[GHC] #15573: Make bindings with multiple occurrences a join point instead of duplicating code during inlining.

27 Aug 2018

      #15573: Make bindings with multiple occurrences a join point instead of duplicating
code during inlining.
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           Reporter:  AndreasK       |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:  JoinPoints     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I have some intermediate core of the form:

 {{{
 -- RHS size: {terms: 9, types: 2, coercions: 0, joins: 0/0}
 cseAlts_s1dD [Occ=Once!T[1]] :: T -> Int#
 [LclId, CallArity=1, Str=]
 cseAlts_s1dD
   = \ (lamVar_s1dw [Occ=Once!, Dmd=, OS=OneShot] :: T) ->
       case lamVar_s1dw of wild_Xc [Dmd=] {
         __DEFAULT -> 1#;
         B -> 2#;
         C -> 3#
       }

 -- RHS size: {terms: 14, types: 3, coercions: 0, joins: 0/0}
 $wfmerge_s1cZ [InlPrag=NOUSERINLINE[0]] :: T -> T -> Int#
 [LclId, Arity=2, CallArity=2, Str=]
 $wfmerge_s1cZ
   = \ (w_s1cU [Occ=Once!, Dmd=] :: T)
       (w_s1cV [Occ=Once*, Dmd=] :: T) ->
       case w_s1cU of wild_XA [Dmd=] {
         __DEFAULT -> -1#;
         A -> 2#;
         B -> cseAlts_s1dD w_s1cV;
         C -> cseAlts_s1dD w_s1cV
       }
 }}}

 Which after the simplifier ran got inlined into the branches to give us:

 {{{
 fmerge
   = \ (w_s1cU :: T) (w_s1cV :: T) ->
       case w_s1cU of {
         __DEFAULT -> GHC.Types.I# -1#;
         A -> GHC.Types.I# 2#;
         B ->
           case w_s1cV of {
             __DEFAULT -> GHC.Types.I# 1#;
             B -> GHC.Types.I# 2#;
             C -> GHC.Types.I# 3#
           };
         C ->
           case w_s1cV of {
             __DEFAULT -> GHC.Types.I# 1#;
             B -> GHC.Types.I# 2#;
             C -> GHC.Types.I# 3#
           }
       }
 }}}

 What I would really like GHC to do instead though is to make
 `cseAlts_s1dD` a join point when possible.
 This would eliminate both the call overhead AND the call duplication.

 The current behavior seems fine when we can't make it a join point. But
 when we can we should try to take advantage of that opportunity.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15573
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