diff options
| -rw-r--r-- | compiler/simplCore/Exitify.hs | 23 |
1 files changed, 19 insertions, 4 deletions
diff --git a/compiler/simplCore/Exitify.hs b/compiler/simplCore/Exitify.hs index a8f02ae461..f67d4bd314 100644 --- a/compiler/simplCore/Exitify.hs +++ b/compiler/simplCore/Exitify.hs @@ -178,9 +178,12 @@ exitifyRec in_scope pairs -- See Note [Interesting expression] is_interesting = anyVarSet isLocalId (fvs `minusVarSet` mkVarSet captured) - -- The possible arguments of this exit join point - -- No need for `sortQuantVars`, `captured` is already in dependency order - abs_vars = map zap $ filter (`elemVarSet` fvs) captured + -- The arguments of this exit join point + -- See Note [Picking arguments to abstract over] + abs_vars = snd $ foldr pick (fvs, []) captured + where + pick v (fvs', acc) | v `elemVarSet` fvs' = (fvs' `delVarSet` v, zap v : acc) + | otherwise = (fvs', acc) -- We are going to abstract over these variables, so we must -- zap any IdInfo they have; see Trac #15005 @@ -261,7 +264,6 @@ addExit in_scope ty join_arity rhs = do put ((v,rhs):fs) return v - {- Note [Interesting expression] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -468,4 +470,17 @@ Positions C and D have their advantages: C decreases allocations in simpl, but D Assuming we have a budget of _one_ run of Exitification, then C wins (but we could get more from running it multiple times, as seen in fish). +Note [Picking arguments to abstract over] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +When we create an exit join point, so we need to abstract over those of its +free variables that are be out-of-scope at the destination of the exit join +point. So we go through the list `captured` and pick those that are actually +free variables of the join point. + +We do not just `filter (`elemVarSet` fvs) captured`, as there might be +shadowing, and `captured` may contain multiple variables with the same Unique. I +these cases we want to abstract only over the last occurence, hence the `foldr` +(with emphasis on the `r`). This is #15110. + -} |