Re: [GHC] #14998: Sort out the strictness mess for exceptions

6 Apr 2018

      ...
...
reverse-complement allocates 11.6% more
That's remarkable.  I expected it to be virtually nothing.  I suppose
#14998: Sort out the strictness mess for exceptions
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.3
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  Exceptions
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by sgraf):

 Replying to [comment:7 simonpj]:
 that some investigation is called for.

 Long story short, there was no difference in generated Core for both
 `reverse-complement` and `sphere`.

 The difference is is due to the
 [https://hackage.haskell.org/package/base-4.11.0.0/docs/src/GHC.IO.Handle.Int...
 do_operation function] that has a call to `catchException`.
 When I compare the strictness signatures when compiling
 `GHC.IO.Handle.Internal` there is a notable difference for the
 `withHandle*` family of functions which calls `do_operation`:

 {{{
 36,38c36,38
 < GHC.IO.Handle.Internals.withAllHandles__: 
 < GHC.IO.Handle.Internals.withHandle: 
 < GHC.IO.Handle.Internals.withHandle':

 ---
...
GHC.IO.Handle.Internals.withAllHandles__: 
GHC.IO.Handle.Internals.withHandle: 
GHC.IO.Handle.Internals.withHandle':

 41c41
 < GHC.IO.Handle.Internals.withHandle__':

...
GHC.IO.Handle.Internals.withHandle__':

 80,82c80,82
 < GHC.IO.Handle.Internals.withAllHandles__: 
 < GHC.IO.Handle.Internals.withHandle: 
 < GHC.IO.Handle.Internals.withHandle':

...
GHC.IO.Handle.Internals.withAllHandles__: 
GHC.IO.Handle.Internals.withHandle: 
GHC.IO.Handle.Internals.withHandle':

 85c85
 < GHC.IO.Handle.Internals.withHandle__':

...
GHC.IO.Handle.Internals.withHandle__':

 }}}
These are called transitively by `hGetBuf` and `hPutBuf`. We could
 annotate the IO action passed to `do_operation` for an easy fix:

 {{{
 diff --git a/libraries/base/GHC/IO/Handle/Internals.hs
 b/libraries/base/GHC/IO/Handle/Internals.hs
 index 48ece1dc5e..3d58f3d3bc 100644
 --- a/libraries/base/GHC/IO/Handle/Internals.hs
 +++ b/libraries/base/GHC/IO/Handle/Internals.hs
 @@ -163,7 +163,8 @@ do_operation :: String -> Handle -> (Handle__ -> IO a)
 -> MVar Handle__ -> IO a
  do_operation fun h act m = do
    h_ <- takeMVar m
    checkHandleInvariants h_
 -  act h_ `catchException` handler h_
 +  let !io = act h_
 +  io `catchException` handler h_
    where
      handler h_ e = do
        putMVar m h_
 }}}

 I'm not sure if that's just navigating around the problem, but this brings
 strictness signatures on par and fixes `sphere` and `reverse-complement`.
 I'll re-run benchmarks now. Also, without this patch, T12996 (#12996)
 broke in `./validate`.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14998#comment:10
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