[GHC] #9188: quot with a power of two is not optimized to a shift

[GHC]

#9188: quot with a power of two is not optimized to a shift
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        Reporter:  tibbe                    |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler                 |          Version:  7.8.2
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:  #8311
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Changes (by rwbarton):

 * related:   => #8311


Comment:

 See also #8311.

 Note that `quot` by 2 rounds towards zero, so for negative `Int`s it's not
 quite just a shift. The LLVM backend does handle this well, producing this
 sequence for the division:
 {{{
   7c:   48 89 d0                mov    %rdx,%rax
   7f:   48 c1 e8 3f             shr    $0x3f,%rax
   83:   48 01 d0                add    %rdx,%rax ; add sign bit to move
 towards 0
   86:   48 d1 f8                sar    %rax
 }}}

 A `div` by 2 really would be a single arithmetic shift, but at the Cmm
 level there is still a function call to `divInt#` :(

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9188#comment:1>
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