------- Comment #7 from ghazi at gcc dot gnu dot org 2009-08-24 14:43 ------- Joseph - back in comment#2, you noted that the results of infinity and zero can be ambiguous. I.e. Infinity in either part of a complex number (perhaps infinity of either sign?) is sufficient, and a pair of zeros, explicitly of either sign is also sufficient for "zero".
This brings up the question that it's possible for GCC to produce a compile-time result (via MPC) that is different from the runtime result (via libgcc2) where both are C99 compliant standard conforming values. Which one the user receives would depend on the context (e.g. static init) or on whether optimizations allowed GCC to fold it at compile-time. Now this happens all the time for finite values, MPFR and MPC are more exact that glibc's math library and often produce different results which are clearly better. However having GCC be more accurate than glibc (where the C library is outside out control) is different IMHO than getting two entirely different results from two parts of GCC, i.e. compile-time folding vs libgcc2. E.g. (Inf NaN) vs (0 -Inf) are both infinities per C99 Annex G. While MPC has committed to producing C99 conforming results for the special cases covered in Annex G, it is entirely possible they may come to different conclusions as to what is best where the standard leaves things ambiguous. Should I continue to pursue having GCC fold the Annex G special cases via MPC if it leads to this kind of discrepancy? Or should be seek to fold these internally, essentially dulpicating the libgcc2 algorithm using real.c functions so that both produce the same result? -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30789
