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mecej4
Joined: 31 Oct 2006
Posts: 1886
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 Posted: Tue Jan 16, 2024 1:33 pm Post subject: Errors in some bit operations with 8-byte integers |
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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 |
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PaulLaidler
Site Admin
Joined: 21 Feb 2005
Posts: 7926
Location: Salford, UK
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| Posted: Tue Jan 16, 2024 1:55 pm Post subject: |
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mecej4
Thanks for the feedback which I have logged for investigation. |
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mecej4
Joined: 31 Oct 2006
Posts: 1886
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| Posted: Tue Jan 16, 2024 4:26 pm Post subject: |
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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. |
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PaulLaidler
Site Admin
Joined: 21 Feb 2005
Posts: 7926
Location: Salford, UK
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| Posted: Tue Jan 16, 2024 7:40 pm Post subject: |
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| No problem either way. |
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PaulLaidler
Site Admin
Joined: 21 Feb 2005
Posts: 7926
Location: Salford, UK
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| Posted: Fri Feb 02, 2024 5:20 pm Post subject: |
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| 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. |
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