Make bindings with multiple occurrences a join point instead of duplicating code during inlining.

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=<S,1*U>]
cseAlts_s1dD
  = \ (lamVar_s1dw [Occ=Once!, Dmd=<S,1*U>, OS=OneShot] :: T) ->
      case lamVar_s1dw of wild_Xc [Dmd=<L,A>] {
        __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=<S,1*U><L,1*U>]
$wfmerge_s1cZ
  = \ (w_s1cU [Occ=Once!, Dmd=<S,1*U>] :: T)
      (w_s1cV [Occ=Once*, Dmd=<L,1*U>] :: T) ->
      case w_s1cU of wild_XA [Dmd=<L,A>] {
        __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.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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