[GHC] #8219: x86 definition of cas() is wrong

3 Sep 2013

      #8219: x86 definition of cas() is wrong
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       Reporter:  parcs             |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Runtime System    |           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:                    |
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 On x86, `cas()` is defined in SMP.h as (paraphrasing)

 {{{
 #!c
 asm ("lock\ncmpxchg %3,%1"
      :"=a"(o), "=m" (*p)
      :"0"(o), "r"(n));
 }}}

 But *p is both read and written to, and therefore should have the '+'
 constraint modifier as opposed to the '=' modifier. Otherwise, the
 compiler is allowed to elide an earlier write to *p because it thinks that
 the call to `cas()` will overwrite it.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8219
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