http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60181
--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- There are no specified accuracy requirements for complex multiplication / division, even under Annex G (parts of which - imaginary types in particular - are not implemented in GCC at present), beyond certain requirements for special cases and avoiding certain cases of overflow from the simplest formulas. Constant folding is correctly rounding, runtime complex multiplication / division isn't (and given complaints about slowness at present, I don't think users would want it to be even slower, though there may well be a case for defining a standard library interface for correctly rounding complex multiplication / division). x86 probably benefits from excess precision being used implicitly by GCC when compiling the implementations of complex multiplication and division.
