[GHC] #15578: Honour INLINE pragmas on 0-arity bindings

[GHC]

#15578: Honour INLINE pragmas on 0-arity bindings
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Looking at `CoreUnfold.mkInlineUnfolding`, a function with an INLINE
 pragma has its unfolding built with `mkInlineUnfoldingWithArity` (the
 arity being the syntactic arity in the source code).  This function gives
 the unfolding an `UnfoldingGuidance` of
 {{{
     guide = UnfWhen { ug_arity = arity
                     , ug_unsat_ok = needSaturated
                     , ug_boring_ok = boring_ok }
     boring_ok = inlineBoringOk expr'
 }}}
 And in fact, in `callSiteInline`, you can see that if `ug_arity = 0` then
 `x` will inline if `boring_ok` is True.

 What is `boring_ok`?  It says whether to inline in an utterly boring
 context.  In
 {{{
   \ y x -> Just (f x y)
 }}}
 we don't inline `f` ''even if `f` has an INLINE pragma'' because the
 context of the call us utterly boring: nothing is known about `x` or `y`
 or the calling context.   So there is really no point in inlining.

 I don't think this applies to 0-ary INLINE pragmas, as Trac #15519 shows.

 So I think it may be enough to say, in `mkInlineUnfoldingWithArity`,
 {{{
     boring_ok | arity == 0 = True
               | otherwise  = inlineBoringOk expr'
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15578#comment:3>
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