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Summary: Ticket #15271 reports a case where 1e1000000000 is incorrectly converted to 0.0. After some investigation, I discovered the number is converted to rational correctly, but converting the ratio into a double introduced an error. Tracking down to how the conversion is done, I found the rts float implementation uses `ldexp`, whose signature is `double ldexp (double x, int exp);` The callsite passes an `I_` to the second argument, which is platform-dependent. On machines where `I_` is 64 bits and `int` is 32 bits, we observe integer overflow behaviour. Here is a mapping from rational to exponent with observations 1e646457008 -> 2147483645 (result = infinity, positive in int32) 1e646457009 -> 2147483648 (result = 0.0, overflow to negative in int32) 1e1000000000 -> 3321928042 (result = infinity, overflow to positive in int32) 1e1555550000 -> 5167425196 (result = 0.0, overflow to negative in int32) We fix this issue by comparing STG_INT_MIN/MAX and INT_MIN/MAX and bound the value appropriately. Test Plan: New test cases Reviewers: bgamari, erikd, simonmar Reviewed By: bgamari Subscribers: rwbarton, carter GHC Trac Issues: #15271 Differential Revision: https://phabricator.haskell.org/D5271
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