[concurrency-interest] Volatile happens before question

David Holmes davidcholmes at aapt.net.au
Tue Jan 17 18:37:13 EST 2012


It may be a silly thing to do but I don't think there is anything to stop
the compiler from reordering as if the code were written:

if (!a) {
  bStore = b;
  print ..
} else {
 bStore = b;
}

David
  -----Original Message-----
  From: Vitaly Davidovich [mailto:vitalyd at gmail.com]
  Sent: Wednesday, 18 January 2012 9:32 AM
  To: dholmes at ieee.org
  Cc: Raph Frank; concurrency-interest at cs.oswego.edu
  Subject: Re: [concurrency-interest] Volatile happens before question


  No. You lost the transitive relationship of happens-before with your
second statement. Your code can be reordered and executed as follows:

  Thread 1 Thread 2 if (!a) // sees false a = true b = 1; bStore =b; print
// prints 1

  This actually isn't valid because in original code nothing would print if
a is false - it doesn't enter the if block.  However, compiler can simply
move b=1 before a =true as that's permissible and maybe it'll do that for
some register allocation purposes, as an example.

  Sent from my phone
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