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Issue with ISHFT and 2 byte integers

 
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QuintinHill



Joined: 13 Feb 2013
Posts: 4
Location: Winchester, United Kingdom

PostPosted: Thu Sep 19, 2013 2:10 pm    Post subject: Issue with ISHFT and 2 byte integers Reply with quote

I have encountered a bug with ISHFT when used with 2 byte integers (I can't reproduce it with 4 byte integers), it sometimes fails to work giving the result zero. Here is a small test case to demonstrate the problem:
Code:

Module TestData

 Integer*2, Save :: i2SH
 Integer*2, Save :: i2DM
 Integer*2, Save :: i2CL

End Module TestData

Module Test

  Contains

  Subroutine ISHFTTest()

    USE TestData
    Integer*2 :: i2SI, i2CP
    Integer*2 :: i2Test

    i2CP = i2CL
    i2SI = i2SH + i2DM
    i2Test = i2SI
    WRITE(*, *) i2SI, i2CP, LS(i2SI, i2CP), ISHFT(260s, 4s) ! 260, 4, 4160, 4160
    i2Test = i2SI
    WRITE(*, *) ISHFT(i2Test, i2CP) ! This should write 4160
    i2Test = i2SI
    WRITE(*, *) i2SI, i2CP, ISHFT(i2Test, i2CP) ! This should write 260, 4, 4160
    i2SI= LS(i2SI, i2CP)
    WRITE(*, *) i2SI ! This should write 4160

  End Subroutine ISHFTTest

End Module Test

Program Main

USE TestData
USE Test

  i2SH = 160
  i2CL = 4
  i2DM = 100

  Call ISHFTTest()
 

End Program


The expected output is:
Code:

260 4 4160 4160
4160
260 4 4160
4160


I actually get:
Code:

260 4 4160 4160
0
260 4 4160
4160


It is strange that the first the two identical ISHFT calls give differt results. This is with FTN95 v6.35 with no switches.
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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 6409
Location: Salford, UK

PostPosted: Thu Sep 19, 2013 2:28 pm    Post subject: Reply with quote

Thanks.
I will log this for investigation.
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QuintinHill



Joined: 13 Feb 2013
Posts: 4
Location: Winchester, United Kingdom

PostPosted: Thu Sep 19, 2013 2:55 pm    Post subject: Reply with quote

Thank you. Note the trivial program:
Code:
Program Main
WRITE(*,*) ISHFT(256, -2)
End Program

Also fails to give the correct output.
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davidb



Joined: 17 Jul 2009
Posts: 557
Location: UK

PostPosted: Thu Sep 19, 2013 4:37 pm    Post subject: Re: Reply with quote

QuintinHill wrote:
Thank you. Note the trivial program:
Code:
Program Main
WRITE(*,*) ISHFT(256, -2)
End Program

Also fails to give the correct output.


In this 256 is a 4 byte integer, so the problem or problems are not restricted to 2 byte integers.

This "may" be evaluated by the compiler at "compile time" since it is a constant expression.

For interest and clue to what it going on both of these alternatives work:

Code:

INTEGER :: L
L=-2
WRITE(*,*) ISHFT(256,L)

!**and**

INTEGER :: W
W=256
WRITE(*,*) ISHFT(W,-2)

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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 6409
Location: Salford, UK

PostPosted: Fri Nov 29, 2013 3:21 pm    Post subject: Reply with quote

There appears to be a problem with the PRINT statement as illustrated by the following test.

Code:
Program Main
  integer*2 j,k,r
  j=260_2
  k=4_2
  r=ishft(j,k)
  print*, r,r,ishft(j,k)
  print*, r,ishft(j,k)
End Program


Can you get ISHFT to fail elsewhere (other than in a PRINT statement)?

With the current FTN95 I get

Code:
         4160        4160        4160
         4160           0
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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 6409
Location: Salford, UK

PostPosted: Fri Nov 29, 2013 3:27 pm    Post subject: Reply with quote

You get different results with

Code:
Program Main
  integer*2 j,k,r
  j=260_2
  k=4_2
  k=k
  r=ishft(j,k)
  print*, r,r,ishft(j,k)
  print*, r,ishft(j,k)
End Program


so previously r was being evaluated at compile time.
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PaulLaidler
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Joined: 21 Feb 2005
Posts: 6409
Location: Salford, UK

PostPosted: Fri Nov 29, 2013 3:40 pm    Post subject: Reply with quote

It is beginning to look like one of the registers is not being freed.
Leave it with me for now.
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davidb



Joined: 17 Jul 2009
Posts: 557
Location: UK

PostPosted: Fri Nov 29, 2013 10:26 pm    Post subject: Reply with quote

It seems to only happen with expressions evaluated by the compiler.

Code:

program main
   integer n, m, k

   n = ishft(256, -2)
   print *, n, "<-- Should be 64. But isn't"

   print *, ishft(256,-2), "<-- Should be 64. But isn't"

   m = 256
   k = -2
   print *, ishft(m,k), "<-- Should be 64. And is!"
end


Thanks for looking!
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PaulLaidler
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Joined: 21 Feb 2005
Posts: 6409
Location: Salford, UK

PostPosted: Mon Feb 10, 2014 2:31 pm    Post subject: Reply with quote

This bug has now been fixed for the next release.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2184
Location: Sydney

PostPosted: Sat May 23, 2020 3:18 pm    Post subject: Reply with quote

Paul,

I can't find recent discussion, but it appears this problem has returned.
The program in the opening post works for
FTN95 /debug /lgo (ok), but
FTN95 /check /lgo (fails)

I have been resurrecting some code that uses FILES@ and manipulates the DOS date and time fields. It fails with YY and also when repacking the adjusted date/time for GMT changes.
Code:
    implicit none
    integer*2 yymmdd, hhmmss, GMT_minutes
!
    integer*2 s2,mi,hh, dd,mo,yy, change
!...
         yy = ishft (yymmdd,-9)            ! 0-127    returns 0 with /check
!...
         hhmmss = s2
         hhmmss = ior ( ishft(mi,5),  hhmmss )
         hhmmss = ior ( ishft(hh,11), hhmmss )
!
         yymmdd = dd
         yymmdd = ior ( ishft(mo,5),  yymmdd )
         yymmdd = ior ( ishft(yy,9),  yymmdd )     !  reports integer overflow with /check

I even tried changing:
yy = ishft (yymmdd,-9) ! 0-127
to
yy = ishft (yymmdd,-9_2) ! 0-127
but this had no effect.

Changing from /check to /debug removed the problem.
Do you need any further info ?
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PaulLaidler
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Joined: 21 Feb 2005
Posts: 6409
Location: Salford, UK

PostPosted: Mon May 25, 2020 8:02 am    Post subject: Reply with quote

This bug has now been fixed for the next release of FTN95 and salflibc.dll.

It concerns 32 bit programs compiled with /undef. The issue is unrelated to that at the start of this thread but rather concerns new code that has been added to FTN95 in order to improve "undef" checking.
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