[GHC] #16254: INLINABLE pragma and newtype wrappers prevent inlining

29 Jan 2019

      #16254: INLINABLE pragma and newtype wrappers prevent inlining
-------------------------------------+-------------------------------------
           Reporter:  monoidal       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #5327
Differential Rev(s):                 |         Wiki Page:
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 In #5327, we allowed case-of-known constructor to see through bindings
 that were marked as INLINABLE and used newtypes. However, this works only
 if the newtype does not have a wrapper. The following code, which is the
 same as T5237 except for the extra type parameters in Size, does not cause
 the optimization to fire.

 {{{
 {-# LANGUAGE GADTs, ExplicitForAll #-}
 module T16254 where

 newtype Size a b where
   Size :: forall b a. Int -> Size a b

 {-# INLINABLE val2 #-}
 val2 = Size 17

 f n = case val2 of Size s -> s + s > n
 }}}

 The reason is that `exprIsConApp_maybe` sees `$WSize (I# 17#)`, which is
 an application with a nontrivial argument. In general, this could cause
 duplication of work, but in this case we're dealing with a newtype
 wrapper, so this is safe.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16254>
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