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Failure at run-time with format F when using real KIND=3

 
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jlb



Joined: 21 Oct 2020
Posts: 75

PostPosted: Thu Aug 01, 2024 12:12 pm    Post subject: Failure at run-time with format F when using real KIND=3 Reply with quote

The following program, compiled with FTN95 v. 9.03.0.0 Win32, raises an exception (Invalid floating point operation) at runtime for i=19.
This error doesn't occur if KIND=2 is used for the real variables or F7.3 instead of F7.2 in the 9000 FORMAT.
Am I missing an obvious pitfall in this case?
Code:
PROGRAM RUNGE
IMPLICIT NONE
INTEGER :: I, N
REAL(KIND=3) :: A, B, T
A=0.0
B=5.0
N=1000
DO I=1,N
  T=(B-A)*FLOAT(I)/FLOAT(N)
  WRITE(*,9000) I,T
END DO
9000 FORMAT(1X,I4,3X,F7.2)
END PROGRAM RUNGE
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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 8067
Location: Salford, UK

PostPosted: Thu Aug 01, 2024 1:11 pm    Post subject: Reply with quote

There appears to be a problem with F7.2 for extended precision in this context but G7.2 seems to work OK.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2593
Location: Sydney

PostPosted: Fri Aug 02, 2024 4:01 am    Post subject: Reply with quote

This is a very strange error !
The following does not show the problem ?
Code:
  PROGRAM RUNGE
  IMPLICIT NONE
  INTEGER :: I, N
  REAL(KIND=3) :: A, B, T

  A=0.0
  B=5.0
  N=1001       !  35  !  1000
  DO I=1,N
    T=(B-A)*FLOAT(I)/FLOAT(N)
    WRITE(*,9000) I,T
  END DO
  9000 FORMAT(1X,I4,3X,F7.2)
 
  END PROGRAM RUNGE


N = 35 or N=1001 work ok, but why not N=1000 ?
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2593
Location: Sydney

PostPosted: Fri Aug 02, 2024 11:09 am    Post subject: Reply with quote

This shows how unique this problem might be.
The following variant shows there is something special about N = 1000 ; i = 19 ?
Does anyone know the reason for the problem with this ?

Code:
  PROGRAM RUNGE
  IMPLICIT NONE
  INTEGER :: I, N
  REAL(KIND=3) :: A, B, T

  A=0.0
  B=5.0
  do n = 4,1004
!  N=1000       !  35  !  1000
  DO I=1,N
    T=(B-A)*FLOAT(I)/FLOAT(N)
    WRITE(*,9000) I,T
  END DO
  end do
  9000 FORMAT(1X,I4,3X,F7.2)
 
  END PROGRAM RUNGE


I am running using PLATO with Checkmate, Win32
[FTN95/x64 Ver. 9.03.0.0 Copyright (c) Silverfrost Ltd 1993-2024]

Even with Release, Win32 it crashes at i=19

The following change skips the error,
do n = 994,1004
if ( n == 1000 ) cycle

??
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jlb



Joined: 21 Oct 2020
Posts: 75

PostPosted: Fri Aug 02, 2024 11:47 am    Post subject: Reply with quote

Paul
A similar error occurs with N=10000 or 100000 or 1000000 and G7.2 at I=199
John
Thanks for confirming the bug. A similar error occurs with N=10000 and F7.2 at I=189 (not 199!).
Neither the DO loop nor the reals A and B are needed, the reproducer can be reduced to the following:
Code:
PROGRAM RUNGE
IMPLICIT NONE
INTEGER :: I, N
REAL(KIND=3) :: T
I=95
N=1000
T=FLOAT(I)/FLOAT(N)
WRITE(*,9000) I,T
9000 FORMAT(1X,I4,3X,F7.2)
END PROGRAM RUNGE
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wahorger



Joined: 13 Oct 2014
Posts: 1238
Location: Morrison, CO, USA

PostPosted: Fri Aug 02, 2024 6:55 pm    Post subject: Reply with quote

No error if you use dble(i)/dble(n).

However, the value displayed for the floating point value is still all ******'s.

The hexadecimal of this is interesting as a repeating pattern:

C28F5C28F5C28F5C
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jlb



Joined: 21 Oct 2020
Posts: 75

PostPosted: Sat Aug 03, 2024 1:34 pm    Post subject: Reply with quote

Bill
I still get the same error with DBLE(I)/DBLE(N) using FTN95 version 9.03.0.0 Win32.
Do you have an easy way to get the full hexadecimal "physical" representation of a KIND=3 real (80-bit) number, since I can only use 64-bit integers (KIND=4) as targets with the TRANSFER method in FTN95?
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wahorger



Joined: 13 Oct 2014
Posts: 1238
Location: Morrison, CO, USA

PostPosted: Mon Aug 05, 2024 5:47 pm    Post subject: Reply with quote

This should work. This causes an error in both Checkmate and Release.

Code:

   PROGRAM RUNGE
   IMPLICIT NONE
   INTEGER :: I, N
        integer (KIND=4):: k=0
        integer(7):: my_loc,indx
   REAL (kind=3) :: T
   I=95
   N=1000
        t=0.0
   T=FLOAT(I)/FLOAT(N)
        my_loc = loc(t)
!        print *,my_loc ! decimal
        write(*,9002)my_loc,(core1(my_loc+indx-1),indx=1,10)
   WRITE(*,9000) I,T
9000   FORMAT(1X,I4,3X,F7.2)
9001   format(z20.20)
9002   format('Start Addr=',z8.8,': ',10z2.2)
   END PROGRAM RUNGE
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wahorger



Joined: 13 Oct 2014
Posts: 1238
Location: Morrison, CO, USA

PostPosted: Mon Aug 05, 2024 6:25 pm    Post subject: Reply with quote

FWIW: Fortran 9.02.00
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jlb



Joined: 21 Oct 2020
Posts: 75

PostPosted: Tue Aug 06, 2024 12:38 pm    Post subject: Reply with quote

Bill
Thanks for the method of showing the physical representation of a 80-bit real number. I had all the knowledge to do it myself, as you kindly gave me a workaround for C_F_POINTER some time ago, but unfortunately I didn't recover the trick. Sorry for that and thanks again for your help.


Last edited by jlb on Tue Aug 20, 2024 1:12 pm; edited 1 time in total
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wahorger



Joined: 13 Oct 2014
Posts: 1238
Location: Morrison, CO, USA

PostPosted: Wed Aug 07, 2024 4:07 pm    Post subject: Reply with quote

No worries! That's what this forum is for, helping others!
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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 8067
Location: Salford, UK

PostPosted: Mon Aug 19, 2024 9:25 am    Post subject: Reply with quote

This failure has now been fixed for the next release of salflibc.dll.
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jlb



Joined: 21 Oct 2020
Posts: 75

PostPosted: Tue Aug 20, 2024 9:34 am    Post subject: Reply with quote

Paul
Thank you for this.
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