[GHC] #16036: expDouble## 0.0## doesn't get complied into 1.0##

12 Dec 2018

      #16036: expDouble## 0.0## doesn't get complied into 1.0##
-------------------------------------+-------------------------------------
           Reporter:  Fuuzetsu       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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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 {{{
 [nix-shell:/tmp]$ cat T.hs
 {-# LANGUAGE MagicHash #-}
 {-# OPTIONS_GHC -O2 -ddump-simpl -ddump-to-file #-}
 module T (f) where

 import GHC.Float
 import GHC.Prim

 f :: Double
 f = D# (expDouble# 0.0## -## 1.0##)

 [nix-shell:/tmp]$ ghc -fforce-recomp T.hs
 [1 of 1] Compiling T                ( T.hs, T.o )

 [nix-shell:/tmp]$ cat T.dump-simpl

 ==================== Tidy Core ====================
 2018-12-12 02:23:40.288094743 UTC

 Result size of Tidy Core
   = {terms: 20, types: 6, coercions: 0, joins: 0/0}

 -- RHS size: {terms: 5, types: 0, coercions: 0, joins: 0/0}
 f :: Double
 [GblId,
  Str=m,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 15 20}]
 f = GHC.Types.D# (-## (expDouble# 0.0##) 1.0##)

 …
 }}}

 This was spotted in real code where we had GHC generate something like

 {{{#!haskell
 1# ->
   jump $j9_s1IVU
     (GHC.Prim.+##
        sc3_s2pVI
        (GHC.Prim.-##
           (GHC.Prim.expDouble#
              0.0##)
           1.0##))
 }}}

 I would expect this (I think reasonably) to be just `{{{jump $j9_s1IVU
 sc3_s2pVI}}}`.

 Maybe {{{expDouble## <literal>##}}} should always evaluate at compile time
 to not block further constant folding?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16036
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