[GHC] #8311: suboptimal code generated for even :: Int -> Bool by NCG (x86, x86_64)

[GHC]

#8311: suboptimal code generated for even :: Int -> Bool by NCG (x86, x86_64)
------------------------------------+-------------------------------------
       Reporter:  rwbarton          |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler (NCG)    |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 This appears to be the x86 assembly for a specialization for `even :: Int
 -> Bool` that's produced by a SPECIALISE pragma for `(^) :: Int -> Int ->
 Int` in GHC.Real. The relevant bit is offsets `0x2c` through `0x3b`.

 {{{

 0000000c <base_GHCziReal_evenzuzdseven1_info>:
    c:   8b 45 00                mov    0x0(%ebp),%eax
    f:   c7 45 00 2c 00 00 00    movl   $0x2c,0x0(%ebp)
                         12: R_386_32    .text
   16:   89 c6                   mov    %eax,%esi
   18:   f7 c6 03 00 00 00       test   $0x3,%esi
   1e:   75 0c                   jne    2c <c8gO_info>
   20:   ff 26                   jmp    *(%esi)
   22:   66 90                   xchg   %ax,%ax
   24:   00 00                   add    %al,(%eax)
   26:   00 00                   add    %al,(%eax)
   28:   20 00                   and    %al,(%eax)
         ...

 0000002c <c8gO_info>:
   2c:   b8 02 00 00 00          mov    $0x2,%eax
   31:   89 c1                   mov    %eax,%ecx
   33:   8b 46 03                mov    0x3(%esi),%eax
   36:   99                      cltd
   37:   f7 f9                   idiv   %ecx
   39:   85 d2                   test   %edx,%edx
   3b:   75 0b                   jne    48 <c8gO_info+0x1c>
   3d:   be 02 00 00 00          mov    $0x2,%esi
                         3e: R_386_32    ghczmprim_GHCziTypes_True_closure
   42:   83 c5 04                add    $0x4,%ebp
   45:   ff 65 00                jmp    *0x0(%ebp)
   48:   be 01 00 00 00          mov    $0x1,%esi
                         49: R_386_32    ghczmprim_GHCziTypes_False_closure
   4d:   83 c5 04                add    $0x4,%ebp
   50:   ff 65 00                jmp    *0x0(%ebp)
 }}}

 It would be much better to do a simple `testb  $0x1,0x3(%esi)` in place of
 the first six instructions of `c8gO_info`.

 The NCG generates similar code on x86_64.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8311>
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