[GHC] #14657: Quadratic constructor tag allocation

[GHC]

#14657: Quadratic constructor tag allocation
-------------------------------------+-------------------------------------
           Reporter:  niteria        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
  genManyConstructors                |
           Blocking:                 |   Related Tickets:
Differential Rev(s):  phab:D4289     |         Wiki Page:
-------------------------------------+-------------------------------------
 With a large data type like:

 {{{
 data A = A0
   | A0001
   | A0002
 ...
   | A9999
 }}}

 GHC spends a lot of time allocating constructor tags. It accounts for half
 of allocations for large data types like this.

 The hot piece of code is in `mkDataCon`:
 {{{
    tag = assoc "mkDataCon" (tyConDataCons rep_tycon `zip` [fIRST_TAG..])
 con
 }}}

 Previous discussion:
 https://mail.haskell.org/pipermail/ghc-devs/2017-October/014974.html

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14657>
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