[GHC] #7206: Implement cheap build

[GHC]

#7206: Implement cheap build
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        Reporter:  simonpj           |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  7.4.2
      Resolution:                    |             Keywords:  FloatOut
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8763, #13422     |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by osa1):

 I rebased the `cheap-build` branch, applied the idea in #13422 to `Word`
 and `Char` enumerations (`Int` was already implemented), verified that it
 works as expected, and run nofib again. The result is the same; only
 allocation difference is in `cacheprof`. Runtimes differ, but when I
 compared STG outputs I see that they're identical even though runtime
 differs a few percent. So they're really just noise. I suspect runtime
 results in comment:13 are also of this nature.

 I was thinking that this optimization can be done with little user
 intervention by adding an `INLINE` pragma, but surprisingly it doesn't
 work. In the example from #13422:

 {{{
 foo :: Int -> Int
 foo n = s + p
   where
     nums = [1..n]
     s = sum nums
     p = product nums
 }}}

 adding `INLINE` to nums (in original GHC, without `cheap-build` patch)
 does not make it fuse. However if I manually inline it (substitute
 `[1..n]` for `nums`) it fuses as expected. Not sure if this is a bug or
 not, but it's certainly surprising to me.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7206#comment:24>
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