[GHC] #8638: Optimize by demoting "denormalized" Integers (i.e. J# -> S#)

[GHC]

#8638: Optimize by demoting "denormalized" Integers (i.e. J# -> S#)
--------------------------------------------+------------------------------
        Reporter:  hvr                      |            Owner:  hvr
            Type:  feature request          |           Status:  infoneeded
        Priority:  normal                   |        Milestone:  7.8.1
       Component:  libraries (other)        |          Version:  7.7
      Resolution:                           |         Keywords:  integer-
Operating System:  Unknown/Multiple         |  gmp
 Type of failure:  Runtime performance bug  |     Architecture:
       Test Case:                           |  Unknown/Multiple
        Blocking:                           |       Difficulty:  Unknown
                                            |       Blocked By:
                                            |  Related Tickets:
--------------------------------------------+------------------------------

Comment (by hvr):

 Replying to [comment:7 simonpj]:
 > Great.  Now, what's happening in `symalg` and `kahan` (+4.7 and 6.7%
 resp)?

 Thunks are happening... (see below)

 > What is puzzling to me is that I don't think `smartJ#` does ''any''
 allocation (apart from its result) so I don't see why allocation should
 ever increase.

 ...because I missed the fact that `(# smartJ# ... , smartJ# ... #)`
 creates a thunk (there was even a source code comment about that pitfall
 in `quotRemInteger#`)

 However, I replaced all `smartJ#`-in-`(#,#)` occurences (incl.
 `decodeDouble`) by `let !x = smartJ# ... in (# x, ... #)` constructs, and
 the results look better now:

 {{{
         Program           Size    Allocs   Runtime   Elapsed  TotalMem
 --------------------------------------------------------------------------------
 ...
          gamteb          +0.1%    -19.0%      0.03      0.03     +0.0%
 ...

           kahan          +0.2%     -1.2%      0.17      0.17     +0.0%
 ...
          mandel          +0.1%     -7.7%      0.05      0.05     +0.0%
 ...
           power          +0.1%    -40.8%    -32.5%    -32.5%     +0.0%
 ...
          symalg          +0.2%     -0.5%      0.01      0.01     +0.0%
 ...
 --------------------------------------------------------------------------------
             Min          +0.0%    -40.8%    -32.5%    -32.5%     -5.1%
             Max          +0.2%     +0.1%     +2.0%     +2.0%     +0.0%
  Geometric Mean          +0.1%     -1.0%     -2.5%     -2.5%     -0.1%
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8638#comment:9>
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