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    <p><strong>A long, in 32bit architectures, is represented by two words</strong>. Now, imagine what could happen if due to insufficient synchronization, the execution of the switch statement observes a long with its high 32 bits from one write, and the 32 low ones from another! It could try to go to ....who knows where! Basically somewhere at random. Even if both writes represented valid cases for the switch statement, their funny combination would probably lead neither to the first nor to the second -- or extremely worse, it could lead to another valid, but unrelated case!</p> <p>At least with an int (or lesser types), no matter how badly you mess up, the switch statement will at least read a value that <em>someone actually wrote</em>, instead of a value "out of thin air".</p> <p>Of course, I don't know the actual reason (it's been more than 15 years, I haven't been paying attention that long!), but if you realize how unsafe and unpredictable such a construct could be, you'll agree that this is a definitely <strong>very good reason</strong> not to <em>ever</em> have a switch on longs (and as long -pun intended- there will be 32bit machines, this reason will remain valid).</p>
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    1. POWhy can't your switch statement data type be long, Java?
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    1. USDimitris Andreou
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      1. VTDownMod
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    1. COI don't think this follows. The value being switched on needs to be calculated and stored in a register or on the stack. If that value is calculated based on data accessed by multiple threads, this *calculation* needs to be made thread-safe, whatever the width of the result. But then, once that result is in a register or on the stack, it's only accessed by the switching thread and is safe.
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      1. USNeil Coffey
    2. CONeil, your argument is quite confused: "But then, once that result is in a register or on the stack, it's only accessed by the switching thread and is safe". Sure, using that value is thread-safe! But my point is that *that value can _already_ be wrong due to synchronization bugs in user code*. Using thread-safely a wrong value is less than useful :) This issue can never be eliminated: buggy concurrent code could already have produced the "out of thin air"/wrong long value, which can be subsequently used in the switch, making the switch go to a case address _nobody ever specified_.
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      1. USDimitris Andreou
    3. CODimitris, maybe there´s something in your argument I'm not understanding. The value switched on could indeed be wrong due to synchronization bugs in user code. But I don't believe there's anything inherent about the switch statement that makes this more likely than in other cases. And thinking it through as best I can, I don't believe that the non-atomicity of hi/low words of long reads/writes to memory is in fact an issue. (Thinking about things antoher way: you could decide that an if comparison on a long was not allowed based on the same argument.)
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      1. USNeil Coffey
 

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