#9811: constant folding with infinities is wrong -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: #9810 | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- {{{ main = let big = 99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999 :: Double in print (big * big - big * big) }}} prints NaN when compiled without optimizations but 0.0 when compiled with optimizations. Or, `print (big * big / 2)` prints Infinity when compiled without optimizations but 8.98846567431158e307 when compiled with optimizations (this number is `fromRational (toRational (1/0) / 2)`). The cause is the conversions that go on between Rational and Double in PrelRules (in the functions doubleOp2, mkDoubleVal, convFloating). Ideally, we would have an accurate model of floating-point arithmetic on the target machine, but for now it would be an improvement to just not constant fold when the result is not a finite floating-point number. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9811 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler