[GHC] #5925: Add inline version of newArray#

[GHC]

#5925: Add inline version of newArray#
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 Reporter:  tibbe             |          Owner:                  
     Type:  feature request   |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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 It'd would be nice to have an inline version of `newArray#` so that
 allocation of small arrays is cheaper (e.g. the heap check could be paired
 up with other heap checks.)

 I see two possible designs:

  1. Add a branch to `newArray#` that checks the requested size and either
 allocates inline or out-of-line.
  2. Add a new primop e.g. `newSmallArray#` that always allocates inline.

 I would lean towards (2). While (1) is equal to (2) in the case when the
 size is statically known (assuming that GHC can discover that fact through
 enough inlining at the Cmm level), there are cases where the programmer
 knows that the size is small, without knowing the exact size. This happens
 e.g. when you use arrays to represent small variable-length tuples (e.g.
 in the `unordered-containers` package.)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5925>
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