[GHC] #9798: Frustrating behaviour of the INLINE pragma

[GHC]

#9798: Frustrating behaviour of the INLINE pragma
-------------------------------------+-------------------------------------
       Reporter:  mojojojo           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:  MacOS X
   Architecture:  x86_64 (amd64)     |         Type of failure:  Runtime
     Difficulty:  Unknown            |  performance bug
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 So I have a function "a", which uses another function "b" from another
 module.

 * Step 1. I benchmark its performance and get 250ns.

 * Step 2. I go and put the "INLINE" pragma on the function "b", run the
 benchmark again and get 500ns. That is twice as long.

 * Step 3. I go and add an explicit invocation of the
 [http://hackage.haskell.org/package/base-4.7.0.1/docs/GHC-
 Exts.html#v:inline inline] function over function "b" in function "a" and
 finally get the desired optimization: 208ns.

 You can reproduce the issue by executing {{{cabal bench decoding
 --benchmark-options=numeric}}} after cloning the trees of the following
 commits:

 * [https://github.com/nikita-volkov/postgresql-
 binary/commit/31e08b4f6887445c28118d25f9da808e7ca9fc86 Step 1]

 * [https://github.com/nikita-volkov/postgresql-
 binary/commit/8f29b8daebc3161dc0fa35fbd70d3f3305370a4e Step 2]

 * [https://github.com/nikita-volkov/postgresql-
 binary/commit/3d3e4378bcd8ea85cda868d6a4bf07a76a9cad2c Step 3]

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9798>
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