[GHC] #10677: slightly silly assembly for testing whether a Word# is 0##

[GHC]

#10677: slightly silly assembly for testing whether a Word# is 0##
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              Reporter:  rwbarton    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  low         |         Milestone:
             Component:  Compiler    |           Version:  7.11
  (CodeGen)                          |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:  x86_64      |   Type of failure:  Runtime
  (amd64)                            |  performance bug
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 {{{
 {-# LANGUAGE MagicHash #-}

 module Zero where

 import GHC.Exts

 f :: Word# -> Word#
 f 0## = 1##
 f x## = x##
 }}}
 in HEAD produces
 {{{
 0000000000000018 <ZZero_f_info>:
   18:   49 83 fe 01             cmp    $0x1,%r14
   1c:   72 06                   jb     24 <ZZero_f_info+0xc>
   1e:   4c 89 f3                mov    %r14,%rbx
   21:   ff 65 00                jmpq   *0x0(%rbp)
   24:   bb 01 00 00 00          mov    $0x1,%ebx
   29:   ff 65 00                jmpq   *0x0(%rbp)
 }}}
 Well, `cmp $0x1,%r14/jb` isn't wrong. But it's a byte longer than `test
 %r14,%r14/je`, so the latter should be preferred.

 GHC 7.10 produced the `test/je` version, so I'm guessing this is a side
 effect of nomeata's work on #10137.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10677>
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