[GHC] #10730: Spectral norm allocations increased 17% between 7.6 and 7.8

[GHC]

#10730: Spectral norm allocations increased 17% between 7.6 and 7.8
-------------------------------------+-------------------------------------
              Reporter:  bgamari     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  low         |         Milestone:
             Component:  Compiler    |           Version:  7.8.3
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 It appears that `spectral-norm` regressed rather severely from 7.6 to 7.8
 in total allocations. Meanwhile the binary size also increased by 7% or
 so. However, it also appears that runtime was unaffected by this change.

 The rule firings are very similar between 7.6 and 7.8. Three rules fire in
 786 that do not fire in 7.6,
 {{{
   1 Class op formatArg
   1 Class op parseFormat
   1 tagToEnum#
 }}}

 The difference in allocation persists even if one reverts the
 `Text.Printf` rewrite that gives rise to the `formatArg` and `parseFormat`
 rules, so these can be safely ignored.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10730>
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