#12776: Panic Simplifier ticks exhausted since ghc 8
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Reporter: sjcjoosten | Owner:
Type: bug | Status: new
Priority: highest | Milestone:
Component: Compiler | Version: 8.0.1
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Compile-time | Unknown/Multiple
crash or panic | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by simonpj):
Thank you for the small test case. I can see what is happening here.
After `SpecConstr`, and occurrence analysis we get
{{{
Rec {
$slast_s_s1J5
:: forall a_a1BZ. a_a1BZ -> Slist a_a1BZ -> Eq a_a1BZ => a_a1BZ
$slast_s_s1J5 =
\ (@ a_a1BZ)
(sc_s1J0 :: a_a1BZ)
(sc_s1J1 :: Slist a_a1BZ)
(sc_s1IZ :: Eq a_a1BZ) ->
case sc_s1J1 of wild_Xp [Dmd=] {
Nil_s -> sc_s1J0;
Cons_s _ [Occ=Dead] _ [Occ=Dead] ->
let {
sc_s1J1 [Occ=Once] :: Slist a_a1BZ
[LclId]
sc_s1J1 = wild_Xp } in
last_s @ a_a1BZ sc_s1IZ sc_s1J1
}
last_s [Occ=LoopBreaker]
:: forall a_a1kp. Eq a_a1kp => Slist a_a1kp -> a_a1kp
[RULES: "SC:last_s0" [ALWAYS]
forall (@ a_a1BZ)
(sc_s1J0 :: a_a1BZ)
(sc_s1J1 :: Slist a_a1BZ)
(sc_s1IZ :: Eq a_a1BZ).
last_s @ a_a1BZ
sc_s1IZ
(Cons_s @ a_a1BZ sc_s1J0 sc_s1J1)
= $slast_s_s1J5 @ a_a1BZ sc_s1J0 sc_s1J1 sc_s1IZ]
last_s =
\ (@ a_a1BC)
($dEq_a1BE :: Eq a_a1BC)
(ds_d1Do [Occ=Once!] :: Slist a_a1BC) ->
case ds_d1Do of {
Nil_s -> lvl_s1Fa @ a_a1BC;
Cons_s x_a1kB [Occ=Once] xs_a1kC [Occ=Once!, Dmd=] ->
case xs_a1kC of {
Nil_s -> x_a1kB;
Cons_s a1_a1zd [Occ=Once] a2_a1ze [Occ=Once] ->
$slast_s_s1J5 @ a_a1BC a1_a1zd a2_a1ze $dEq_a1BE
}
}
end Rec }
}}}
Then in `last_s`:
* `$s_last_s_s1J5` is inlined
* The RULE fires, yielding a new cal to `$s_last_s_s1J5`
* `$s_last_s_s1J5` is inlined again
* and so on.
The problem is that `$s_last_s_s1J5` should be a loop breaker. See the
extensive notes under `Note [Choosing loop breakers]`.
But alas in `occAnalRec` we see this
{{{
pairs :: [(Id,CoreExpr)]
pairs | isEmptyVarSet weak_fvs = reOrderNodes 0 bndr_set weak_fvs
tagged_nodes []
| otherwise = loopBreakNodes 0 bndr_set weak_fvs
loop_breaker_edges []
}}}
We want to go via the `loopBreakNodes` call, but actually `weak_fvs` is
empty so we go via the former call.
I'm not yet sure exactly why... just recording breadcrumbs for now.
How urgent is this for you, Stef?
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12776#comment:3
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler
