diff options
| -rw-r--r-- | compiler/simplCore/FloatIn.lhs | 40 |
1 files changed, 26 insertions, 14 deletions
diff --git a/compiler/simplCore/FloatIn.lhs b/compiler/simplCore/FloatIn.lhs index 0dd318b420..8dbec27bf5 100644 --- a/compiler/simplCore/FloatIn.lhs +++ b/compiler/simplCore/FloatIn.lhs @@ -157,8 +157,8 @@ fiExpr to_drop (_,AnnApp fun arg) [drop_here, fun_drop, arg_drop] = sepBindsByDropPoint False [freeVarsOf fun, freeVarsOf arg] to_drop \end{code} -We are careful about lambdas: - +Note [Floating in past a lambda group] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * We must be careful about floating inside inside a value lambda. That risks losing laziness. The float-out pass might rescue us, but then again it might not. @@ -174,24 +174,30 @@ We are careful about lambdas: This is bad as now f is an updatable closure (update PAP) and has arity 0. +* Hack alert! We only float in through one-shot lambdas, + not (as you might guess) through lone big lambdas. + Reason: we float *out* past big lambdas (see the test in the Lam + case of FloatOut.floatExpr) and we don't want to float straight + back in again. + + It *is* important to float into one-shot lambdas, however; + see the remarks with noFloatIntoRhs. + So we treat lambda in groups, using the following rule: - Float inside a group of lambdas only if - they are all either type lambdas or one-shot lambdas. + Float in if (a) there is at least one Id, + and (b) there are no non-one-shot Ids + + Otherwise drop all the bindings outside the group. - Otherwise drop all the bindings outside the group. +This is what the 'go' function in the AnnLam case is doing. + +Urk! if all are tyvars, and we don't float in, we may miss an + opportunity to float inside a nested case branch \begin{code} - -- Hack alert! We only float in through one-shot lambdas, - -- not (as you might guess) through big lambdas. - -- Reason: we float *out* past big lambdas (see the test in the Lam - -- case of FloatOut.floatExpr) and we don't want to float straight - -- back in again. - -- - -- It *is* important to float into one-shot lambdas, however; - -- see the remarks with noFloatIntoRhs. fiExpr to_drop lam@(_, AnnLam _ _) - | all is_one_shot bndrs -- Float in + | go False bndrs -- Float in = mkLams bndrs (fiExpr to_drop body) | otherwise -- Dump it all here @@ -199,6 +205,12 @@ fiExpr to_drop lam@(_, AnnLam _ _) where (bndrs, body) = collectAnnBndrs lam + + go seen_one_shot_id [] = seen_one_shot_id + go seen_one_shot_id (b:bs) + | isTyVar b = go seen_one_shot_id bs + | isOneShotBndr b = go True bs + | otherwise = False -- Give up at a non-one-shot Id \end{code} We don't float lets inwards past an SCC. |