RFR: 8163014: Mysterious/wrong value for "long" frame local variable on 64-bit

Max Ockner max.ockner at oracle.com
Thu Sep 8 16:26:46 UTC 2016

Here is version 2 of this fix.
  - Added copyright to LocalLongHelper.java
  - Switched LocalLongTest.java to run with -Xint instead of -Xcomp, and 
cleaned up some copypaste garbage.
  - changed the test directory name from locallong to LocalLong.

Reran test with -Xint.


On 9/7/2016 9:47 AM, Coleen Phillimore wrote:
> Max,
> http://cr.openjdk.java.net/~mockner/8163014.01/test/runtime/locallong/LocalLongHelper.java.html 
> There's a lot in this test, but I can't think of a different way to 
> inspect the stack to find out if the long unused slot is zeroed. This 
> file is missing a copyright, and I think the directory should be 
> LocalLong (I think our new convention of test directory names is 
> capitalized).  I think we should keep the test in the hotspot 
> repository even though it uses the StackWalk API because otherwise it 
> won't be run with hotspot changes on an ad-hoc or nightly basis.
> http://cr.openjdk.java.net/~mockner/8163014.01/test/runtime/locallong/LocalLongTest.java.html 
> I thought the idea was to run this with -Xint not -Xcomp?
> The interpreter changes look good.
> Coleen
> On 9/6/16 5:33 PM, Max Ockner wrote:
>> Hello,
>> Please review this multi-platform fix for the stack walk API.
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8163014
>> Webrev: http://cr.openjdk.java.net/~mockner/8163014.01/
>> In 64 bits, long values can fit into a single slot, but two slots are 
>> still used. The high slot contains garbage. This normally wouldn't 
>> matter since it is never read from but the stack walk API expects to 
>> display useful information. This is an issue when displaying longs 
>> from local variables, so this means we can kill any garbage off by 
>> zeroing it when it is pushed to the stack in the previous frame. This 
>> solution zeroes the high byte of a long value when it is being pushed 
>> to the stack (in push_l).
>> This applies to x86, aarch64, and sparc. This change does not apply 
>> to ppc, though the bug almost certainly does affect it. I have left 
>> ppc untouched since I don't have access to the hardware required to 
>> reproduce the bug and test the fix.
>> I have adapted the reproducer from the bug into a test. It is 
>> curently sitting in runtime/locallong, but I believe there must be a 
>> better place for it.
>> Thanks,
>> Max

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