diff options
Diffstat (limited to 'compiler/GHC/CoreToStg.hs')
| -rw-r--r-- | compiler/GHC/CoreToStg.hs | 28 |
1 files changed, 25 insertions, 3 deletions
diff --git a/compiler/GHC/CoreToStg.hs b/compiler/GHC/CoreToStg.hs index ead7a12da0..a450f342b0 100644 --- a/compiler/GHC/CoreToStg.hs +++ b/compiler/GHC/CoreToStg.hs @@ -198,6 +198,26 @@ import Control.Monad (ap) -- do we set CCCS from it; so we just slam in -- dontCareCostCentre. +-- Note [Coercion tokens] +-- ~~~~~~~~~~~~~~~~~~~~~~ +-- In coreToStgArgs, we drop type arguments completely, but we replace +-- coercions with a special coercionToken# placeholder. Why? Consider: +-- +-- f :: forall a. Int ~# Bool -> a +-- f = /\a. \(co :: Int ~# Bool) -> error "impossible" +-- +-- If we erased the coercion argument completely, we’d end up with just +-- f = error "impossible", but then f `seq` () would be ⊥! +-- +-- This is an artificial example, but back in the day we *did* treat +-- coercion lambdas like type lambdas, and we had bug reports as a +-- result. So now we treat coercion lambdas like value lambdas, but we +-- treat coercions themselves as zero-width arguments — coercionToken# +-- has representation VoidRep — which gets the best of both worlds. +-- +-- (For the gory details, see also the (unpublished) paper, “Practical +-- aspects of evidence-based compilation in System FC.”) + -- -------------------------------------------------------------- -- Setting variable info: top-level, binds, RHSs -- -------------------------------------------------------------- @@ -357,8 +377,10 @@ coreToStgExpr (App (Lit LitRubbish) _some_unlifted_type) -- We lower 'LitRubbish' to @()@ here, which is much easier than doing it in -- a STG to Cmm pass. = coreToStgExpr (Var unitDataConId) -coreToStgExpr (Var v) = coreToStgApp v [] [] -coreToStgExpr (Coercion _) = coreToStgApp coercionTokenId [] [] +coreToStgExpr (Var v) = coreToStgApp v [] [] +coreToStgExpr (Coercion _) + -- See Note [Coercion tokens] + = coreToStgApp coercionTokenId [] [] coreToStgExpr expr@(App _ _) = coreToStgApp f args ticks @@ -554,7 +576,7 @@ coreToStgArgs (Type _ : args) = do -- Type argument (args', ts) <- coreToStgArgs args return (args', ts) -coreToStgArgs (Coercion _ : args) -- Coercion argument; replace with place holder +coreToStgArgs (Coercion _ : args) -- Coercion argument; See Note [Coercion tokens] = do { (args', ts) <- coreToStgArgs args ; return (StgVarArg coercionTokenId : args', ts) } |