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(This bug report is about the incorrect result not the poor performance.)

Very large positive exponent in floating point literal at Double type gives 0 instead of Infinity in ghci-8.4.3 (self-compiled on Debian Buster x86_64 / amd64):

$ ghci
GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
Prelude> :set +s
Prelude> 1e100000000 :: Double
Infinity
(5.70 secs, 68,552 bytes)
Prelude> 1e1000000000 :: Double
0.0
(69.35 secs, 60,088 bytes)

Writing 10^ instead of 1e completes almost instantly with the correct result (Infinity) in both cases.

More precisely,

Prelude> 1e646457008 :: Double
Infinity
(40.80 secs, 64,272 bytes)
Prelude> 1e646457009 :: Double
0.0
(40.46 secs, 60,088 bytes)

Note:

(floor $ 2^31 / logBase 2 10 + 16) == 646457009

This vague numerology makes me think something C int-related is overflowing somewhere (GMP? integer-gmp? GHC?).

Standalone test program:

main = do
  print 1e646457008
  print 1e646457009

Interestingly, it doesn't occur, or at least not near the same threshold, in 32-bit ghci-8.0.1 (Debian Stretch i686), though it aborts when getting too large (the 32-bit i686 machine has 1GB RAM and 4GB swap, the 64-bit x86_64/amd64 has 32GB RAM). The sheer time it takes to run makes bisecting the exact threshold on i686 not something I want to take on (though if someone writes some code that can do it programmatically I'd be happy to run it overnight if it would help).

$ ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> :set +s
Prelude> 1e646457008 :: Double
Infinity
(415.26 secs, 17,908 bytes)
Prelude> 1e646457009 :: Double
Infinity
(417.66 secs, 17,820 bytes)
Prelude> 1e746457298 :: Double
Infinity
(490.00 secs, 17,820 bytes)
Prelude> 1e1000000000 :: Double
GNU MP: Cannot allocate memory (size=419438600)
Aborted
$

ghci-8.0.2 on Debian Buster amd64 exhibits the problem also.