[GHC] #10346: Cross-module SpecConstr

[GHC]

#10346: Cross-module SpecConstr
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        Reporter:  simonpj           |                Owner:  ckoparkar
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.1
      Resolution:                    |             Keywords:  SpecConstr,
                                     |  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13016            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by sgraf):

 Replying to [comment:18 ckoparkar]:
 > Now that I'm looking at the proper thing, I'm a bit confused and have
 the same question as ak3n did.

 I blame the example.

 1. After SpecConstr runs on M, the call isn't really recursive anymore:
 There will be a specialisation for the call pattern `[x, y] |> [True, (x,
 y)]`. The resulting function is non-recursive, inlined, and suddenly foo
 itself isn't recursive anymore and simply gets inlined in X.
 2. The `baz` binding in M will make it so that there is a specialisation
 for the call pattern `[x,y] |> [False, (x,y)]` anyway. The call in X
 matches this pattern, so will be specialised appropriately.

 Try this instead:

 {{{#!hs
 module M where

 {-# INLINABLE foo #-}
 foo 0 y = y
 foo n y = foo (n-1) y

 --baz = foo 14 (2,2)

 ---------------------

 module X where

 import M

 bar = foo 2 (3,4)
 }}}

 This works better, because integer literals are not currently considered
 values (probably to avoid code bloat through endless loop unrolling).
 Notice that the call in X specialises iff you comment in `baz` in M which
 has the same call pattern.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10346#comment:19>
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