[GHC] #12743: Profiling wrongly attributes allocations to a function with Int# result

20 Oct 2016


      #12743: Profiling wrongly attributes allocations to a function with Int# result
-------------------------------------+-------------------------------------
           Reporter:                 |             Owner:
  MikolajKonarski                    |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Profiling can't decide whether the given function allocates or not at all.
 The code snippet says it all:

 {{{
 {-# LANGUAGE MagicHash #-}
 import GHC.Exts

 -- ghc -prof --make bork.hs -fprof-auto -O1
 -- ./bork +RTS -s -P -RTS

 main = do
   let f :: Int -> Int#
 --      {-# NOINLINE f #-}
       f x = case x `div` 17 of I# i -> i
 --      {-# NOINLINE g #-}
       g h = sum $ map (\y -> I# (h y)) [0..1000000]
   g f `seq` return ()

 -- COST CENTRE MODULE  %time %alloc  ticks     bytes
 -- main.g      Main     73.6   93.3    159 224000328
 -- main.f      Main     24.1    6.7     52  16000016
 -- main.g.\    Main      2.3    0.0      5         0

 -- When either of the NOINLINE is enabled, the profile becomes:

 -- COST CENTRE MODULE  %time %alloc  ticks     bytes
 -- main.g      Main     72.9   93.3    161 224000328
 -- main.f      Main     16.7    0.0     37         0
 -- main.g.\    Main     10.4    6.7     23  16000016

 -- So, does f actually allocate or not?
 -- Profiling is quite a bit less useful if it can't answer that reliably,
 -- because in larger code snippets it's hard to decide from source
 -- or from Core/STG.
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12743
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