#15271: 1e1000000000 :: Double yields 0.0 instead of Infinity -------------------------------------+------------------------------------- Reporter: claude | Owner: fangyizhou Type: bug | Status: patch Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D5271 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"8e1a5593f940b9d2882a5e81abf028e7b99aff3a/ghc" 8e1a559/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="8e1a5593f940b9d2882a5e81abf028e7b99aff3a" Fix integer overflow when encoding doubles (Trac #15271) 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 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15271#comment:13> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler