https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82143
--- Comment #6 from Steve Kargl <sgk at troutmask dot apl.washington.edu> --- On Fri, Sep 08, 2017 at 04:09:24PM +0000, dominiq at lps dot ens.fr wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82143 > > --- Comment #5 from Dominique d'Humieres <dominiq at lps dot ens.fr> --- > > -fdefault-real-8 promotes some things that are REAL(4) > > to REAL(8), and if available it promotes some things from REAL(8) to > > REAL(16). > > Not exactly: -fdefault-real-8 promotes some things that are REAL > to REAL(8) and DOUBLE PRECISION to REAL(16). Variables with a > KIND (say REAL(4) or REAL(8) are left unchanged. Which is what I said as in "some things" are promoted. REAL and REAL(4) are the exact same type unless one use a broken-by-design compiler option that maps REAL to DOUBLE PRECISION while leaving REAL(4) unchanged or one has a myoptic view of Fortran's type system. > This is the difference with -freal-4-real-8 which converts all > the REAL* to REAL(8). The interest of -fdefault-real-8 compared to > -freal-4-real-8 is when you use libraries expecting REAL(4) > arguments (my use a long time ago was with etime). This, in fact, is a great example of why -fdefault-real-8 should never be used. The proper method of porting a code/library is to inspect and write the necessary intrinsic procedures. For this case, etime is a (nonstandard) specific intrinsic subprogram for REAL and by extension REAL(4) (unless one uses a broken-by-design option that maps REAL to DOUBLE PRECISION while leaving REAL(4) unchanged) and dtime is for DOUBLE PRECISION. etime is not a generic subprogram. Blindly using -fdefault-real-8 and expecting things to just work is a sure fire way of shooting oneself in the foot. One also needs to keep in mind that (old crusty) codes/libraries can contain code that intermixes REAL, REAL*4, and REAL(4). These types of libraries sometimes/typically use COMMON and EQUIVALENCE. -fdefault-real-8 may not (or more likely does not) preserve storage associate while -freal-4-real-8 does.
