RFR: 8058255: Native jbyte Atomic::cmpxchg for supported x86 platforms

[Andrew Haley]

On 12/09/14 02:40, David Holmes wrote:
> Can we pause and give some more thought to a clean mechanism for 
> allowing a shared implementation if desired with the ability to override 
> if desired. I really do not like to see CPU specific ifdefs being added 
> to shared code. (And I would also not like to see all platforms being 
> forced to reimplement this natively).

Indeed.  Could we put the code for things like cmpxchg in an abstract
class and override them?

Also, this doesn't look right:

+  // Support fori jbyte Atomic::cmpxchg(jbyte exchange_value,
+  //                                    volatile jbyte *dest,
+  //                                    jbyte compare_value)
+  // An additional bool (os::is_MP()) is passed as the last argument.
+      .inline _Atomic_cmpxchg_byte,4
+      movb     8(%esp), %al   // compare_value
+      movb     0(%esp), %cl   // exchange_value
+      movl     4(%esp), %edx   // dest
+      cmp      $0, 12(%esp)    // MP test
+      jne      1f
+      cmpxchgb %cl, (%edx)
+      jmp      2f
+1:    lock
+      cmpxchgl %cl, (%edx)
+2:
+      .end

What is this cmpxchgl for?

Andrew.