[GHC] #10922: String inlining is inconsistent

[GHC]

#10922: String inlining is inconsistent
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           Reporter:  xnyhps         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:  x86_64         |   Type of failure:  Runtime
  (amd64)                            |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |
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 I have the following code:

 A.hs:
 {{{#!hs
 module A where

 foo :: String
 {-# INLINE foo #-}
 foo = "foo"
 }}}

 B.hs:
 {{{#!hs
 import A

 main = if foo == "foo" then return () else putStrLn "Wrong"
 }}}

 Compiling this with `-O2` shows the if-statement has not been eliminated
 (`"Wrong"` is still part of the output with `-ddump-simpl`).

 However, if I rewrite the code to:

 A.hs:
 {{{#!hs
 module A where

 foo :: () -> String
 {-# INLINE foo #-}
 foo () = "foo"
 }}}

 B.hs:
 {{{#!hs
 import A

 main = if foo () == "foo" then return () else putStrLn "Wrong"
 }}}

 Then the if-statement disappears and the generated Core has no string
 literals left.

 Even when the literal can not be eliminated from B it seems GHC is fine
 with duplicating it accross two modules if it occurs somewhere inside a
 function. But once I make it a top-level expression of type `String`,
 inlining it does not happen. I can not think of how this could be working
 as intended.

 (The case where `A.foo` is `""` seems to work differently, but also does
 not get optimized away.)

 (This happened to me when rewriting some code that previously used `CPP`
 for conditional compilation to using `if System.Info.os == "darwin" then
 ... else ...` and expecting the other OS cases to get optimized away.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10922>
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