Remove in-place lowering. (#2055)

This is one of our oldest optimisations, and mainly served to clean up
the code produced by sequentialisation, as well as a few other simple
things that we would today call "memory short circuiting".

In-place lowering is a really complicated optimisation, so now that we
have a propert short circuiting pass, we can get rid of it.
Short circuiting is even more complicated, but it
is also properly principled, and more general.

We still largely have the sequentialisation problems that in-place
lowering was supposed to address (mostly nested maps), but at least on
the GPU backends there seems to be no real impact from it anymore. I
would still like to one day fix sequentialisation so that it produces
better code for nested SOACs.

futhark.cabal

@@ -313,9 +313,6 @@ library
       Futhark.Optimise.Fusion.TryFusion
       Futhark.Optimise.GenRedOpt
       Futhark.Optimise.HistAccs
-      Futhark.Optimise.InPlaceLowering
-      Futhark.Optimise.InPlaceLowering.LowerIntoStm
-      Futhark.Optimise.InPlaceLowering.SubstituteIndices
       Futhark.Optimise.InliningDeadFun
       Futhark.Optimise.MemoryBlockMerging
       Futhark.Optimise.MemoryBlockMerging.GreedyColoring

src/Futhark/CLI/Dev.hs

@@ -35,7 +35,6 @@ import Futhark.Optimise.CSE
 import Futhark.Optimise.DoubleBuffer
 import Futhark.Optimise.Fusion
 import Futhark.Optimise.HistAccs
-import Futhark.Optimise.InPlaceLowering
 import Futhark.Optimise.InliningDeadFun
 import Futhark.Optimise.MemoryBlockMerging qualified as MemoryBlockMerging
 import Futhark.Optimise.ReduceDeviceSyncs (reduceDeviceSyncs)
@@ -330,23 +329,6 @@ allocateOption short =
     long = [passLongOption pass]
     pass = Seq.explicitAllocations
 
-iplOption :: String -> FutharkOption
-iplOption short =
-  passOption (passDescription pass) (UntypedPass perform) short long
-  where
-    perform (GPU prog) config =
-      GPU
-        <$> runPipeline (onePass inPlaceLoweringGPU) config prog
-    perform (Seq prog) config =
-      Seq
-        <$> runPipeline (onePass inPlaceLoweringSeq) config prog
-    perform s _ =
-      externalErrorS $
-        "Pass '" ++ passDescription pass ++ "' cannot operate on " ++ representation s
-
-    long = [passLongOption pass]
-    pass = inPlaceLoweringSeq
-
 cseOption :: String -> FutharkOption
 cseOption short =
   passOption (passDescription pass) (UntypedPass perform) short long
@@ -616,7 +598,6 @@ commandLineOptions =
     kernelsPassOption reduceDeviceSyncs [],
     typedPassOption soacsProg GPU extractKernels [],
     typedPassOption soacsProg MC extractMulticore [],
-    iplOption [],
     allocateOption "a",
     kernelsMemPassOption doubleBufferGPU [],
     mcMemPassOption doubleBufferMC [],