Fix inc. ranges with start == end and negative step (2) (#2195)

This changes behavior of inclusive ranges for the case where
`start == end` and `start != second`, for compiled programs only.
In this case the result is now `[start]` rather than a runtime error.

Example: `1..0...1` now produces `[1]` rather than an "invalid range"
runtime error.

This makes behavior for compiled programs similar to that of the Futhark
interpreter (and e.g. Haskell ranges), for at least this particular
case (I have not looked for other discrepancies).

src/Futhark/Internalise/Exps.hs

@@ -215,10 +215,10 @@ internaliseAppExp desc _ (E.Range start maybe_second end loc) = do
                )
             ++ [ErrorVal int64 end'_i64, " is invalid."]
 
-  (it, le_op, lt_op) <-
+  (it, lt_op) <-
     case E.typeOf start of
-      E.Scalar (E.Prim (E.Signed it)) -> pure (it, CmpSle it, CmpSlt it)
-      E.Scalar (E.Prim (E.Unsigned it)) -> pure (it, CmpUle it, CmpUlt it)
+      E.Scalar (E.Prim (E.Signed it)) -> pure (it, CmpSlt it)
+      E.Scalar (E.Prim (E.Unsigned it)) -> pure (it, CmpUlt it)
       start_t -> error $ "Start value in range has type " ++ prettyString start_t
 
   let one = intConst it 1
@@ -245,7 +245,7 @@ internaliseAppExp desc _ (E.Range start maybe_second end loc) = do
   bounds_invalid_downwards <-
     letSubExp "bounds_invalid_downwards" $
       I.BasicOp $
-        I.CmpOp le_op start' end'
+        I.CmpOp lt_op start' end'
   bounds_invalid_upwards <-
     letSubExp "bounds_invalid_upwards" $
       I.BasicOp $

tests/range0.fut

@@ -5,24 +5,20 @@
 -- input { 1 5 } output { [1i32, 2i32, 3i32, 4i32, 5i32] }
 -- input { 5 1 } error: 5...1
 -- input { 5 0 } error: 5...0
--- input { 0 5 } output { [0i32, 1i32, 2i32,
--- 3i32, 4i32, 5i32] }
 
 -- ==
 -- entry: test1
 -- input { 0 5 } output { [0i32, 1i32, 2i32, 3i32, 4i32] }
 -- input { 1 5 } output { [1i32, 2i32, 3i32, 4i32] }
 -- input { 5 1 } error: 5..<1
 -- input { 5 0 } error: 5..<0
--- input { 0 5 } output { [0i32, 1i32, 2i32, 3i32, 4i32] }
 
 -- ==
 -- entry: test2
 -- input { 0 5 } error: 0..>5
 -- input { 1 5 } error: 1..>5
 -- input { 5 1 } output { [5i32, 4i32, 3i32, 2i32] }
 -- input { 5 0 } output { [5i32, 4i32, 3i32, 2i32, 1i32] }
--- input { 0 5 } error: 0..>5
 
 -- ==
 -- entry: test3
@@ -31,6 +27,9 @@
 -- input { 5 4 1 } output { [5i32, 4i32, 3i32, 2i32, 1i32] }
 -- input { 5 0 0 } output { [5i32, 0i32] }
 -- input { 0 2 5 } output { [0i32, 2i32, 4i32] }
+-- input { 1 1 5 } error: 1..1...5
+-- input { 1 0 1 } output { [1i32] }
+-- input { 1 2 1 } output { [1i32] }
 
 -- ==
 -- entry: test4