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| author | Sebastian Graf <sebastian.graf@kit.edu> | 2022-02-21 18:30:41 +0100 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2022-03-11 19:59:24 -0500 |
| commit | 524795feabf3a977d074f40b482ed7d8893f8710 (patch) | |
| tree | 07e21c55f4d18ee0077e016149314331fa13d995 | |
| parent | a6d8facbbd0cb87374e2afd8a0b908e5cbec399d (diff) | |
| download | haskell-524795feabf3a977d074f40b482ed7d8893f8710.tar.gz | |
Demand: Document why we need three additional equations of multSubDmd
| -rw-r--r-- | compiler/GHC/Types/Demand.hs | 37 |
1 files changed, 20 insertions, 17 deletions
diff --git a/compiler/GHC/Types/Demand.hs b/compiler/GHC/Types/Demand.hs index 0cfb0dae29..a27667c21e 100644 --- a/compiler/GHC/Types/Demand.hs +++ b/compiler/GHC/Types/Demand.hs @@ -820,6 +820,26 @@ lubSubDmd _ _ = topSubDmd lubDmd :: Demand -> Demand -> Demand lubDmd (n1 :* sd1) (n2 :* sd2) = lubCard n1 n2 :* lubSubDmd sd1 sd2 +multSubDmd :: Card -> SubDemand -> SubDemand +multSubDmd C_11 sd = sd +-- The following three equations don't have an impact on Demands, only on +-- Boxity. They are needed so that we don't trigger the assertions in `:*` +-- when called from `multDmd`. +multSubDmd C_00 _ = seqSubDmd -- Otherwise `multSubDmd A L == A /= !A` +multSubDmd C_10 (Poly _ n) = if isStrict n then botSubDmd else seqSubDmd -- Otherwise `multSubDmd B L == B /= !B` +multSubDmd C_10 (Call n _) = if isStrict n then botSubDmd else seqSubDmd -- Otherwise we'd call `mkCall` with absent cardinality +multSubDmd n (Poly b m) = Poly b (multCard n m) +multSubDmd n (Call n' sd) = mkCall (multCard n n') sd -- See Note [Call demands are relative] +multSubDmd n (Prod b ds) = mkProd b (strictMap (multDmd n) ds) + +multDmd :: Card -> Demand -> Demand +-- The first two lines compute the same result as the last line, but won't +-- trigger the assertion in `:*` for input like `multDmd B 1L`, which would call +-- `B :* A`. We want to return `B` in these cases. +multDmd C_10 (n :* _) = if isStrict n then BotDmd else AbsDmd +multDmd n (C_10 :* _) = if isStrict n then BotDmd else AbsDmd +multDmd n (m :* sd) = multCard n m :* multSubDmd n sd + -- | Denotes '+' on 'SubDemand'. plusSubDmd :: SubDemand -> SubDemand -> SubDemand -- Handle seqSubDmd (just an optimisation, the general case would do the same) @@ -849,23 +869,6 @@ plusSubDmd _ _ = topSubDmd plusDmd :: Demand -> Demand -> Demand plusDmd (n1 :* sd1) (n2 :* sd2) = plusCard n1 n2 :* plusSubDmd sd1 sd2 -multSubDmd :: Card -> SubDemand -> SubDemand -multSubDmd C_11 sd = sd -multSubDmd C_00 _ = seqSubDmd -multSubDmd C_10 (Poly _ n) = if isStrict n then botSubDmd else seqSubDmd -multSubDmd C_10 (Call n _) = if isStrict n then botSubDmd else seqSubDmd -multSubDmd n (Poly b m) = Poly b (multCard n m) -multSubDmd n (Call n' sd) = mkCall (multCard n n') sd -- See Note [Call demands are relative] -multSubDmd n (Prod b ds) = mkProd b (strictMap (multDmd n) ds) - -multDmd :: Card -> Demand -> Demand --- The first two lines compute the same result as the last line, but won't --- trigger the assertion in `:*` for input like `multDmd B 1L`, which would call --- `B :* A`. We want to return `B` in these cases. -multDmd C_10 (n :* _) = if isStrict n then BotDmd else AbsDmd -multDmd n (C_10 :* _) = if isStrict n then BotDmd else AbsDmd -multDmd n (m :* sd) = multCard n m :* multSubDmd n sd - -- | Used to suppress pretty-printing of an uninformative demand isTopDmd :: Demand -> Bool isTopDmd dmd = dmd == topDmd |