#4391: forkIO threads do not properly save/restore the floating point
environment
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Reporter: draconx | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.2.1
Component: Runtime System | Version: 6.12.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: x86_64 (amd64) | Failure: None/Unknown
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Comment(by rwbarton):
Perhaps a better question is whether it affects "ordinary" programs, where
by "ordinary" programs I mean ones that don't invoke the `fe*` family of
functions (via the FFI) or the equivalent. My understanding is that it
does not, but I am not an expert on the subject; please correct me if I am
wrong.
If your program ''does'' write the rounding mode register or read from the
exception register, then this behavior of `forkIO` is not the only problem
you'll have: the Prelude exports the floating point arithmetic operations
as pure functions, even though they depend on the rounding mode and can
raise exceptions. This obviously breaks referential transparency and
compiler optimizations, so you could say that even the Prelude is "broken"
under these conditions. Asking "how much code on hackage is broken in
this regard" is meaningless, unless you think there a lot of code on
hackage which goes and mucks with the floating point state on unsuspecting
programs (seems unlikely to me).
It would be nice to mention in the documentation, maybe in section
"12.1.2. GHC's interpretation of undefined behaviour in Haskell 98" under
"Unchecked float arithmetic", that GHC does not manage the floating point
rounding or exception registers in any way; in particular, they are not
saved or restored by Haskell thread switches, and accessing them directly
in conjunction with using built-in floating point operations causes
undefined behavior.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4391#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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