Re: [GHC] #9630: compile-time performance regression (probably due to Generics)

17 Dec 2015

      #9630: compile-time performance regression (probably due to Generics)
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.9
      Resolution:                    |             Keywords:  deriving-perf
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9583, #10293     |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by ezyang):

 I did some investigation related to this bug on GHC 7.10.3-ish and binary
 0.7.5.0 (yes it's old, but I didn't see any more recent relevant commits;
 and it's what GHC is bootstrapping with). Here are some partial findings:

 1. I can trigger bad constant factors this data type:
 {{{
 {-# LANGUAGE DeriveGeneric #-}
 module A where
 import Data.Binary
 import GHC.Generics
 data T = T
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  deriving Generic
 instance Binary T
 }}}
 {{{
 ezyang@sabre:~/Dev/labs$ time $DEV/ghc-7.10-frontend-plugins/usr/bin/ghc
 --make Bin.hs -O2  -fforce-recomp -fliberate-case-threshold=1000
 [1 of 1] Compiling A                ( Bin.hs, Bin.o )

 real    0m7.369s
 user    0m6.408s
 sys     0m0.224s
 ezyang@sabre:~/Dev/labs$ time $DEV/ghc-7.10-frontend-plugins/usr/bin/ghc
 --make Bin.hs -O0  -fforce-recomp -fliberate-case-threshold=1000
 [1 of 1] Compiling A                ( Bin.hs, Bin.o )

 real    0m0.881s
 user    0m0.776s
 sys     0m0.080s
 }}}

 2. The problem gets worse if you have explicit field names, something like
 a x2 factor.
 {{{
 data T = T
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  () () () () () () () () () ()
  deriving Generic

 -- versus

 data T = T {
     f0 :: (),
     f1 :: (),
     f2 :: (),
     f3 :: (),
     f4 :: (),
     f5 :: (),
     f6 :: (),
     f7 :: (),
     f8 :: (),
     f9 :: (),
     f10 :: (),
     f11 :: (),
     f12 :: (),
     f13 :: (),
     f14 :: (),
     f15 :: (),
     f16 :: (),
     f17 :: (),
     f18 :: (),
     f19 :: (),
     f20 :: (),
     f21 :: (),
     f22 :: (),
     f23 :: (),
     f24 :: (),
     f25 :: (),
     f26 :: (),
     f27 :: (),
     f28 :: (),
     f29 :: (),
     f30 :: (),
     f31 :: (),
     f32 :: (),
     f33 :: (),
     f34 :: (),
     f35 :: (),
     f36 :: (),
     f37 :: (),
     f38 :: (),
     f39 :: ()
     }
     deriving (Generic)
 }}}
 {{{
 ezyang@sabre:~/Dev/labs$ time $DEV/ghc-7.10-frontend-plugins/usr/bin/ghc
 --make Bin.hs -O  -fforce-recomp -fliberate-case-threshold=1000
 [1 of 1] Compiling A                ( Bin.hs, Bin.o )

 real    0m2.649s
 user    0m2.520s
 sys     0m0.080s
 ezyang@sabre:~/Dev/labs$ time $DEV/ghc-7.10-frontend-plugins/usr/bin/ghc
 --make Bin.hs -O  -fforce-recomp -fliberate-case-threshold=1000
 [1 of 1] Compiling A                ( Bin.hs, Bin.o )

 real    0m4.990s
 user    0m4.824s
 sys     0m0.144s
 }}}
    It's NOT just generic deriving; the generic deriving is still pretty
 speedy, honestly.

 3. If you inline all of Binary's class definitions into the same file,
 things speed up. Comparing the two cases, it seems that when we cross-
 module compile, GHC does more inlining and more optimization, and it takes
 longer. This is something like a 30% factor.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9630#comment:21
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