Fix #1142.

CHANGELOG.md

@@ -28,6 +28,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
   * Fix in monomorphisation of types with constant sizes.
 
+  * Fix in in-place loweing (#1142).
+
 ## [0.17.2]
 
 ### Added

src/Futhark/IR/Prop/Aliases.hs

@@ -15,6 +15,7 @@ module Futhark.IR.Prop.Aliases
   ( subExpAliases,
     expAliases,
     patternAliases,
+    lookupAliases,
     Aliased (..),
     AliasesOf (..),
 
@@ -31,7 +32,8 @@ where
 
 import Control.Arrow (first)
 import qualified Data.Kind
-import Futhark.IR.Prop (IsOp)
+import qualified Data.Map as M
+import Futhark.IR.Prop (IsOp, NameInfo (..), Scope)
 import Futhark.IR.Prop.Names
 import Futhark.IR.Prop.Patterns
 import Futhark.IR.Prop.Types
@@ -169,6 +171,13 @@ instance AliasesOf Names where
 instance AliasesOf dec => AliasesOf (PatElemT dec) where
   aliasesOf = aliasesOf . patElemDec
 
+-- | Also includes the name itself.
+lookupAliases :: AliasesOf (LetDec lore) => VName -> Scope lore -> Names
+lookupAliases v scope =
+  case M.lookup v scope of
+    Just (LetName dec) -> oneName v <> aliasesOf dec
+    _ -> oneName v
+
 -- | The class of operations that can produce aliasing and consumption
 -- information.
 class IsOp op => AliasedOp op where

src/Futhark/Optimise/InPlaceLowering.hs

@@ -13,10 +13,9 @@
 -- @
 --   let r =
 --     loop (r1 = r0) = for i < n do
---       let a = r1[i] in
---       let r1[i] = a * i in
---       r1
---       in
+--       let a = r1[i]
+--       let r1[i] = a * i
+--       in r1
 --   ...
 --   let x = y with [k] <- r in
 --   ...
@@ -27,11 +26,10 @@
 -- @
 --   let x0 = y with [k] <- r0
 --   loop (x = x0) = for i < n do
---     let a = a[k,i] in
---     let x[k,i] = a * i in
---     x
---     in
---   let r = x[y] in
+--     let a = a[k,i]
+--     let x[k,i] = a * i
+--     in x
+--   let r = x[k] in
 --   ...
 -- @
 --

src/Futhark/Optimise/InPlaceLowering/LowerIntoStm.hs

@@ -13,7 +13,6 @@ import Control.Monad
 import Control.Monad.Writer
 import Data.Either
 import Data.List (find, unzip4)
-import qualified Data.Map as M
 import Data.Maybe (mapMaybe)
 import Futhark.Analysis.PrimExp.Convert
 import Futhark.Construct
@@ -205,7 +204,7 @@ lowerUpdateIntoLoop scope updates pat ctx val form body = do
   forM_ (zip val $ bodyAliases body) $ \((p, _), als) ->
     guard $ not $ paramName p `nameIn` als
 
-  mk_in_place_map <- summariseLoop updates usedInBody resmap val
+  mk_in_place_map <- summariseLoop scope updates usedInBody resmap val
 
   Just $ do
     in_place_map <- mk_in_place_map
@@ -217,10 +216,8 @@ lowerUpdateIntoLoop scope updates pat ctx val form body = do
     let body' = mkBody (newbnds <> res_bnds) body_res
     return (prebnds, postbnds, ctxpat, valpat, ctx, val', body')
   where
-    usedInBody = mconcat $ map expandAliases $ namesToList $ freeIn body <> freeIn form
-    expandAliases v = case M.lookup v scope of
-      Just (LetName dec) -> oneName v <> aliasesOf dec
-      _ -> oneName v
+    usedInBody =
+      mconcat $ map (`lookupAliases` scope) $ namesToList $ freeIn body <> freeIn form
     resmap = zip (bodyResult body) $ patternValueIdents pat
 
     mkMerges ::
@@ -277,18 +274,22 @@ lowerUpdateIntoLoop scope updates pat ctx val form body = do
         Left (inPatternAs summary)
 
 summariseLoop ::
-  MonadFreshNames m =>
+  ( Aliased lore,
+    MonadFreshNames m
+  ) =>
+  Scope lore ->
   [DesiredUpdate (als, Type)] ->
   Names ->
   [(SubExp, Ident)] ->
   [(Param DeclType, SubExp)] ->
   Maybe (m [LoopResultSummary (als, Type)])
-summariseLoop updates usedInBody resmap merge =
+summariseLoop scope updates usedInBody resmap merge =
   sequence <$> zipWithM summariseLoopResult resmap merge
   where
     summariseLoopResult (se, v) (fparam, mergeinit)
       | Just update <- find (updateHasValue $ identName v) updates =
-        if updateSource update `nameIn` usedInBody
+        -- Safety condition (7)
+        if usedInBody `namesIntersect` lookupAliases (updateSource update) scope
           then Nothing
           else
             if hasLoopInvariantShape fparam

src/Futhark/TypeCheck.hs

@@ -60,7 +60,7 @@ import Data.Maybe
 import qualified Data.Set as S
 import Futhark.Analysis.PrimExp
 import Futhark.Construct (instantiateShapes)
-import Futhark.IR.Aliases
+import Futhark.IR.Aliases hiding (lookupAliases)
 import Futhark.Util
 import Futhark.Util.Pretty (Pretty, align, indent, ppr, prettyDoc, text, (<+>))
 

tests/issue1142.fut

@@ -0,0 +1,7 @@
+let main [n][m] (xsss: *[2][n][m]i32) =
+  #[unsafe]
+  let xss = xsss[0]
+  let ys = loop acc = replicate m 0 for i < m do
+           let acc[i] = xsss[0,0,i]+1
+           in acc
+  in xss with [0] = ys