[GHC] #13589: Possible inconsistency in CSE's treatment of NOINLINE

[GHC]

#13589: Possible inconsistency in CSE's treatment of NOINLINE
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           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           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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 While debugging #13535 I noticed the following inconsistency in CSE:

 `Note [CSE for INLINE and NOINLINE]` states that we need to take care when
 adding expressions bound to binders with inline pragmas to the `CSEnv`. To
 see why, consider the following,
 {{{#!hs
 {-# NOINLINE bar #-}
 bar = <rhs>     -- Same rhs as foo

 foo = <rhs>
 }}}
 Given this program, we need to avoid producing `foo = bar` since doing so
 would mean that we would lose the ability to inline `foo`'s original RHS.

 The note then goes on to give the following rule,
 > We should not add
 >
 > {{{<rhs> :-> bar}}}
 >
 > to the CSEnv if `bar` has any constraints on when it can inline;
 > that is, if its 'activation' not always active.  Otherwise we
 > might replace `<rhs>` by `bar`, and then later be unable to see that it
 > really was `<rhs>`.
 This rule is implemented in `noCSE` with,
 {{{#!hs
   not (isAlwaysActive (idInlineActivation id))
 }}}

 However, it's quite unclear to me that this rule avoids the issue we set
 out to solve. Afterall, `NOINLINE bar` is always active, but it still
 means that rewriting `foo` to `foo=bar` would lose us the ability to see
 `foo`'s original RHS.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13589>
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