[GHC] #3198: inliner fails to kick in for Double (*)

[GHC]

#3198: inliner fails to kick in for Double (*)
---------------------------------+------------------------------------------
    Reporter:  JulesBean         |        Owner:                  
        Type:  bug               |       Status:  new             
    Priority:  normal            |    Milestone:                  
   Component:  Compiler          |      Version:  6.11            
    Severity:  normal            |   Resolution:                  
    Keywords:                    |   Difficulty:  Unknown         
    Testcase:                    |           Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |  
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Changes (by simonpj):

  * difficulty:  => Unknown

Comment:

 Suppose you have
 {{{
 f :: Double -> Double -> Double
 f x y = x * y
 }}}
 What is gained if we inline `(*)` here?  Virtually nothing: simply a jump
 from `f` to `*`.  After all, `f` is just `(*)`, and we know nothing about
 `x` or `y`.  (It'd be quite differnet if it was `x*1`!.)  Moreover, if we
 don't inline `*` here, we'll replace `f` by `(*)` at every call site.

 (Remember too that this is Haskell, so it `(*)` was inlined you'd get
 {{{
 f x y = case x of D# a -> case y of D# b -> D# (plusDouble# a b)
 }}}

 In general, if GHC sees a call `(foo x y)`, where
  * nothing whatsoever is known about `x` or `y` (for example, they are
 lambda-bound), ''and''
  * there is nothing interesting about the context of the cal
 then GHC does not inline `foo`, ''even if `foo` has an INLINE pragma''.
 Why?  Because it simply give code bloat with zero or small benefit.

 This is an adjustable decision, but I'm not yet convinced it needs
 adjusting.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3198#comment:3>
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