[GHC] #12116: No easy way to add cost-centre to top-levels

25 May 2016

      #12116: No easy way to add cost-centre to top-levels
-------------------------------------+-------------------------------------
           Reporter:  osa1           |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 As far as I can see from the user manual (I also tried placing some
 `SCC`s), there's no way to easily add cost-centres to top-level
 definitions. We can do things like:

 {{{
 f x y z = ...
 =>
 f = {-# SCC f #-} \x y z -> ...
 }}}

 but it becomes too much work once we have "equations" style definitions
 like

 {{{
 head [] = {-# SCC head #-} Nothing
 head (x : _) = {-# SCC head #-} Just x
 }}}

 Also, `where` clauses work differently after the transformations. Example:

 {{{
 f x y z = ...
   where <can use x and y here>
 =>
 f = {-# SCC f #-} \x y z -> ...
   where <can't use x and y here>
 }}}

 So it gets annoying real quick. It'd be really great if we could just add
 `{-# SCC ... #-}` just like `{-# INLINE ... #-}` pragmas.

 Any ideas/opinions?

 Looking at the code, it seems like `-fprof-auto` and similar flags are
 adding ticks to `FunBind.fun_tick`. We could do the same with this new
 `SCC` pragma.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12116>
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