[GHC] #12357: Increasing maximum constraint tuple size significantly blows up compiler allocations

[GHC]

#12357: Increasing maximum constraint tuple size significantly blows up compiler
allocations
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by bgamari:

@@ -7,0 +7,5 @@
+
+ Judging by ticky and the cost-center profiler the performance regression
+ appears to simply be the result of the loading the new declarations from
+ the interface file, as one might expect. Nevertheless, it's quite silly to
+ pay such a large price for a change that only gets occasionally exercised.

New description:

 In July 2015 (dd3080fe0263082f65bf2570f49189c277b12e28) the maximum
 constraint tuple size was raised from 16 to 62 to address #10451. It turns
 out that this change is apparently one of the larger compile-time
 regressions in recent GHC history. For instance, the nofib
 `real/fulsom/Shapes.hs` module regresses by around 15% in both compiler
 allocations and compile time.

 Judging by ticky and the cost-center profiler the performance regression
 appears to simply be the result of the loading the new declarations from
 the interface file, as one might expect. Nevertheless, it's quite silly to
 pay such a large price for a change that only gets occasionally exercised.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12357#comment:2>
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