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REAL*10 or selected_real_kind(18,4931)

 
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Kenneth_Smith



Joined: 18 May 2012
Posts: 506
Location: Hamilton, Lanarkshire, Scotland.

PostPosted: Wed May 18, 2016 9:48 am    Post subject: REAL*10 or selected_real_kind(18,4931) Reply with quote

Out of curiosity I downloaded the updated compiler and tried to compile some existing code. Unfortunately I did not get very far (line 83 of 35,000).

Can somebody advise me on the status of extended precision floating point REAL*10 or selected_real_kind(18,4931) within the new 64 bit compiler?

Thanks

Ken
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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 7142
Location: Salford, UK

PostPosted: Wed May 18, 2016 10:48 am    Post subject: Reply with quote

Extended precision is not support in the 64 bit compiler.

Details can be found in the file NotesOn64BitFtn95.txt that should be in the installed ftn95\doc folder.
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Kenneth_Smith



Joined: 18 May 2012
Posts: 506
Location: Hamilton, Lanarkshire, Scotland.

PostPosted: Wed May 18, 2016 11:05 am    Post subject: Reply with quote

Thanks Paul,

Is this a temporary position. Will real*10 be added later?

Regards

Ken
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PaulLaidler
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Joined: 21 Feb 2005
Posts: 7142
Location: Salford, UK

PostPosted: Wed May 18, 2016 11:27 am    Post subject: Reply with quote

No it isn't temporary. I am not sure that it is feasible. Certainly it is not on the current list of things to do. Right now we are working on optimisation and run-time checking.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2353
Location: Sydney

PostPosted: Wed May 18, 2016 11:49 am    Post subject: Reply with quote

Ken,

Real*10 is not well supported in 64 bit. gFortran provides real*10, but stores the values in a 16 byte format !! As far as I know, both real*10 and real*16 are only software supported in any windows 64 bit compiler. There is no hardware support for real*10 in 64 bit.

If you need further precision, you would need to find a high precision library for software support. There are strategies to accumulate errors in a sum (see Kahan summation algorithm), but again there can be problems with the precision of the numbers you are providing.

If you need further range; a number larger than 1.7976931E+308 then I have no solution.

I have removed all real*10 usage and have not found a problem with the precision of the real*8 alternative. This was not a hard decision, as my use of real*10 did not improve precision in practical terms.
I once tested a real*10 skyline solver but the real*10 matrix coefficients were the sum of element real*8 matrices, (which were probably based on real*4 parameters). The structural finite element test problem showed bad round-off errors with real*8, but the net result of the real*10 test was no change to the calculated precision of the solution.

John
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Kenneth_Smith



Joined: 18 May 2012
Posts: 506
Location: Hamilton, Lanarkshire, Scotland.

PostPosted: Wed May 18, 2016 4:12 pm    Post subject: Reply with quote

John,

Thanks for your useful insight on this.

Generally, for the last decade or so I have adopted REAL*8 as my default approach.

Recently I undertook some work using sum, products and quotients of Bessel/Kelvin functions and quickly got mired in precision overflow/underflow problems (In(x) tends to infinity and Kn(x) tends to zero).

Adopting REAL*10 KINDS was a very quick fix that avoided having to recast the equations in my solution to avoid these issues.

Regards

Ken
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2353
Location: Sydney

PostPosted: Fri May 20, 2016 11:15 am    Post subject: Reply with quote

Ken,

I have a program to report the available KIND values a compiler supports for integer and real.
I suggest you could try it with FTN95 using the options:
ftn95 kind.f90 /lgo
ftn95 kind.f90 /lgo /64
ftn95 kind.f90 /lgo /alt_kinds
ftn95 kind.f90 /lgo /alt_kinds /64

You could also test it on other compilers to see what is available.
For example, with gfortran I use:
del %1.exe
del %1.o
gfortran %1.f90 -o %1.exe
%1

my test kind.f90 is:
Code:
Program kind_f90
integer*4 p,r, ik, lk
!
  write (*,*) 'kind(0)          ', kind(0)
  write (*,*) 'kind(999999999)  ', kind(999999999)
! write (*,*) 'kind(9999999999) ', kind(9999999999)
  write (*,*) 'kind(0.)         ', kind(0.)
  write (*,*) 'kind(0.e0)       ', kind(0.e0)
  write (*,*) 'kind(0.d0)       ', kind(0.d0)
!
11 format (a,' :digits = ',i0,' Kind = ',i0 )
12 format (a,' :digits = ',i0,' :range = ',i0,' Kind = ',i0)
  lk = -1
  do p = 1,10000
    ik = selected_int_kind ( p )
    if ( lk == ik ) cycle
    write (*,11) 'Int ',p-1,lk
    write (*,11) 'Int ',p,ik
    lk = ik
    if ( ik < 0 ) exit
  end do
!
  lk = -1
  do p = 1,10000
    ik = selected_real_kind ( p )
    if ( lk == ik ) cycle
    write (*,11) 'Real',p-1,lk
    write (*,11) 'Real',p,ik
    lk = ik
    if ( ik < 0 ) exit
  end do
!
  lk = -1
  p = 0
  do r = 1,10000
    ik = selected_real_kind ( p,r )
    if ( lk == ik ) cycle
    write (*,12) 'Real',p,r-1,lk
    write (*,12) 'Real',p,r,ik
    lk = ik
    if ( ik < 0 ) exit
  end do
end


I find that gFortran's approach to real*10 of storing it in a 16 byte variable removes a lot of the advantage. You may as well use real*16 as both are supported by an extended precision library and so not suitable for a compute intensive problem. I expect others will disagree.

John
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