Re: [GHC] #11131: Eta reduction/expansion loop

25 Nov 2015

      #11131: Eta reduction/expansion loop
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by simonpj:

Old description:
...
In the build log for HEAD today you'll see:
{{{
WARNING: file compiler\simplCore\SimplCore.hs, line 604
  Simplifier bailing out after 10 iterations [148, 4, 4, 4, 4, 4, 4, 4,
4, 4]
    Size = {terms: 290, types: 237, coercions: 56}
}}}
when compiling`Data/ByteString/Builder/Prim/Internal/Floating.hs`.
There is a loop here: in each simplifier pass we get an eta-expansion and
an eta reduction, which cancel each other out.  This is bad.
Here is the offending snippet of code
{{{
((bindIO @ () @ ()
    ($fStorableDouble_$cpoke
       (castPtr @ Word8 @ Double op)
       x)
    ((\ (ds :: ()) ->
        (\ (eta_B1 :: State# RealWorld) ->
           (k `cast` (NTCo:IO[0] <()>_R
                     :: IO ()
                        ~R# (State# RealWorld -> (# State# RealWorld, ()
#))))
            eta_B1)
        `cast` (Sym (NTCo:IO[0] <()>_R)
                :: (State# RealWorld -> (# State# RealWorld, () #))
                   ~R# IO ()))
     `cast` (<()>_R
             -> NTCo:IO[0] <()>_R ; Sym (NTCo:IO[0] <()>_R)
             :: (() -> IO ()) ~R# (() -> IO ()))))
 `cast` (NTCo:IO[0] <()>_R
         :: IO () ~R# (State# RealWorld -> (# State# RealWorld, () #))))
}}}
At this point `k` is in scope with arity 2.  Actually its binding is
{{{
k :: IO ()
k = bindIO @ Word64 @ () a_s2As a_s2Au
}}}
I'm not precisely sure why this goes wrong, but it's very clearly bogus,
so I'm opening a ticket to keep track.
New description:

 In the build log for HEAD today you'll see:
 {{{
 WARNING: file compiler\simplCore\SimplCore.hs, line 604
   Simplifier bailing out after 10 iterations [148, 4, 4, 4, 4, 4, 4, 4, 4,
 4]
     Size = {terms: 290, types: 237, coercions: 56}
 }}}
 when compiling`Data/ByteString/Builder/Prim/Internal/Floating.hs`.

 There is a loop here: in each simplifier pass we get an eta-expansion and
 an eta reduction, which cancel each other out.  This is bad.

 Here is the offending snippet of code
 {{{
 ((bindIO @ () @ ()
     ($fStorableDouble_$cpoke
        (castPtr @ Word8 @ Double op)
        x)
     ((\ (ds :: ()) ->
         (\ (eta_B1 :: State# RealWorld) ->
            (k `cast` (NTCo:IO[0] <()>_R
                      :: IO ()
                         ~R# (State# RealWorld -> (# State# RealWorld, ()
 #))))
             eta_B1)
         `cast` (Sym (NTCo:IO[0] <()>_R)
                 :: (State# RealWorld -> (# State# RealWorld, () #))
                    ~R# IO ()))
      `cast` (<()>_R
              -> NTCo:IO[0] <()>_R ; Sym (NTCo:IO[0] <()>_R)
              :: (() -> IO ()) ~R# (() -> IO ()))))
  `cast` (NTCo:IO[0] <()>_R
          :: IO () ~R# (State# RealWorld -> (# State# RealWorld, () #))))
 }}}
 At this point `k` is in scope with arity 1.  Actually its binding is
 {{{
 k :: IO ()
 k = bindIO @ Word64 @ () a_s2As a_s2Au
 }}}
 I'm not precisely sure why this goes wrong, but it's very clearly bogus,
 so I'm opening a ticket to keep track.

--

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11131#comment:1
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