[Bug rtl-optimization/50904] [4.7 regression] pessimization when -fno-protect-parens is enabled by -Ofast

["burnus at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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