[GHC] #14532: Missing constant folding for Numeric.Natural

27 Nov 2017

      #14532: Missing constant folding for Numeric.Natural
-------------------------------------+-------------------------------------
           Reporter:  reinerp        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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:
-------------------------------------+-------------------------------------
 The following Haskell code

 {{{#!hs
 module Zero where

 import Numeric.Natural

 zero :: Natural
 zero = 0
 }}}

 generates this Core:

 {{{#!hs
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 zero1
 zero1 = 0

 -- RHS size: {terms: 39, types: 12, coercions: 0, joins: 0/0}
 zero
 zero
   = case zero1 of {
       S# i#_a2c6 ->
         case tagToEnum# (>=# i#_a2c6 0#) of {
           False -> underflowError;
           True -> NatS# (int2Word# i#_a2c6)
         };
       Jp# dt_a2cg ->
         case uncheckedIShiftRL# (sizeofByteArray# dt_a2cg) 3# of {
           __DEFAULT ->
             case sizeofByteArray# dt_a2cg of {
               __DEFAULT -> NatJ# dt_a2cg;
               0# -> underflowError
             };
           1# ->
             case indexWordArray# dt_a2cg 0# of wild2_a2ck { __DEFAULT ->
             NatS# wild2_a2ck
             }
         };
       Jn# ipv_a2cn -> underflowError
     }
 }}}

 It should instead generate this:

 {{{#!hs
 zero = NatS# 0##
 }}}

 This kind of code is generated whenever one uses `GHC.TypeLits.natVal`,
 even at a constant `Nat`.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14532
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler