Add support for memoized streams to the term model evaluator.

Although it is correct to treat streams according to their
SAWCore datatype definition, the naive implementation as a function
looses sharing that users expect given the behavior of Cryptol
streams.  This patch makes sure to memoize concrete evaluations
of stream values, and blocks evaluation of stream lookups for
values that are not concrete.

This significantly improves the performance of the kinds of
stream programs that arise from Cryptol.

saw-core/src/Verifier/SAW/Simulator/TermModel.hs

@@ -25,6 +25,9 @@ module Verifier.SAW.Simulator.TermModel
 import Control.Monad
 import Control.Monad.Fix
 import Control.Monad.IO.Class
+import Control.Monad.Trans
+import Control.Monad.Trans.Except
+
 import Data.IORef
 import Data.Maybe (fromMaybe)
 import qualified Data.Vector as V
@@ -34,8 +37,10 @@ import Data.Set (Set)
 import qualified Data.Set as Set
 import Numeric.Natural
 
+
 import Verifier.SAW.Prim (BitVector(..))
 import qualified Verifier.SAW.Prim as Prim
+import Verifier.SAW.Prelude.Constants
 import qualified Verifier.SAW.Simulator as Sim
 import Verifier.SAW.Simulator.Value
 import qualified Verifier.SAW.Simulator.Prims as Prims
@@ -188,11 +193,17 @@ type instance Extra  TermModel = VExtra
 
 data VExtra
   = VExtraTerm
-       (TValue TermModel) -- type of the term
-       Term               -- term value (closed term!)
+       !(TValue TermModel) -- type of the term
+       !Term               -- term value (closed term!)
+  | VExtraStream
+       !(TValue TermModel) -- type of the stream elements
+       !(Thunk TermModel -> MValue TermModel) -- function to compute stream values
+       !(IORef (Map Natural (Value TermModel))) -- cache of concrete values
+       !(Lazy IO Term) -- stream value as a term (closed term!)
 
 instance Show VExtra where
   show (VExtraTerm ty tm) = "<extra> " ++ showTerm tm ++ " : " ++ show ty
+  show (VExtraStream ty _ _ _) = "<stream of " ++ show ty ++ ">"
 
 data TermModelArray =
   TMArray
@@ -376,6 +387,7 @@ readBackValue sc cfg = loop
     loop _ (TValue tv) = readBackTValue sc cfg tv
 
     loop _ (VExtra (VExtraTerm _tp tm)) = return tm
+    loop _ (VExtra (VExtraStream _tp _fn _ref tm)) = liftIO (force tm)
 
     loop tv@VPiType{} v@VFun{} =
       do (ecs, tm) <- readBackFuns tv v
@@ -460,9 +472,6 @@ intTerm :: SharedContext -> VInt TermModel -> IO Term
 intTerm _ (Left tm) = pure tm
 intTerm sc (Right i) = scIntegerConst sc i
 
-extraTerm :: VExtra -> IO Term
-extraTerm (VExtraTerm _ tm) = pure tm
-
 unOp ::
   SharedContext ->
   (SharedContext -> t -> IO t') ->
@@ -663,8 +672,8 @@ prims sc cfg =
        case c of
          Right b -> if b then pure x else pure y
          Left tm ->
-           do x' <- extraTerm x
-              y' <- extraTerm y
+           do x' <- readBackValue sc cfg tp (VExtra x)
+              y' <- readBackValue sc cfg tp (VExtra y)
               a  <- readBackTValue sc cfg tp
               VExtraTerm tp <$> scIte sc a tm x' y'
 
@@ -892,6 +901,10 @@ constMap sc cfg = Map.union (Map.fromList localPrims) (Prims.constMap pms)
     , ("Prelude.intModMul" , intModMulOp sc)
     , ("Prelude.intModNeg" , intModNegOp sc)
 
+    -- Streams
+    , ("Prelude.MkStream", mkStreamOp sc cfg)
+    , ("Prelude.streamGet", streamGetOp)
+
     -- Miscellaneous
     , ("Prelude.expByNat", Prims.expByNatOp pms)
     ]
@@ -1008,7 +1021,7 @@ bvShiftOp sc cfg szf tmOp bvOp =
         do let n = szf n0
            n0'  <- scNat sc n0
            w'   <- readBackValue sc cfg (VVecType n VBoolType) w
-           dt   <- scRequireDataType sc "Prelude.Nat"
+           dt   <- scRequireDataType sc preludeNatIdent
            pn   <- traverse (evalType cfg) (dtPrimName dt)
            amt' <- readBackValue sc cfg (VDataType pn [] []) amt
            tm   <- tmOp sc n0' w' amt'
@@ -1104,3 +1117,41 @@ intModBinOp sc termOp valOp n = binOp sc toTerm termOp' valOp
     termOp' _ x y =
       do n' <- scNat sc n
          termOp sc n' x y
+
+-- MkStream :: (a :: sort 0) -> (Nat -> a) -> Stream a;
+mkStreamOp :: (?recordEC :: BoundECRecorder) =>
+  SharedContext -> Sim.SimulatorConfig TermModel -> TmPrim
+mkStreamOp sc cfg =
+  Prims.tvalFun $ \ty ->
+  Prims.strictFun $ \f ->
+  Prims.PrimExcept $
+    case f of
+      VFun nm fn ->
+        do ref <- liftIO (newIORef mempty)
+           stm <- liftIO $ delay $ do
+                     natDT <- scRequireDataType sc preludeNatIdent
+                     natPN <- traverse (evalType cfg) (dtPrimName natDT)
+                     ty' <- readBackTValue sc cfg ty
+                     ftm <- readBackValue sc cfg (VPiType nm (VDataType natPN [] []) (VNondependentPi ty)) f
+                     scCtorApp sc (mkIdent preludeModuleName "MkStream") [ty',ftm]
+           return (VExtra (VExtraStream ty fn ref stm))
+
+      _ -> throwE "expected function value"
+
+-- streamGet :: (a :: sort 0) -> Stream a -> Nat -> a;
+streamGetOp :: TmPrim
+streamGetOp =
+  Prims.tvalFun   $ \_ty ->
+  Prims.strictFun $ \xs ->
+  Prims.natFun $ \ix ->
+  Prims.PrimExcept $
+    case xs of
+      VExtra (VExtraStream _ fn ref _tm) ->
+        liftIO (Map.lookup ix <$> readIORef ref) >>= \case
+          Just v  -> return v
+          Nothing -> lift $
+            do v <- fn (ready (VNat ix))
+               liftIO (atomicModifyIORef' ref (\m' -> (Map.insert ix v m', ())))
+               return v
+
+      _ -> throwE "expected stream value"