https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114803
--- Comment #16 from Alexandre Oliva <aoliva at gcc dot gnu.org> --- That macro is used within libstdc++ to avoid relying on C99 functions when the target C library is not C99-compliant. It involves leaving out C++ standard bits that would depend on functions that the C library doesn't offer, including some of the <cmath> declarations in ::std::. IMHO it would make sense to omit simd functions corresponding to C99/TR1 ones known to be missing. Now, it's unclear to me whether we'd be better off omitting the entire set of C99/TR1 math functions as a block, or whether there'd be any benefit from considering individual functions separately. newlib, for example, exports a number of the C99/TR1 functions, but not all of them, so the macro is not defined.
