Inconsistent results between Native and PTX backends when slicing an Array with a Scalar Int

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Description

The following computation gives different results when run with the Native and PTX backends:

mat :: Acc (Matrix Int)
mat = use $ fromList (Z :. 4 :. 3) [1..]

s :: Acc (Scalar Int)
s = A.unit (A.constant 0)

A.slice mat . lift $ Any :. the s :. All

Expected behaviour

The expected result is given by the Native backend:

Vector (Z :. 3) [1,2,3]

Current behaviour

The PTX backend gives the following result:

Vector (Z :. 3) [4,5,6]

Note that this result is given regardless of the value of s. For example, if we change

s :: Acc (Scalar Int)
s = A.unit (A.constant 3)

the native backend gives the expected result of

Vector (Z :. 3) [10,11,12]

while the PTX backend gives

Vector (Z :. 3) [4,5,6]

Steps to reproduce (for bugs)

Here is a test program:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE RebindableSyntax #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GADTs #-}
module Main where

import Prelude hiding (Num, zipWith, Eq, (==), (!!))
import qualified Prelude as P
import Data.Array.Accelerate as A hiding ((++))
import qualified Data.Array.Accelerate.LLVM.Native as Native
import qualified Data.Array.Accelerate.LLVM.PTX as PTX

mat :: Acc (Matrix Int)
mat = use $ fromList (Z :. 4 :. 3) [1..]

s :: Acc (Scalar Int)
s = A.unit (A.constant 3)

testCase :: (forall a . Arrays a => Acc a -> a) -> Acc (Scalar Int) -> Acc (Matrix Int) -> IO ()
testCase run ix m = do
  putStrLn "given a matrix m ="
  print (run m)
  putStrLn $ "\n... and an index ix = " ++ show (run ix)
  let sliced = A.slice mat . lift $ Any :. the s :. All
  putStrLn $ "\nthe row at index " ++ show (run ix) ++ " is:\n" ++ show (run sliced)

main = do
  putStrLn "==============================="
  putStrLn "Native backend...\n"
  testCase Native.run s mat
  putStrLn "==============================="

  putStrLn "\n\n\n"

  putStrLn "==============================="
  putStrLn "PTX backend...\n"
  testCase PTX.run s mat
  putStrLn "==============================="

... and here is what it prints on my system:

===============================
Native backend...

given a matrix m =
Matrix (Z :. 4 :. 3)
  [  1,  2,  3,
     4,  5,  6,
     7,  8,  9,
    10, 11, 12]

... and an index ix = Scalar Z [3]

the row at index Scalar Z [3] is:
Vector (Z :. 3) [10,11,12]
===============================




===============================
PTX backend...

given a matrix m =
Matrix (Z :. 4 :. 3)
  [  1,  2,  3,
     4,  5,  6,
     7,  8,  9,
    10, 11, 12]

... and an index ix = Scalar Z [3]

the row at index Scalar Z [3] is:
Vector (Z :. 3) [4,5,6]
===============================

Your environment

• Version used: accelerate == 1.3
• Backend(s) used and version: accelerate-llvm == 1.3, accelerate-llvm-native == 1.3, accelerate-llvm-ptx == 1.3
• GHC version: 8.8.4
• Operating system and version: Arch Linux
• If this is a bug with the GPU backend, include the output of nvidia-device-query: I don't seem to have this program. Where do I get it?

GPU details:

• GPU: GeForce GTX 1650
• NVIDIA driver: 455.45.01