GHC HEAD panic: collectNBinders

repa-eval-4.2.3.1 currently fails to build on GHC HEAD because of this issue. Trying to build it leads to several collectNBinders panics is various modules. You can reproduce this by compiling this module:

{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE MagicHash #-}
module Data.Repa.Eval.Generic.Seq.Chunked where

import GHC.Exts (Int#, (+#), (*#), (>=#))

-------------------------------------------------------------------------------
-- | Fill a block in a rank-2 array, sequentially.
--
--   * Blockwise filling can be more cache-efficient than linear filling for
--     rank-2 arrays.
--
--   * The block is filled in row major order from top to bottom.
--
fillBlock2
        :: (Int# -> a -> IO ()) -- ^ Update function to write into result buffer.
        -> (Int# -> Int# -> a)  -- ^ Function to get the value at an (x, y) index.
        -> Int#                 -- ^ Width of the whole array.
        -> Int#                 -- ^ x0 lower left corner of block to fill.
        -> Int#                 -- ^ y0
        -> Int#                 -- ^ w0 width of block to fill
        -> Int#                 -- ^ h0 height of block to fill
        -> IO ()

fillBlock2
        write getElem
        !imageWidth !x0 !y0 !w0 h0

 = do   fillBlock y0 ix0
 where  !x1     = x0 +# w0
        !y1     = y0 +# h0
        !ix0    = x0 +# (y0 *# imageWidth)

        {-# INLINE fillBlock #-}
        fillBlock !y !ix
         | 1# <- y >=# y1     = return ()
         | otherwise
         = do   fillLine1 x0 ix
                fillBlock (y +# 1#) (ix +# imageWidth)

         where  {-# INLINE fillLine1 #-}
                fillLine1 !x !ix'
                 | 1# <- x >=# x1             = return ()
                 | otherwise
                 = do   write ix' (getElem x y)
                        fillLine1 (x +# 1#) (ix' +# 1#)

{-# INLINE [0] fillBlock2 #-}

This compiles on GHC 8.0.2, but on GHC HEAD:

$ ~/Software/ghc4/inplace/bin/ghc-stage2 -fforce-recomp Bug.hs
[1 of 1] Compiling Data.Repa.Eval.Generic.Seq.Chunked ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20170201 for x86_64-unknown-linux):
        collectNBinders
  2
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1179:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1183:37 in ghc:Outputable
        pprPanic, called at compiler/coreSyn/CoreSyn.hs:1970:25 in ghc:CoreSyn

Interestingly, compiling this triggers the panic at any optimization level, but loading the module into GHCi does not cause it to panic.

This regression was introduced in 8d5cf8bf (Join points).

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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