[GHC] #13725: Remove false dependency on the destination of the popcnt instruction

[GHC]

#13725: Remove false dependency on the destination of the popcnt instruction
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  Phab:D3539     |         Wiki Page:
-------------------------------------+-------------------------------------
 fryguybob writes in Phab:D3539,

 > Some Intel processors appear to have a false dependency on the
 destination of the popcnt instruction. This could lead to poor
 performance. A simple way to prevent this is to clear the destination
 register immediately before the popcnt instruction. Currently I can't
 produce code from GHC where the source and destination registers are not
 the same (perhaps someone is interested in producing a test case that
 does). I'm putting this here in case anyone is interested in investigating
 further.

 I'm opening this ticket so I can bump the diff out of the review queue, in
 hopes that someone might some day pick it up.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13725>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler