RFR (S) JDK-8009575 - Reduce Symbol::_refcount from 4 bytes to 2 bytes

[Ioi Lam]

Please review:

http://cr.openjdk.java.net/~iklam/8009575/symbol_refcount_001/ 
<http://cr.openjdk.java.net/%7Eiklam/8009575/symbol_refcount_001/>


Bug: Reduce Symbol::_refcount from 4 bytes to 2 bytes

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8009575
https://jbs.oracle.com/bugs/browse/JDK-8009575


Summary of fix:

(1) Symbol::_refcount has been changed to a short.

     Note that _refcount needs to be incremented atomically using
     Atom::add, which requires a 32-bit word in memory. So I glue the
     _refcount and _length fields together using union and ENDIAN
     tricks, which is kind of ugly but faster.

     The prettier alternative would be to use shift and masking to
     access the _refcount and _length components, but performance
     would be slightly slower.

     If anyone has a more elegant solution, please let me know.

     #------------------------------------------------------
       union {
         volatile int   _refcount_and_length;
         struct {
     #ifdef VM_LITTLE_ENDIAN
           unsigned short _length; // number of UTF8 characters in the 
symbol
           volatile short _refcount;
     #else
           volatile short _refcount;
           unsigned short _length; // number of UTF8 characters in the 
symbol
     #endif
         } s;
       } u;

     void Symbol::increment_refcount() {
       if (u.s._refcount >= 0) {
         // Adding 0x10000 to _refcount_and_length is equivalent to 
adding 0x1 to
         // _refcount (w.r.t overflow/underflow).
         (void)Atomic::add(0x10000, &u._refcount_and_length);
       ....
     }
#------------------------------------------------------

(2) Symbol::size(int) was wasting one byte per Symbol instance. I fixed 
it by
     splitting part of the fields from Symbol into SymbolBase.


Footprint result:

     The theoretical reduction is 3 bytes per symbol -- 2 bytes
     from _refcount, and 1 byte from the fixed Symbol::size(int)
     function.

     In actual measurement, CDS image (about 3000 classes, with about
     50000 symbols) is reduced by about 140KB, or about 2.8 bytes per
     Symbol, or about 0.6% of the whole CDS image.


Testing:

     * UTE (vm.runtime.testlist, vm.quick.testlist 
vm.parallel_class_loading.testlist)

Thanks,
Ioi

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