#7360: Case-of-identical-alts optimisation fails abjectly
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Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonpj):
I was very surprised to see this but you are absolutely right. The reason
the occurrence info does not show the alternative's binders as dead shows
up in `Simplify.simplAlt`. Look at the call to `zapBndrOccInfo`,
controlled by the boolean `keep_occ_info`. Notice that `keep_occ_info` is
false if the scrutinee is a variable, which it usually is. This is
enough to defeat the optimisation.
The reason is described in `Note [zapOccInfo]` in `Simplify.lhs`. If we
have
{{{
case x of { C a b -> ....case x of { C p q -> p }.... }
}}}
Now, `a` and `b` are not mentioned, and are hence dead. But if we were to
optimise the inner case, we'd reduce
{{{
case x of { C p q -> p } ===> a
}}}
and now `a` is alive. Moreover `mkCase1` is applied use '''after''' the
alternative has been simplified, so now it in fact looks like
{{{
case x of { C a b -> ....a.... }
}}}
That's why a's occurrence info is zapped: it might not be dead any more.
But in fact the occurrence analyser goes to great trouble (the "binder-
swap" optimisation) to ensure that if the scrutinee is a variable, we
replace its uses in the alternatives with the case-binder. So the
occurrence analyser will produce this
{{{
case x of y { C a b -> ....case y of { C p q -> p }.... }
}}}
So we really don't need to take the scrutinee into account.
I think I see how to ''both'' simplify the code ''and'' make the
optimisation work again. I need enough time to do some comparative nofib
runs to make sure I hvae not messed up, though.
Thanks for pointing this out.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7360#comment:1>
GHC <http://www.haskell.org/ghc/>
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