Opened 2 years ago
Closed 2 years ago
#13410 closed bug (fixed)
GHC HEAD regression: Template variable unbound in rewrite rule
Reported by: | RyanGlScott | Owned by: | simonpj |
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Priority: | highest | Milestone: | 8.2.1 |
Component: | Compiler | Version: | 8.0.1 |
Keywords: | JoinPoints, SpecConstr | Cc: | lukemauer, goldfire |
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | Compile-time crash or panic | Test Case: | simplCore/should_compile/T13410 |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Rev(s): | ||
Wiki Page: |
Description (last modified by )
hybrid-vectors-0.2.1
currently fails to build with GHC HEAD. I've attached a minimized repro file with no dependencies (it's 410 lines long, since I had to copy-paste a lot of code from vector
in order to reproduce this). The attached file builds on GHC 7.10.3 and 8.0.2, but fails on GHC HEAD with this panic:
$ ~/Software/ghc2/inplace/bin/ghc-stage2 -O2 -fforce-recomp Bug.hs [1 of 1] Compiling Data.Vector.Hybrid.Internal ( Bug.hs, Bug.o ) Bug.hs:389:10: warning: [-Wmissing-methods] • No explicit implementation for ‘gmbasicOverlaps’, ‘gmbasicInitialize’, and ‘gmbasicUnsafeRead’ • In the instance declaration for ‘GMVector (MVector u v) (a, b)’ | 389 | instance (GMVector u a, GMVector v b) => GMVector (MVector u v) (a, b) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:407:10: warning: [-Wmissing-methods] • No explicit implementation for ‘gbasicUnsafeFreeze’, ‘gbasicUnsafeThaw’, ‘gbasicLength’, ‘gbasicUnsafeSlice’, and ‘gbasicUnsafeIndexM’ • In the instance declaration for ‘GVector (Vector u v) (a, b)’ | 407 | instance (GVector u a, GVector v b) => GVector (Vector u v) (a, b) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.3.20170310 for x86_64-unknown-linux): Template variable unbound in rewrite rule Variable: sc_s8FV Rule "SC:$j0" Rule bndrs: [sc_s8FV, sc_s8FW, sc_s8FX, sg_s8FY, sc_s8FU] LHS args: [sc_s8FU, (MV @ (GMutable u_a4oM) @ (GMutable v_a4oO) @ (PrimState (ST RealWorld)) @ (a_a4oN, b_a4oP) @ a_a4oN @ b_a4oP @~ (<(a_a4oN, b_a4oP)>_N :: ((a_a4oN, b_a4oP) :: *) ~# ((a_a4oN, b_a4oP) :: *)) sc_s8FW sc_s8FX) `cast` (sg_s8FY :: (MVector (GMutable u_a4oM) (GMutable v_a4oO) (PrimState (ST RealWorld)) (a_a4oN, b_a4oP) :: *) ~R# (GMutable (Vector u_a4oM v_a4oO) (PrimState (ST RealWorld)) c_a4oQ :: *))] Actual args: [sc_s8FS, wild_X3x `cast` (Sub (Sym (D:R:GMutableVector[0] <u_a4oM>_N <v_a4oO>_N)) <PrimState (ST RealWorld)>_N (Sym cobox_a4Gz) :: (MVector (GMutable u_a4oM) (GMutable v_a4oO) (PrimState (ST RealWorld)) (a_a4oN, b_a4oP) :: *) ~R# (GMutable (Vector u_a4oM v_a4oO) (PrimState (ST RealWorld)) c_a4oQ :: *))] Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable pprPanic, called at compiler/specialise/Rules.hs:579:19 in ghc:Rules
Some important things to note:
-O2
is required to trigger this bug; using-O1
or below works fine.- The problematic code is the
Read (Vector u v c)
instance at the very bottom. In particular, commenting out the last linereadPrec = greadPrec
makes the panic go away. - Moreover, there's a particular
INLINE
pragma in theGMVector (MVector u v) (a, b)
instance that is critical to triggering the panic (it's on line 395; I've added a commentRemoving this INLINE pragma makes it compile
above it).
Attachments (1)
Change History (13)
Changed 2 years ago by
comment:1 Changed 2 years ago by
Description: | modified (diff) |
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comment:2 Changed 2 years ago by
Keywords: | JoinPoints added |
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comment:3 Changed 2 years ago by
Cc: | lukemauer added |
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comment:4 Changed 2 years ago by
I reduced the file from ~400 lines to ~150 lines:
{-# LANGUAGE BangPatterns #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} module Data.Vector.Hybrid.Internal (Vector) where import Control.Monad (liftM2) import Data.Functor.Identity (Identity(..)) import GHC.ST (ST, runST) import Text.Read (ReadPrec, readPrec) ----- class Monad m => PrimMonad m where type PrimState m instance PrimMonad (ST s) where type PrimState (ST s) = s class GMVector v a where gmbasicLength :: v s a -> Int gmbasicUnsafeSlice :: Int -> Int -> v s a -> v s a gmbasicUnsafeNew :: PrimMonad m => Int -> m (v (PrimState m) a) gmbasicUnsafeWrite :: PrimMonad m => v (PrimState m) a -> Int -> a -> m () type family GMutable (v :: * -> *) :: * -> * -> * class GMVector (GMutable v) a => GVector v a where gbasicUnsafeFreeze :: PrimMonad m => GMutable v (PrimState m) a -> m (v a) data Step s a where Yield :: a -> s -> Step s a instance Functor (Step s) where {-# INLINE fmap #-} fmap f (Yield x s) = Yield (f x) s data Stream m a = forall s. Stream (s -> m (Step s a)) s data Chunk v a = Chunk Int (forall m. (PrimMonad m, GVector v a) => GMutable v (PrimState m) a -> m ()) data New v a = New { newrun :: forall s. ST s (GMutable v s a) } type MBundle m v a = Stream m (Chunk v a) type Bundle v a = MBundle Identity v a mbfromStream :: Monad m => Stream m a -> MBundle m v a {-# INLINE mbfromStream #-} mbfromStream (Stream step t) = Stream step' t where step' s = do r <- step s return $ fmap (\x -> Chunk 1 (\v -> gmbasicUnsafeWrite v 0 x)) r mbunsafeFromList :: Monad m => [a] -> MBundle m v a {-# INLINE [1] mbunsafeFromList #-} mbunsafeFromList xs = mbfromStream (sfromList xs) blift :: Monad m => Bundle v a -> MBundle m v a {-# INLINE [1] blift #-} blift (Stream vstep t) = Stream (return . runIdentity . vstep) t sfromList :: Monad m => [a] -> Stream m a {-# INLINE sfromList #-} sfromList zs = Stream step zs where step (x:xs) = return (Yield x xs) step _ = undefined sfoldlM :: Monad m => (a -> b -> m a) -> a -> Stream m b -> m a {-# INLINE [1] sfoldlM #-} sfoldlM m w (Stream step t) = foldlM_loop w t where foldlM_loop z s = do r <- step s case r of Yield x s' -> do { z' <- m z x; foldlM_loop z' s' } gmvunstream :: (PrimMonad m, GVector v a) => Bundle v a -> m (GMutable v (PrimState m) a) {-# INLINE [1] gmvunstream #-} gmvunstream s = gmvmunstreamUnknown (blift s) gmvmunstreamUnknown :: (PrimMonad m, GVector v a) => MBundle m v a -> m (GMutable v (PrimState m) a) {-# INLINE gmvmunstreamUnknown #-} gmvmunstreamUnknown s = do v <- gmbasicUnsafeNew 0 (_, _) <- sfoldlM copyChunk (v,0) s return undefined where {-# INLINE [0] copyChunk #-} copyChunk (v,i) (Chunk n f) = do let j = i+n v' <- if gmbasicLength v < j then gmbasicUnsafeNew undefined else return v f (gmbasicUnsafeSlice i n v') return (v',j) newunstream :: GVector v a => Bundle v a -> New v a {-# INLINE [1] newunstream #-} newunstream s = s `seq` New (gmvunstream s) gnew :: GVector v a => New v a -> v a {-# INLINE [1] gnew #-} gnew m = m `seq` runST (gbasicUnsafeFreeze =<< newrun m) gunstream :: GVector v a => Bundle v a -> v a {-# INLINE gunstream #-} gunstream s = gnew (newunstream s) gfromList :: GVector v a => [a] -> v a {-# INLINE gfromList #-} gfromList = gunstream . mbunsafeFromList greadPrec :: (GVector v a, Read a) => ReadPrec (v a) {-# INLINE greadPrec #-} greadPrec = do xs <- readPrec return (gfromList xs) ----- data MVector :: (* -> * -> *) -> (* -> * -> *) -> * -> * -> * where MV :: !(u s a) -> !(v s b) -> MVector u v s (a, b) instance (GMVector u a, GMVector v b) => GMVector (MVector u v) (a, b) where gmbasicLength (MV ks _) = gmbasicLength ks gmbasicUnsafeSlice s e (MV ks vs) = MV (gmbasicUnsafeSlice s e ks) (gmbasicUnsafeSlice s e vs) gmbasicUnsafeNew n = liftM2 MV (gmbasicUnsafeNew n) (gmbasicUnsafeNew n) -- Removing this INLINE pragma makes it compile {-# INLINE gmbasicUnsafeNew #-} gmbasicUnsafeWrite (MV ks vs) n (k,v) = do gmbasicUnsafeWrite ks n k gmbasicUnsafeWrite vs n v data Vector :: (* -> *) -> (* -> *) -> * -> * type instance GMutable (Vector u v) = MVector (GMutable u) (GMutable v) instance (GVector u a, GVector v b) => GVector (Vector u v) (a, b) where gbasicUnsafeFreeze = undefined instance (GVector u a, GVector v b, Read a, Read b, c ~ (a, b)) => Read (Vector u v c) where readPrec = greadPrec
comment:5 Changed 2 years ago by
Version: | 8.1 → 8.0.1 |
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Oh dear, there's actually a distinction between the attached program and the program in comment:4. The attached program compiles without issue on GHC 8.0.2 and earlier, but panics on GHC HEAD (post–join points). The program in comment:4, however, compiles without issue on GHC 7.10.3 and earlier, but panics on GHC 8.0.1, 8.0.2, and HEAD.
This isn't terribly surprising, as others have reported seeing this error on earlier versions of GHC, but AFAICT, no one was able to minimize the panic down to a single file until now. So the issue likely predates join points, but join points have made it easier to trigger in GHC HEAD.
I'll see if I can figure out what commit lead to the panic surfacing in the program in comment:4.
comment:6 Changed 2 years ago by
Huh, the commit which caused the regression in comment:4 is 6746549772c5cc0ac66c0fce562f297f4d4b80a2 (Add kind equalities to GHC). What this means, I have no idea.
comment:7 Changed 2 years ago by
Keywords: | SpecConstr added |
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comment:8 Changed 2 years ago by
Cc: | goldfire added |
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comment:9 Changed 2 years ago by
Owner: | set to simonpj |
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I know exactly what is happening here. Just starved of time to fix it -- and next week looks little better.
In brief: TyCoRep.substCoVarBndr
is replacing occurrences of the binder with Refl
in the LHS of a rule. Solution: mkCoVarCo
should not replace varaibles with Refl
; but optCoercion
should do so instead.
comment:11 Changed 2 years ago by
Status: | new → merge |
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Test Case: | → simplCore/should_compile/T13410 |
OK, done!
comment:12 Changed 2 years ago by
Resolution: | → fixed |
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Status: | merge → closed |
Merged to ghc-8.2
as 7087abfe2ba944d6609eee399c5ad2ca44cd388c.
Commit 8d5cf8bf584fd4849917c29d82dcf46ee75dd035 (Join points) introduced this regression.