#14610: newtype wrapping of a monadic stack kills performance
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Reporter: mrkkrp | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone: 8.6.1
Component: Compiler | Version: 8.2.2
Resolution: | Keywords: JoinPoints
Operating System: Unknown/Multiple | Architecture:
Type of failure: Runtime | Unknown/Multiple
performance bug | Test Case:
Blocked By: | Blocking:
Related Tickets: #14620 | Differential Rev(s):
Wiki Page: |
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Comment (by nomeata):
Here is a contrived example. This code:
{{{
{-# LANGUAGE KindSignatures, DataKinds, GADTs, TypeFamilies #-}
module RecCast (foo) where
import Data.Coerce
data Nat = Z | S Nat
data Sing (n :: Nat) where
FZ :: Sing Z
FS :: Sing n -> Sing (S n)
-- The NoConst to avoid worker-wrapper
data Const (y::Nat) = Const Nat | NoConst
mapConst :: (Nat -> Nat) -> Const n -> Const n
mapConst f (Const x) = Const (f x)
mapConst f NoConst = NoConst
{-# NOINLINE mapConst #-}
inc :: Const n -> Const (S n)
inc = coerce
type family Plus n m :: Nat where
Plus Z m = m
Plus (S n) m = S (Plus n m)
foo :: Sing n -> Const m -> Const (Plus n m)
foo FZ c = c
foo (FS n) c = inc (foo n (mapConst S c))
}}}
produces this function that would be a join point when we consider casted
expressions as tail-recursive:
{{{
Rec {
-- RHS size: {terms: 14, types: 25, coercions: 18, joins: 0/0}
foo [Occ=LoopBreaker]
:: forall (n :: Nat) (m :: Nat).
Sing n -> Const m -> Const (Plus n m)
[GblId, Arity=2, Caf=NoCafRefs, Str=