[PATCH] 8202414: Unsafe crash in C2

[Andy Law]

Hi Lutz and Andrew,

Thank you for your reply and sorry for my typos :)

TL;DR
I think it is the optimization of `clear_memory()`which cause this problem, in my understanding it may not be a user fault :)

When running the example on the bug list using `-XX:DisableIntrinsic=_putInt`, or use interpreter/C1 only will make this problem go away, due to the fact that program will go to another branch.

In function `clear_memory()`, it will make an optimization which will clear the context of the memory, in fact

    if (done_offset > start_offset) {  // [1]
        // it will clear the memory from start_offset to done_offset
    }

    if (done_offset < end_offset) {  // [2]
        // it will clear the memory by using a Int (0) to clear the memory of done_offset
    }

|<--------------- 16-byte header —--——>| <—— 4-byte arr length (new byte[397]) ———>
                                                              |       0000     0001     1000     1101     |
If it is aligned, it won’t have any problem but, if it is not aligned as the example, this optimization will mis-clear the `0000 0001` to `0000 0000`, so the array length becomes 141. Then it will crash when gc happens.

It is the optimization which cause this problem, so when it is not aligned, we don’t do this optimization for this unaligned address may solve the problem.
By the way I didn’t find the unaligned message on the doc:( but I think you’re right and it should be aligned when using Unsafe, though it is an deprecated API :) It won’t be reproduced using the templateInterpreter or C1 compiler, due to the fact that they won’t do this optimization.

Thank you:),
Andy
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