Re: [GHC] #9020: Massive blowup of code size on trivial program

24 Apr 2014

      #9020: Massive blowup of code size on trivial program
-------------------------------------+------------------------------------
        Reporter:  simonpj           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"79e46aea1643b4dfdc7c846bbefe06b83b535efd/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="79e46aea1643b4dfdc7c846bbefe06b83b535efd"
 Don't eta-expand PAPs (fixes Trac #9020)

 See Note [Do not eta-expand PAPs] in SimplUtils.  This has a tremendously
 good effect on compile times for some simple benchmarks.

 The test is now where it belongs, in perf/compiler/T9020 (instead of
 simpl015).

 I did a nofib run and got essentially zero change except for cacheprof
 which
 gets 4% more allocation.  I investigated.  Turns out that we have

     instance PP Reg where
        pp ppm ST_0 = "%st"
        pp ppm ST_1 = "%st(1)"
        pp ppm ST_2 = "%st(2)"
        pp ppm ST_3 = "%st(3)"
        pp ppm ST_4 = "%st(4)"
        pp ppm ST_5 = "%st(5)"
        pp ppm ST_6 = "%st(6)"
        pp ppm ST_7 = "%st(7)"
        pp ppm r    = "%" ++ map toLower (show r)

 That (map toLower (show r) does a lot of map/toLowers.  But if we inline
 show
 we get something like

        pp ppm ST_0 = "%st"
        pp ppm ST_1 = "%st(1)"
        pp ppm ST_2 = "%st(2)"
        pp ppm ST_3 = "%st(3)"
        pp ppm ST_4 = "%st(4)"
        pp ppm ST_5 = "%st(5)"
        pp ppm ST_6 = "%st(6)"
        pp ppm ST_7 = "%st(7)"
        pp ppm EAX  = map toLower (show EAX)
        pp ppm EBX  = map toLower (show EBX)
        ...etc...

 and all those map/toLower calls can now be floated to top level.
 This gives a 4% decrease in allocation.  But it depends on inlining
 a pretty big 'show' function.

 With this new patch we get slightly better eta-expansion, which makes
 a function look slightly bigger, which just stops it being inlined.
 The previous behaviour was luck, so I'm not going to worry about
 losing it.

 I've added some notes to nofib/Simon-nofib-notes
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9020#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler