|
forums.silverfrost.com Welcome to the Silverfrost forums
|
View previous topic :: View next topic |
Author |
Message |
mecej4
Joined: 31 Oct 2006 Posts: 1898
|
Posted: Tue Jan 16, 2024 1:33 pm Post subject: Errors in some bit operations with 8-byte integers |
|
|
The late George Marsaglia posted his SuperKiss-64 RNG code in the "General" forum in 2009 ( http://forums.silverfrost.com/viewtopic.php?t=1480 ). At the time (and, perhaps, even now), 32-bit code generated by FTN95 used the FPU for 8-byte integer operations, which leads to lots of FPU exceptions, so we use only /64 for this code.
Marsaglia's test program generates a billion 8-byte random integers and compares the last of those to a known value. With FTN95 8.972 and 9.0, the test fails. With 8.9, 8.92, 8.95, the test passes. Here is a very short program that contains a small portion of Marsaglia's code, just enough to display the bug.
Code: | ! Suitable only for 64-bit compilation
! Some of the last digits are wrong with Versions 8.972, 9.0
! Correct output with Versions 8.9, 8.92, 8.95, as follows
! i h z carry q(i)
! 1 1 -2945433577893571930 863862222379 5890867155787143858
! 2 1 6661552707265476373 1293535672257 5123638659178598868
! 3 1 -4642211112981496864 93621267173 -9162321847746557890
! 4 1 -8101377823381285518 1808705431682 -2243988426946980582
!
program S64BITOPS
implicit None
Integer, Parameter :: QSIZ = 4
Integer, Parameter :: i8 = selected_int_kind(18)
Integer (Kind=i8) :: i, z, h, q(QSIZ)
Integer (Kind=i8), Parameter :: CMUL = 6906969069_i8, COFFS = 123_i8
Integer (Kind=i8) :: carry = 36243678541_i8, &
xcng = 12367890123456_i8, xs = 521288629546311_i8
Do i = 1, qsiz ! fill Q with Congruential + Xorshift
xcng = xcng*CMUL + COFFS ! ignore integer overflow here
xs = ieor(xs, ishft(xs,13))
xs = ieor(xs, ishft(xs,-17))
xs = ieor(xs, ishft(xs,43))
q(i) = xcng + xs
End Do
print *,' i h z carry q(i)'
Do i = 1, QSIZ
h = iand(carry, 1_i8)
z = ishft(ishft(q(i),41), -1) + &
ishft(ishft(q(i),39), -1) + &
ishft(carry, -1)
carry = ishft(q(i), -23) + ishft(q(i), -25) + ishft(z, -63)
q(i) = not(ishft(z,1)+h)
print '(2i3,i21,i15,i21)',i,h,z,carry,q(i)
End Do
end program |
Last edited by mecej4 on Sat Feb 03, 2024 8:53 am; edited 1 time in total |
|
Back to top |
|
|
PaulLaidler Site Admin
Joined: 21 Feb 2005 Posts: 8065 Location: Salford, UK
|
Posted: Tue Jan 16, 2024 1:55 pm Post subject: |
|
|
mecej4
Thanks for the feedback which I have logged for investigation. |
|
Back to top |
|
|
mecej4
Joined: 31 Oct 2006 Posts: 1898
|
Posted: Tue Jan 16, 2024 4:26 pm Post subject: |
|
|
Paul, I did not mean to post this in the FTN77 Support forum!
I can repost in the 64-bit forum, where it belongs, but I am afraid that if I did so your reply may be left behind in the FTN77 forum. |
|
Back to top |
|
|
PaulLaidler Site Admin
Joined: 21 Feb 2005 Posts: 8065 Location: Salford, UK
|
Posted: Tue Jan 16, 2024 7:40 pm Post subject: |
|
|
No problem either way. |
|
Back to top |
|
|
PaulLaidler Site Admin
Joined: 21 Feb 2005 Posts: 8065 Location: Salford, UK
|
Posted: Fri Feb 02, 2024 5:20 pm Post subject: |
|
|
This regression (for 64 bit FTN95) has been fixed for the next release of FTN95. It concerns ishft(z, -63) and ishft(z, 63) for INTEGER*8 and ishft(z, -31) and ishft(z, 31) for INTEGER*4. |
|
Back to top |
|
|
|
|
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|