#4867: Incorrect result from trig functions
---------------------------------+------------------------------------------
Reporter: gwright | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.0.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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Trigonometric functions give the wrong answer in some cases. I have
verified this bug on ghc-7.0.1 and ghc-7.0.1-rc1 on OS X 10.6 64 bit.
(The bug may be limited to 64 bit platforms; I've not been able to
reproduce it on ghc-6.10.4/OS X 10.5/32 bit.)
Here's an example (ghc-7.0.2-rc1, OS X 10.6, 64 bit):
{{{
plumbbob-franklin> inplace/bin/ghc-stage2 --interactive
GHCi, version 7.0.1.20101221: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> tan (0.5 * pi + 0.01)
-4.563974029425858e214
Prelude> tan (0.5 * pi - 0.01)
99.99666664444354
Prelude>
}}}
The first result (`tan (0.5 * pi + 0.01)`) is wrong. The correct answer is
close to -100.0. The `sin` and `cos` functions also give incorrect
answers in some cases, e.g.,
{{{
Prelude> cos (3.0 * pi)
-3.666940035476786e76
}}}
At first glance, the bug seems related to the normalization of the
argument (i.e., mapping the argument to the range `(-pi/2, pi/2)`).
This is a nasty one. It ought to be fixed before 7.0.2 goes out.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4867>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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