[GHC] #14830: Use test instead of cmp for comparison against zero.

[GHC]

#14830: Use test instead of cmp for comparison against zero.
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           Reporter:  AndreasK       |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:
  (NCG)                              |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 For comparisons `test r, r` and `cmp r, $0` are interchangeable at least
 on x86.

 However using test leads to slightly smaller code. (Saves a byte per
 comparison).

 A good starting point is stmtToInstrs in compiler/nativeGen/X86/CodeGen
 starting with how CmmCondBranch is compiled to Instructions.

 From there it should be not too hard to track down the required changes.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14830>
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