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

David Holmes david.holmes at oracle.com
Sun Dec 10 05:39:44 UTC 2017

bug: https://bugs.openjdk.java.net/browse/JDK-8193222
webrev: http://cr.openjdk.java.net/~dholmes/8193222/webrev/

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
       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.


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