https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99708
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> --- It looks like trunk now defines __SIZEOF_FLOAT128__ on powerpc-ibm-aix* and powerpc64*-*-linux-gnu, but it seems to be defined unconditionally, even if the __float128 type *isn't* available! On power8-aix (e.g. gcc119 in the cfarm): ./gcc/cc1plus -E -dM /dev/null -quiet -maix64 | fgrep FLOAT128#define __SIZEOF_FLOAT128__ 16 ./gcc/cc1plus - -quiet <<< '__float128 f;' <stdin>:1:1: error: '__float128' does not name a type; did you mean '__int128'? You need to add -mfloat128 -mvsx to be able to use the type. On power8 ppc64le linux (e.g. gcc112 in the cfarm): ./gcc/cc1plus -E -dM /dev/null -quiet | fgrep FLOAT128 #define __FLOAT128__ 1 #define __SIZEOF_FLOAT128__ 16 #define __FLOAT128_TYPE__ 1 That's fine, the type is usable. But for power7 ppc64 (BE) linux (e.g. gcc110 in the cfarm): ./gcc/cc1plus -E -dM /dev/null -quiet | fgrep FLOAT128 #define __SIZEOF_FLOAT128__ 16 That means __SIZEOF_FLOAT128__ still can't be used to detect whether a type called "__float128" is supported for the current target. On power it looks like we can use __FLOAT128__ for that purpose, but other targets such as x86 don't define that one, only __SIZEOF_FLOAT128__. So you have to use a different macro to detect __float128 depending on the target.
