Add warning to documentation of fromPtrs

[tomsmeding]

In accelerate 1.3, code that should have been dead-code eliminated, but was not, may be executed at runtime while nothing waits for it. The fact that this is possible makes fromPtrs essentially impossible to use safely.

[tomsmeding]

Reproduction for the issue in question:

{-# LANGUAGE RankNTypes #-}
module FromPtrsUnsafe where

import qualified Data.Array.Accelerate as A
import Data.Array.Accelerate.Array.Unique (newUniqueArray)
import qualified Data.Array.Accelerate.Representation.Array as R
import Data.Array.Accelerate.Sugar.Array (Array(..))
import Data.Array.Accelerate.Test.NoFib.Base

import Control.Monad
import Foreign
import Test.Tasty
import Test.Tasty.HUnit

-- Source: reduced from https://github.com/LCSB-BioCore/remotesom/blob/516581a3e2973a5561a08b0abcffbb9d7afb261e/Numeric/RemoteSOM.hs
somIter :: A.Acc (A.Matrix Float) -> A.Acc (A.Matrix Float) -> A.Acc (A.Matrix Float)
somIter points som =
  let A.I2 ptsn _ = A.shape points
      A.I2 somn dim = A.shape som
      closest = A.generate (A.I1 ptsn) $ \_ -> 0
      A.T2 s _s = A.T2 
                    (A.permute
                      (+)
                      (A.I2 somn dim `A.fill` A.constant (0 :: Float))
                      (\(A.I2 pix dimi) -> A.Just_ $ A.I2 (closest A.! A.I1 pix) dimi)
                      points)
                    (A.permute
                      (+)
                      (A.I2 somn dim `A.fill` A.constant (0 :: Float))
                      (\(A.I2 pix dimi) -> A.Just_ $ A.I2 (closest A.! A.I1 pix) dimi)
                      (A.map (\x -> x * x) points))
  in s

test_fromPtrsUnsafe :: RunN -> TestTree
test_fromPtrsUnsafe runN = testCase "fromPtrsUnsafe" $ do
  let ptsn = 200000
      dim = 128

  pointsbuf <- mallocBytes (ptsn * dim * sizeOf (undefined :: Float))
  forM_ [0 .. ptsn*dim - 1] $ \i ->
    pokeElemOff pointsbuf i 0.0
  pointsUA <- newUniqueArray =<< newForeignPtr_ pointsbuf
  let points = Array (R.Array (((), ptsn), dim) pointsUA)

  let som = A.fromFunction (A.Z A.:. 400 A.:. dim) (\_ -> 0.0)
  let res = runN somIter points som
  when (res /= A.fromFunction (A.Z A.:. 400 A.:. dim) (\_ -> 0.0)) $
    assertFailure "unequal"

  free pointsbuf