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.
- 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
