[GHC] #16005: Don't use a generic apply thunk for known calls

6 Dec 2018

      #16005: Don't use a generic apply thunk for known calls
-------------------------------------+-------------------------------------
           Reporter:  sgraf          |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  Phab:D5414     |         Wiki Page:
-------------------------------------+-------------------------------------
 Currently, an AP thunk like `sat = f a b c` will not have its own entry
 point and info pointer and will instead reuse a generic apply thunk
 like `stg_ap_4_upd`.

 That's great from a code size perspective, but if `f` is a known
 function, a specialised entry point with a plain call can be much faster
 than figuring out the arity and doing dynamic dispatch.

 ----

 I prepared a patch over at Phab:D5414 that fixes this by checking if the
 arity of `f` is unknown (e.g. 0). If not, it will no longer delegate to a
 generic apply thunk and generate regular entry code and info tables
 instead.

 No impact on allocations, but on counted instructions and code size.
 Significant changes to counted instructions:

 ||cryptarithm1 || -2.5%||
 ||lcss || -2.3%||
 ||paraffins || -3.8%||
 ||wheel-sieve2 || -3.4%||
 ||||||
 ||Min || -3.8%||
 ||Max || +0.0%||
 || Geometric Mean || -0.2%||

 And changes to binary size greater than 0.1% (all the other programs,
 basically):

 ||       Program           ||Size      || Instrs||
 ||           anna          ||+0.3%     ||-0.2%||
 ||         expert          ||+0.2%     ||-0.0%||
 ||          fluid          ||+0.2%     ||-0.1%||
 ||           grep          ||+0.2%     ||-0.0%||
 ||          infer          ||+0.2%     ||-0.4%||
 ||     last-piece          ||+0.2%     ||-0.1%||
 ||           lift          ||+0.2%     ||-0.0%||
 ||      paraffins          ||+0.2%     ||-3.8%||
 ||         prolog          ||+0.2%     ||-0.1%||
 ||            scs          ||+0.3%     ||-0.0%||
 ||      transform          ||+0.2%     ||-0.2%||
 ||        veritas          ||+0.2%     ||+0.0%||
 ||||||||
 ||            Min          ||+0.1%     ||-3.8%||
 ||            Max          ||+0.3%     ||+0.0%||
 || Geometric Mean          ||+0.1%     ||-0.2%||

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16005
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