[GHC] #13413: GHC HEAD panic: collectNBinders

[Simon Peyton Jones]

I know what is going on here.  I'm in a meeting all day, but I hope to fix tomorrow.

Simon

| -----Original Message-----
| From: ghc-tickets [mailto:ghc-tickets-bounces at haskell.org] On Behalf Of
| GHC
| Sent: 11 March 2017 20:12
| Cc: ghc-tickets at haskell.org
| Subject: [GHC] #13413: GHC HEAD panic: collectNBinders
| 
| #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>
| GHC <http://www.haskell.org/ghc/>
| The Glasgow Haskell Compiler