#3259: A module-local combinator using Control.Parallel.par behaves differently
than when it's imported
-------------------------+--------------------------------------------------
Reporter: blamario | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 6.11
Severity: normal | Resolution: fixed
Keywords: | Difficulty: Unknown
Testcase: | Os: Linux
Architecture: x86 |
-------------------------+--------------------------------------------------
Changes (by simonpj):
* status: new => closed
* resolution: => fixed
Comment:
OK so after talking to Simon I indeed moved the exposure of `lazy` into
`CorePrep`. That certainly fixes the bug.
{{{
Fri May 29 00:20:20 PDT 2009 simo...@microsoft.com
* Fix Trac #3259: expose 'lazy' only after generating interface files
Ignore-this: 3c762bda546981c4b4f01d28b8586ff8
This patch fixes an insidious and long-standing bug in the way that
parallelism is handled in GHC. See Note [lazyId magic] in MkId.
...snip...
The downside is that a little less optimisation may happen on programs
that use 'lazy'. And you'll only see this in the results -ddump-prep
not in -ddump-simpl. So KEEP AN EYE OUT (Simon and Satnam especially).
Still, it should work properly now. Certainly fixes #3259.
M ./compiler/basicTypes/MkId.lhs -18 +23
M ./compiler/coreSyn/CorePrep.lhs -2 +10
M ./compiler/stranal/WorkWrap.lhs -10 +3
}}}
I have not added a test case, because I can't see a simple way to test it.
But please do see if things improve.
Do watch out for loss of optimisation as a result. Simon and I do not
think it'll be important, but we've been wrong before.
For example, in your program you have way too many `lazy` calls:
{{{
parallelize a b = lazy a `par` (lazy b `pseq` (a, b))
}}}
Here the `lazy` calls are completely unnecessary, and indeed they (now)
make the code a bit less efficient. Instead you can say
{{{
parallelize a b = a `par` (b `pseq` (a, b))
}}}
Thank you for identifying such an insidious bug.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3259#comment:2>
GHC <http://www.haskell.org/ghc/>
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