_builtin_type
(float128_type_node, "__float128");
ia64/ia64.c: "__float128");
ia64/ia64.c: "__float128");
pa/pa.c: "__float128");
rs6000/rs6000-c.c: rs6000_define_or_undefine_macro (false, "__float128");
grep SIZEOF_FLOAT128 */*
i386/i386-c.c: cpp_define (parse_in, "__SIZEOF_FLOAT128__=16");
ia64/ia64.h: builtin_define("__SIZEOF_FLOAT128__=16");\
pa/pa.h: builtin_define("__SIZEOF_FLOAT128__=16"); \
See also PR56540
