[GHC] #13720: INLINE pragma semantics changed since 8.0.2

[GHC]

#13720: INLINE pragma semantics changed since 8.0.2
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             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:
-------------------------------------+-------------------------------------
 It now seems that unfoldings for INLINE things are optimised slightly.
 This causes different interactions with `RULES` than before.

 {{{#!hs
 module A where

 {-# INLINE f #-}
 f x = h x

 h x = x

 {-# RULES "h x" forall x . h x = error "REWRITE" #-}
 }}}

 {{{#!hs
 module B where

 import A

 qux = f 5
 }}}

 Then running

 {{{
 > ghc-8.0.2 B.hs -O2 -fforce-recomp -ddump-simpl | grep qux -A5
 qux :: Integer
 [GblId, Str=DmdType x]
 qux =
   error ....
 }}}

 {{{
 > ghc-8.2.0.20170507 B.hs -O2 -fforce-recomp -ddump-simpl  | grep qux -A5
 qux :: Integer
 [GblId,
  Caf=NoCafRefs,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 100 0}]
 qux = 5
 }}}

 Inspecting the unfoldings, we see that 8.2 optimises the unfoldings to
 inline `h` before the rule can apply.

 {{{
 d61439f58ce9c5a268304423a43b9b44
   f :: p -> p
   {- Arity: 1, HasNoCafRefs, Strictness: <S,1*U>,
      Inline: (sat-args=1),
      Unfolding: InlineRule (1, False, True) (\ @ p (x :: p) -> x) -}
 }}}


 Is this new behaviour intentional? It seems possible that it will break
 some programs which use rewrite rules.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13720>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler