diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/Arity.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/Arity.hs | 33 |
1 files changed, 11 insertions, 22 deletions
diff --git a/compiler/GHC/Core/Opt/Arity.hs b/compiler/GHC/Core/Opt/Arity.hs index cfa0ad93c3..795d256917 100644 --- a/compiler/GHC/Core/Opt/Arity.hs +++ b/compiler/GHC/Core/Opt/Arity.hs @@ -776,12 +776,12 @@ andArityType :: ArityType -> ArityType -> ArityType andArityType (AT (os1:oss1) div1) (AT (os2:oss2) div2) | AT oss' div' <- andArityType (AT oss1 div1) (AT oss2 div2) = AT ((os1 `bestOneShot` os2) : oss') div' -- See Note [Combining case branches] -andArityType (AT [] div1) at2 +andArityType at1@(AT [] div1) at2 | isDeadEndDiv div1 = at2 -- Note [ABot branches: max arity wins] - | otherwise = takeWhileOneShot at2 -- See Note [Combining case branches] -andArityType at1 (AT [] div2) + | otherwise = at1 -- See Note [Combining case branches] +andArityType at1 at2@(AT [] div2) | isDeadEndDiv div2 = at1 -- Note [ABot branches: max arity wins] - | otherwise = takeWhileOneShot at1 -- See Note [Combining case branches] + | otherwise = at2 -- See Note [Combining case branches] {- Note [ABot branches: max arity wins] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -794,24 +794,13 @@ So we need \??.⊥ for the whole thing, the /max/ of both arities. Note [Combining case branches] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Consider - go = \x. let z = go e0 - go2 = \x. case x of - True -> z - False -> \s(one-shot). e1 - in go2 x -We *really* want to respect the one-shot annotation provided by the -user and eta-expand go and go2. -When combining the branches of the case we have - T `andAT` \1.T -and we want to get \1.T. -But if the inner lambda wasn't one-shot (\?.T) we don't want to do this. -(We need a usage analysis to justify that.) - -So we combine the best of the two branches, on the (slightly dodgy) -basis that if we know one branch is one-shot, then they all must be. -Surprisingly, this means that the one-shot arity type is effectively the top -element of the lattice. + +Unless we can conclude that **all** branches are safe to eta-expand then we +must pessimisticaly conclude that we can't eta-expand. See #21694 for where this +went wrong. +We can do better in the long run, but for the 9.4/9.2 branches we choose to simply +ignore oneshot annotations for the time being. + Note [Arity trimming] ~~~~~~~~~~~~~~~~~~~~~ |