[GHC] #11735: Optimize coercionKind

[GHC]

#11735: Optimize coercionKind
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               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:                    |
-------------------------------------+-------------------------------------

Comment (by tdammers):

 OK, new profiling result:

 {{{
         Thu Jan 25 13:11 2018 Time and Allocation Profiling Report
 (Final)

            ghc-stage2 +RTS -p -RTS -B/home/tobias/well-
 typed/devel/ghc/inplace/lib Grammar.hs -ddump-stg -ddump-simpl -ddump-to-
 file -fforce-recomp

         total time  =       20.99 secs   (20989 ticks @ 1000 us, 1
 processor)
         total alloc = 29,250,375,256 bytes  (excludes profiling overheads)

 COST CENTRE           MODULE     SRC
 %time %alloc

 CoreTidy              HscMain    compiler/main/HscMain.hs:1253:27-67
 24.2   28.5
 Stg2Stg               HscMain    compiler/main/HscMain.hs:1489:12-44
 20.3   24.2
 simplCast             Simplify   compiler/simplCore/Simplify.hs:871:62-87
 18.7   15.9
 tc_rn_src_decls       TcRnDriver
 compiler/typecheck/TcRnDriver.hs:(494,4)-(556,7)      9.0    6.9
 addCoerce-pushCoTyArg Simplify
 compiler/simplCore/Simplify.hs:(1236,12)-(1237,72)    7.4    6.4
 subst_ty              TyCoRep    compiler/types/TyCoRep.hs:2237:28-32
 4.3    5.1
 zonkTopDecls          TcRnDriver
 compiler/typecheck/TcRnDriver.hs:(445,16)-(446,43)    1.4    1.1
 coercionKind          Coercion   compiler/types/Coercion.hs:1725:3-7
 1.3    3.0
 simplExprF1-Lam       Simplify   compiler/simplCore/Simplify.hs:896:5-39
 1.0    1.1
 }}}

 This is the same `Grammar.hs`, compiled with the same GHC code as before,
 but with `NthCo` extended with an extra `Role` field, and the kind
 calculation from `coercionRole` moved out into `mkNthCo`. I ended up
 having to make changes in 16 modules, but most of them were
 straightforward, discarding or forwarding the extra field in a pattern
 match. I think this shouldn't have any negative impact, because forwarding
 the role can only make things better (avoiding future calls to
 `coercionRole`), and discarding it retains the old status.

 Conclusions:

 - We're shaving off another 4 seconds of execution time, and allocations
 remain the same. So this doesn't seem to make things worse for the
 `Grammar.hs` case.
 - We are not actually reducing allocations any further.
 - CoreTidy is worth looking into.
 - In order to verify that this change really makes an impact for the
 better, I would still love to test this against source code that would
 perform badly without it. Test cases very welcome.
 - 20 seconds is still awfully long.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11735#comment:27>
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