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Date: Tue, 25 Oct 2005 13:21:00 -0000
Subject: [Bug fortran/24518]  New: Intrinsic MOD incorrect for large arg1/arg2 and slow.
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Message-ID: <bug-24518-9236@http.gcc.gnu.org/bugzilla/>
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To: gcc-bugs@gcc.gnu.org
From: "paul dot richard dot thomas at cea dot fr" <gcc-bugzilla@gcc.gnu.org>
X-SW-Source: 2005-10/txt/msg03237.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

This:

  real*8 :: x = 10.0e9
  do i = 10, 22
    x = 10d0 * x
    print '(a,i2,a,g14.8)', "mod (10**",i,", 1.7_8) = ",mod (x, 1.7_8)
  end do
end

does this with cvs gfortran

mod (10**10, 1.7_8) =  1.3000026
mod (10**11, 1.7_8) =  1.1000261
mod (10**12, 1.7_8) = 0.80026150
mod (10**13, 1.7_8) =  1.2026138
mod (10**14, 1.7_8) = 0.12609863
mod (10**15, 1.7_8) =  1.2607422
mod (10**16, 1.7_8) =  5.8125000
mod (10**17, 1.7_8) = -50.687500
mod (10**18, 1.7_8) =  364.00000
mod (10**19, 1.7_8) = -.70000000E+20
mod (10**20, 1.7_8) = -.70000000E+21
mod (10**21, 1.7_8) = -.70000000E+22
mod (10**22, 1.7_8) = -.70000000E+23

and this with a leading commercial brand:

mod (10**10, 1.7_8) =  1.3000026
mod (10**11, 1.7_8) =  1.1000261
mod (10**12, 1.7_8) = 0.80026123
mod (10**13, 1.7_8) =  1.2026123
mod (10**14, 1.7_8) = 0.12612289
mod (10**15, 1.7_8) =  1.2612289
mod (10**16, 1.7_8) = 0.71228947
mod (10**17, 1.7_8) = 0.32289470
mod (10**18, 1.7_8) =  1.5289470
mod (10**19, 1.7_8) =  1.6894697
mod (10**20, 1.7_8) =  1.5946971
mod (10**21, 1.7_8) = 0.64697063
mod (10**22, 1.7_8) = 0.86970625

gfortran loses it at around 10**16 and starts producing values larger than the
second argument. Whilst, to some degree, one deserves to be hit on the nose for
trying to calculate mod for these values, a slightly less deviant behaviour
might be better.

As noted in the list http://gcc.gnu.org/ml/fortran/2005-10/msg00482.html, this
leading brand is about twice as fast in calulating mod, as gfortran, in spite
of its apparently using 64 bit arithmetic.


-- 
           Summary: Intrinsic MOD incorrect for large arg1/arg2 and slow.
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: paul dot richard dot thomas at cea dot fr


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24518