X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Fcmm%2FDFMonad.hs;h=4c254e68aad251d279738f44cd5dcf247798610e;hb=5d52d9b64c21dcf77849866584744722f8121389;hp=0cf1ead0fc9b7673a71c505bac2aace0bf650783;hpb=5dc8b425443200a5160b9d1399aca1808bfcffee;p=ghc-hetmet.git

diff --git a/compiler/cmm/DFMonad.hs b/compiler/cmm/DFMonad.hs
index 0cf1ead..4c254e6 100644
--- a/compiler/cmm/DFMonad.hs
+++ b/compiler/cmm/DFMonad.hs
@@ -16,7 +16,6 @@ import CmmTx
 import PprCmm()
 import OptimizationFuel
 
-import Control.Monad
 import Maybes
 import Outputable
 import UniqSupply
@@ -167,8 +166,7 @@ instance Monad m => DataflowAnalysis (DFM' m) where
                                     text "changed from", nest 4 (ppr old_a), text "to",
                                     nest 4 (ppr new),
                                     text "after supposedly reaching fixed point;",
-                                    text "env is", pprFacts facts]) 
-                  ; setFact id a }
+                                    text "env is", pprFacts facts]) }
          }
     where pprFacts env = vcat (map pprFact (blockEnvToList env))
           pprFact (id, a) = hang (ppr id <> colon) 4 (ppr a)
@@ -192,6 +190,10 @@ instance Monad m => Monad (DFM' m f) where
   DFM' f >>= k = DFM' (\l s -> do (a, s') <- f l s
                                   s' `seq` case k a of DFM' f' -> f' l s')
   return a = DFM' (\_ s -> return (a, s))
+ -- The `seq` is essential to ensure that entire passes of the dataflow engine 
+ -- aren't postponed in a thunk. By making the sequence strict in the state,
+ -- we ensure that each action in the monad is executed immediately, preventing
+ -- stack overflows that previously occurred when finally forcing the old state thunks.
 
 instance FuelUsingMonad (DFM' FuelMonad f) where
   fuelRemaining = liftToDFM' fuelRemaining