http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50904
--- Comment #29 from rguenther at suse dot de <rguenther at suse dot de> 2011-12-02 16:13:25 UTC --- On Fri, 2 Dec 2011, burnus at gcc dot gnu.org wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50904 > > --- Comment #27 from Tobias Burnus <burnus at gcc dot gnu.org> 2011-12-02 > 16:02:45 UTC --- > (In reply to comment #26) > > The trivial example is (x + 2**52) - 2**52 which rounds x to > > an integer. Without parens we optimize away that rounding effect. > > Corrected example. The result I get with other compilers matches the current > behaviour of GCC/gfortran: > > - GCC: Gives (of course independent of -fno-protect-parens): 1.3 with "-O1 > -ffast-math", 1.0 without -ffast-math. Indeed GCC does not perform FP association without some sub-flags enabled by -ffast-math (it assumes then intermediate rounding is to be preserved). > - Intel ifort 12.2: -O1 has 1.0, -O2 has 1.3, -assume protect_parens does not > help but -fp-model strict does (with -O2: 1.0). > > - PGI pgf95 11.5-0: 1.0 with up to -O4. > > - Crayftn 7.1.4.111: 1.0 for -O0, 1.3 for -O1. Option "-O fp0" gives 1.0 while > already "-O fp1" gives 1.3. > > - PathScale pathf95 3.2.99: 1.0 for up to -O3, -Ofast prints 1.3. As with GCC, > -OPT:fast_math={on,off} toggles between 1.0 and 1.3 > > - NAG f95: 1.0 for up to -O4, 1.3 with -Ounsafe. > > - Sun Fortran 95 8.3: 1.0 for -O4, 1.3 for -fast. > > program test > implicit none > real(8), volatile :: y > y = 1.3d0 > call sub(y) > print *, y > ! if (y /= 1.0d0) & > ! call abort > contains > subroutine sub(x) > real*8 x, tem > tem = x + 2.d0**52 > x = tem - 2.d0**52 > end subroutine sub > end program test And for the sake of completeness the evaluation of sub above and subroutine sub2(x) real*8 x x = (x + 2.d0**52) - 2.d0**52 end subroutine sub2 should behave consistently if I read your Fortran standard quotations correctly.
