RFR 8169389 : Use a bitmap to control StackTraceElement::toString format and save footprint

[Brent Christian]

Hi,

Please review my fix for 8169389.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8169389
Webrev:
http://cr.openjdk.java.net/~bchristi/8169389/webrev.03/

The addition of classloader names (JDK-6749237) updated the format of 
StackTraceElement.toString().  A field was added to store the 
"[[class-loader-name]/[module-name[@module-version]]]" portion of the 
returned String.

Instead of storing a String, we can cut down on footprint by using a 
bitfield to indicate which fields to include in the toString() output.

Looking at the heap usage of a simple test case indicates ~25% reduction 
in space used by Throwable's StackTraceElement[].  (Of course this could 
vary quite a bit, depending on the specifics of the call stack.)

The additional byte field does not increase the instance size of a 
StackTraceElement; it is within space already lost for alignment (per 
linux_x64 w/ CompressedOops).

While working on this, it was noticed that with the new toString() 
format, there are circumstances in which two StackTraceElements can be 
equals(), but return different values from toString().  I thought it was 
worth updating the JavaDoc to call out this unusual possibility.

Likewise, there can be information reflected in the StackTraceElement 
getter methods that is not reflected in the toString() value.  I updated 
the test code to check for this.

Thanks,
-Brent