Re: [GHC] #13209: ghc panic with optimization.

30 Jan 2017


      #13209: ghc panic with optimization.
-------------------------------------+-------------------------------------
        Reporter:  1chb              |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:  wontfix           |             Keywords:
Operating System:  Linux             |         Architecture:  x86
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by mpickering):

 * status:  new => closed
 * resolution:   => wontfix


@@ -1,6 +1,1 @@
- {{{
- > ghc eqFn && ./eqFn
- [1 of 1] Compiling Main             ( eqFn.hs, eqFn.o )
- Linking eqFn ...
- True
- }}}
+ ghc eqFn && ./eqFn
@@ -13,2 +8,1 @@
- {{{
- > ghc -fsimpl-tick-factor=2000 -O eqFn && ./eqFn
+ ghc -fsimpl-tick-factor=2000 -O eqFn && ./eqFn
@@ -24,1 +18,0 @@
- }}}

New description:

 ghc eqFn && ./eqFn

 Works fine and the program runs fine too, but a bit slow.

 Adding optimization (-O) makes ghc panic (even after setting a bigger
 'tick' factor):

 ghc -fsimpl-tick-factor=2000 -O eqFn && ./eqFn
 [1 of 1] Compiling Main             ( eqFn.hs, eqFn.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for i386-unknown-linux):
         Simplifier ticks exhausted
   When trying UnfoldingDone f_XOO
   To increase the limit, use -fsimpl-tick-factor=N (default 100)
   If you need to do this, let GHC HQ know, and what factor you needed
   To see detailed counts use -ddump-simpl-stats
   Total ticks: 700804

 My program eqFn.hs:
 {{{
 {-# LANGUAGE FlexibleInstances #-}

 instance Eq a => Eq (Bool -> a) where
     f == g = f True == g True &&
              f False == g False

 nand a b = not (a && b)

 xor a b = nand (nand a c) (nand b c) where c=nand a b

 halfAdder a b = (xor a b, a && b) -- (sum, cOut)
 fullAdder cIn a b = let (s, c') = halfAdder a b
                         (sum, c'') = halfAdder cIn s
                     in (sum, c' || c'')

 adder2 cIn a0 b0 a1 b1 =
     let (o0, cTmp) = fullAdder cIn a0 b0
         (o1, cOut) = fullAdder cTmp a1 b1
     in (o0, o1, cOut)

 adder4 cIn a0 b0 a1 b1 a2 b2 a3 b3 =
     let (o0, o1, cTmp) = adder2 cIn a0 b0 a1 b1
         (o2, o3, cOut) = adder2 cTmp a2 b2 a3 b3
     in (o0, o1, o2, o3, cOut)

 adder8 cIn a0 b0 a1 b1 a2 b2 a3 b3 a4 b4 a5 b5 a6 b6 a7 b7 =
     let (o0, o1, o2, o3, cTmp) = adder4 cIn a0 b0 a1 b1 a2 b2 a3 b3
         (o4, o5, o6, o7, cOut) = adder4 cTmp a4 b4 a5 b5 a6 b6 a7 b7
     in (o0, o1, o2, o3, o4, o5, o6, o7, cOut)

 main = print (adder8 == adder8)
 }}}

 Btw, it works if I replace adder8 with adder4 in the last line.

 I think this is a bug because, if the problem is too complicated for the
 optimizer it should just skip that part instead of failing.

--

Comment:

 If you actually want the inliner to work really hard for you then you can
 set the tick factor to 0.

 Just looking at the definition of `==` and `adder8` it is clear that very
 aggressive inlining will at some point create an exponentially big
 program. When `==` is inlined at the first step we get

 {{{
 adder8 == adder8
 =
 adder8 True == adder8 True && adder8 False == adder8 False
 =
 adder8 True True == adder8 True True && adder8 True False == adder8 True
 False && adder8 False True == adder8 False True && adder8 False False ==
 adder8 False False
 =
 ....
 }}}

 There is little difference between the compiler taking a very long time
 (as it does in this example) and looping forever. Some kind of limit on
 the simplifier is prudent to stop the compiler appearing to hang.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13209#comment:2
GHC http://www.haskell.org/ghc/
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