[GHC] #12964: Runtime regression to RTS change

[GHC]

#12964: Runtime regression to RTS change
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           Reporter:  nomeata        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.2.1
          Component:  Runtime        |           Version:  8.1
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The commit to fix #12799, namely changeset:1732d7ac43ca, seems to have
 introduced a runtime regression. The hidden and integer nofib benchmarks
 regress by 5.5% resp. 2.5%:

 https://perf.haskell.org/ghc/#revision/1732d7ac43ca578deca39ea5a63cbf34f3cd9dd5

 I re-measured this commit and its parent once, and the numbers aid not
 change much.

 The `hidden` regression seems to be offset almost completely by the later
 changeset:6f7ed1e51bf360621a3c2a447045ab3012f68575 while the `integer`
 regression got “fixed” by the later
 changeset:1732d7ac43ca578deca39ea5a63cbf34f3cd9dd5.

 The perf builder is still new, so this might be bogus. But we should still
 check.

 Also, maybe random changes to the RTS cause random changes to the code
 layout which surface here. Or there is a real cause.

 Simon Marlow [https://phabricator.haskell.org/rGHC1732d7ac43ca#56419
 suggested] to open a high priority ticket for this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12964>
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