#2756: state hack causes unneeded value to be evaluated
------------------------------+---------------------------------------------
Reporter: int-e | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.10.2
Component: Compiler | Version: 6.10.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown/Multiple
Os: Unknown/Multiple |
------------------------------+---------------------------------------------
Changes (by simonmar):
* difficulty: => Unknown
* milestone: => 6.10.2
Comment:
Good bug. I don't think this is caused by `-fno-state-hack`. The code
coming out of the simplifier is fine; this is `newX`:
{{{
a_rus =
\ (n_afy :: ()) (eta_ssU :: GHC.Prim.State# GHC.Prim.RealWorld) ->
(# eta_ssU,
Main.X
(let {
value_sub [NEVER Just S] :: ()
[Str: DmdType]
value_sub = n_afy } in
value_sub) #)
}}}
The prep phase lifts out that `let`, but doesn't reset its strictness
annotation, which changes the meaning of the program:
{{{
a_rus =
\ (n_suA :: ()) (eta_suy :: GHC.Prim.State# GHC.Prim.RealWorld) ->
let {
sat_suE :: Main.X
[]
sat_suE =
case n_suA of value_suC [NEVER Just S] { __DEFAULT ->
Main.X value_suC
} } in
(# eta_suy, sat_suE #)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2756#comment:1>
GHC <http://www.haskell.org/ghc/>
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