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

23 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
      Resolution:                    |         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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Comment (by Patrick Palka ):

 In [changeset:84dff71075b915938e2457f5bff2d127eac3b3cc/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="84dff71075b915938e2457f5bff2d127eac3b3cc"
 Fix the definition of cas() on x86 (#8219)

 *p is both read and written to by the cmpxchg instruction, and therefore
 should be given the '+' constraint modifier.

 (In GCC's extended ASM language, '+' means that the operand is both read
 and written to whereas '=' means that it is only written to.)

 Otherwise, the compiler is allowed to rewrite something like

 SpinLock lock;
 initSpinLock(&lock);       /* sets lock = 1 */
 ACQUIRE_SPIN_LOCK(&lock);

 into

 SpinLock lock;
 ACQUIRE_SPIN_LOCK(&lock);

 because according to the asm statement, the previous value of 'lock' is
 not important.
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8219#comment:4
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