RFR: 8193222: EnsureLocalCapacity() should maintain capacity requests through multiple calls

[David Holmes]

Thanks Dan! Nits will be fixed.

David

On 12/12/2017 11:00 AM, Daniel D. Daugherty wrote:
> On 12/10/17 12:39 AM, David Holmes wrote:
>> bug: https://bugs.openjdk.java.net/browse/JDK-8193222
>> webrev: http://cr.openjdk.java.net/~dholmes/8193222/webrev/
> 
> make/test/JtregNativeHotspot.gmk
>      No comments.
> 
> src/hotspot/share/prims/jniCheck.cpp
>      No comments.
> 
> test/hotspot/jtreg/runtime/jni/checked/TestCheckedEnsureLocalCapacity.java
>      L42:     // If copies > capacity + warning-threshold then we still 
> get a warning
>          Nit: Needs a period at the end.
> 
>      L75:         //Warning
>          Nit: Needs a space before 'W'.
> 
> test/hotspot/jtreg/runtime/jni/checked/libTestCheckedEnsureLocalCapacity.c
>      No comment.
> 
> Thumbs up! Your choice on whether to fix the nits.
> Don't need to see another webrev even if you do...
> 
> Dan
> 
> 
>>
>> In product mode, for hotspot, EnsureLocalCapacity is a no-op as there 
>> is no artificial limit applied for local refs.
>>
>> With -Xcheck:jni the checked mode was augmented to watch for 
>> "excessive" use of local refs and to produce a warning if that 
>> happened e.g.:
>>
>>  WARNING: JNI local refs: 41, exceeds capacity: 40
>>
>> That was added under JDK-8043224.
>>
>> The problem is that the code always modifies the planned capacity 
>> (that expected before warnings should be used) without regard for the 
>> fact the existing capacity may be higher than that requested. As a 
>> result if you have a call chain like:
>>
>> void foo() {  // C code
>>   env->EnsureLocalCapacity(60); // needs lots of local refs
>>   ...
>>   JNU_GetPlatformsString(...)
>>       env->EnsureLocalCapacity(5); // lower than 60!
>>    ...
>>    // create 60 local refs
>> }
>>
>> upon return the warning will be issued because the number of local 
>> refs exceeds the most recent call to EnsureLocalCapacity.
>>
>> A simple fix is for EnsureLocalCapacity to only raise the planned 
>> capacity, not lower it. That fits with the notion of "ensuring" there 
>> is sufficient space - the function is not SetLocalcapacity. It also 
>> fits with the way PushLocalFrame(capacity) increases the planned 
>> capacity by "capacity" but PopLocalFrame does not reduce it again.
>>
>> New test added.
>>
>> Tested through JPRT.
>>
>> Thanks,
>> David
>>
>