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

Max Ockner max.ockner at oracle.com
Fri Sep 9 16:51:43 UTC 2016

Oops. Here is version 2: 


On 9/8/2016 12:26 PM, Max Ockner wrote:
> 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.
> Thanks,
> Max
> 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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