[GHC] #14373: Introduce PTR-tagging for big constructor families

[GHC]

#14373: Introduce PTR-tagging for big constructor families
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           Reporter:  heisenbug      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  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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 Currently only ''small'' constructor families come into the benefit of
 pointer tagging.

 In big fams, 1 is the tag that says "I am evaluated". I suggest to do
 best-effort pointer tagging on big families too by this scheme:

 Ptr-tag 1..6 for the first 6 constructors, 7 would signify "look into the
 info table and branch on that tag". In the info table the tags will then
 be 6..(familySize - 1).

 I have an implementation which I'll submit to fabricator soon.

 TODOs: update wiki pages.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14373>
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