[GHC] #7198: New codegen more than doubles compile time of T3294

[GHC]

#7198: New codegen more than doubles compile time of T3294
-------------------------------------+-------------------------------------
        Reporter:  simonmar          |                Owner:  simonmar
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.4.2
  (CodeGen)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #4258             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"6a2264d2bd47e993c43a592bd614ab7917184e22/ghc" 6a2264d/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="6a2264d2bd47e993c43a592bd614ab7917184e22"
 cmm/CmmLayoutStack: avoid generating unnecessary reloads

 This tries to be more precise when generating reloads of local
 registers in proc points. Previously we'd reload all local registers
 that were live. But we used liveness information that assumed local
 registers survive native calls. For the purpose of reloading registers
 this is an overapproximation and might lead to generating huge amounts
 of unnecessary reloads (in case there's another proc point before the
 register is used).

 This change takes the approach of moving the generation of reloads to
 a second pass over the Cmm, which allows to recompute the liveness and
 can use the knowledge that local registers do *not* survive calls.
 This leads to generating only useful reloads. For an extreme example
 where this helps a lot please see T3294. This should also fix #7198

 Finally, this re-introduces the code to do Cmm rewriting using in
 `Dataflow` module (with the difference that we know operate on a whole
 block at a time).

 Signed-off-by: Michal Terepeta <michal.terepeta at gmail.com>

 Reviewers: austin, bgamari, simonmar

 Reviewed By: simonmar

 Subscribers: kavon, rwbarton, thomie

 GHC Trac Issues: #7198

 Differential Revision: https://phabricator.haskell.org/D3586
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7198#comment:15>
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