[GHC] #13413: GHC HEAD panic: collectNBinders

11 Mar 2017

      #13413: GHC HEAD panic: collectNBinders
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
           Keywords:  JoinPoints     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 `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:

 {{{#!hs
 {-# 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 8d5cf8bf584fd4849917c29d82dcf46ee75dd035
 (Join points).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13413
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