Significant compilation time regression between 8.4 and HEAD due to coverage checking
Consider the following program:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
module Bug where
data family Sing (z :: k)
class SEq k where
(%==) :: forall (a :: k) (b :: k). Sing a -> Sing b -> ()
infix 4 %==
data Foo a b c d
= A a b c d |
B a b c d |
C a b c d |
D a b c d |
E a b c d |
F a b c d
data instance Sing (z_awDE :: Foo a b c d) where
SA :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('A a b c d)
SB :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('B a b c d)
SC :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('C a b c d)
SD :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('D a b c d)
SE :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('E a b c d)
SF :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('F a b c d)
$([d| instance (SEq a, SEq b, SEq c, SEq d) => SEq (Foo a b c d) where
(%==) (SA _ _ _ _) (SA _ _ _ _) = ()
(%==) (SA _ _ _ _) (SB _ _ _ _) = ()
(%==) (SA _ _ _ _) (SC _ _ _ _) = ()
(%==) (SA _ _ _ _) (SD _ _ _ _) = ()
(%==) (SA _ _ _ _) (SE _ _ _ _) = ()
(%==) (SA _ _ _ _) (SF _ _ _ _) = ()
(%==) (SB _ _ _ _) (SA _ _ _ _) = ()
(%==) (SB _ _ _ _) (SB _ _ _ _) = ()
(%==) (SB _ _ _ _) (SC _ _ _ _) = ()
(%==) (SB _ _ _ _) (SD _ _ _ _) = ()
(%==) (SB _ _ _ _) (SE _ _ _ _) = ()
(%==) (SB _ _ _ _) (SF _ _ _ _) = ()
(%==) (SC _ _ _ _) (SA _ _ _ _) = ()
(%==) (SC _ _ _ _) (SB _ _ _ _) = ()
(%==) (SC _ _ _ _) (SC _ _ _ _) = ()
(%==) (SC _ _ _ _) (SD _ _ _ _) = ()
(%==) (SC _ _ _ _) (SE _ _ _ _) = ()
(%==) (SC _ _ _ _) (SF _ _ _ _) = ()
(%==) (SD _ _ _ _) (SA _ _ _ _) = ()
(%==) (SD _ _ _ _) (SB _ _ _ _) = ()
(%==) (SD _ _ _ _) (SC _ _ _ _) = ()
(%==) (SD _ _ _ _) (SD _ _ _ _) = ()
(%==) (SD _ _ _ _) (SE _ _ _ _) = ()
(%==) (SD _ _ _ _) (SF _ _ _ _) = ()
(%==) (SE _ _ _ _) (SA _ _ _ _) = ()
(%==) (SE _ _ _ _) (SB _ _ _ _) = ()
(%==) (SE _ _ _ _) (SC _ _ _ _) = ()
(%==) (SE _ _ _ _) (SD _ _ _ _) = ()
(%==) (SE _ _ _ _) (SE _ _ _ _) = ()
(%==) (SE _ _ _ _) (SF _ _ _ _) = ()
(%==) (SF _ _ _ _) (SA _ _ _ _) = ()
(%==) (SF _ _ _ _) (SB _ _ _ _) = ()
(%==) (SF _ _ _ _) (SC _ _ _ _) = ()
(%==) (SF _ _ _ _) (SD _ _ _ _) = ()
(%==) (SF _ _ _ _) (SE _ _ _ _) = ()
(%==) (SF _ _ _ _) (SF _ _ _ _) = () |])
It takes significantly longer to compile this program on 8.4 and HEAD:
$ /opt/ghc/8.4.1/bin/ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.4.1
$ time /opt/ghc/8.4.1/bin/ghc Bug.hs -fforce-recomp
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
real 0m0.285s
user 0m0.236s
sys 0m0.036s
$ /opt/ghc/head/bin/ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.5.20180306
$ time /opt/ghc/head/bin/ghc Bug.hs -fforce-recomp
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
real 0m29.684s
user 0m29.656s
sys 0m0.060s
The reason for this regression is somewhat incidental—it's due to commit ffb2738f (Fix #14838 by marking TH-spliced code as FromSource
). Before that commit, we were supressing pattern-match coverage checking entirely on TH-quoted code. We no longer do this, which means that we coverage-check the TH-quoted instance in that program, which appears to be why it takes so long to compile.
This is a serious issue in practice because a good chunk of singletons
-generated code is of this form, which means that a good amount of code is effectively uncompilable on GHC HEAD now. (See, for instance, this Travis failure on GHC HEAD.)
Trac metadata
Trac field | Value |
---|---|
Version | 8.5 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | highest |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |