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[Bug target/36241] Executable compiled with -m64 almost three times faster than the one compiled with -m32 on Core2Duo
- From: "kargl at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 15 Jun 2009 18:44:07 -0000
- Subject: [Bug target/36241] Executable compiled with -m64 almost three times faster than the one compiled with -m32 on Core2Duo
- References: <bug-36241-12313@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Comment #4 from kargl at gcc dot gnu dot org 2009-06-15 18:44 -------
(In reply to comment #3)
> This is not darwin-specific, I also see it happening on x86_64-linux.
>
> And what's more, the output changes between -m32 and -m64.
The code is invalid Fortran, so gfortran is not required to give
any sensible output.
> $ cat u.f90
> integer(8), parameter :: l = z'5fe6eb3be0000000'
> integer, parameter :: ni = 3
> integer :: i, j, n
> integer(8) :: k
> real(8) :: a, b, e, m, s
> equivalence (b, k)
> a = 1.0d0
> e = epsilon(1.0)/2.0d0**4
> m = 0.0d0
> s = 0.0d0
> n = 0
> do
> n = n + 1
> b = a
When you do the assignment to b, k is/becomes undefined.
> k = l - ishft(k, -1_8)
When you do the assignment to k, b becomes undefined.
Not to mention, that the RHS uses k, which is undefined.
See Section 14.7.6 (1) and (9).
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36241