[GHC] #9435: x86 sse4.2 popCnt16# needs to zero-extend its result

[GHC]

#9435: x86 sse4.2 popCnt16# needs to zero-extend its result
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              Reporter:  rwbarton    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.9
  (NCG)                              |         Keywords:
            Resolution:              |     Architecture:  x86_64 (amd64)
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Incorrect   |  Related Tickets:
  result at runtime                  |
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D147   |
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Comment (by Reid Barton <rwbarton@…>):

 In [changeset:"64151913f1ed32ecfe17fcc40f7adc6cbfbb0bc1/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="64151913f1ed32ecfe17fcc40f7adc6cbfbb0bc1"
 x86: zero extend the result of 16-bit popcnt instructions (#9435)

 Summary:
 The 'popcnt r16, r/m16' instruction only writes the low 16 bits of
 the destination register, so we have to zero-extend the result to
 a full word as popCnt16# is supposed to return a Word#.

 For popCnt8# we could instead zero-extend the input to 32 bits
 and then do a 32-bit popcnt, and not have to zero-extend the result.
 LLVM produces the 16-bit popcnt sequence with two zero extensions,
 though, and who am I to argue?

 Test Plan:
  - ran "make TEST=cgrun071 EXTRA_HC_OPTS=-msse42"
  - then ran again adding "WAY=optasm", and verified that
    the popcnt sequences we generate match the ones produced
    by LLVM for its @llvm.ctpop.* intrinsics

 Reviewers: austin, hvr, tibbe

 Reviewed By: austin, hvr, tibbe

 Subscribers: phaskell, hvr, simonmar, relrod, ezyang, carter

 Differential Revision: https://phabricator.haskell.org/D147

 GHC Trac Issues: #9435
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9435#comment:3>
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