src/cpu/x86/vm/macroAssembler_x86.cpp

@@ -1779,10 +1779,11 @@
          lock();
        }
        cmpxchgptr(scrReg, Address(boxReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(owner)));
     } else
     if ((EmitSync & 128) == 0) {                      // avoid ST-before-CAS
+       // register juggle because we need tmpReg for cmpxchgptr below
        movptr(scrReg, boxReg);
        movptr(boxReg, tmpReg);                   // consider: LEA box, [tmp-2]
 
        // Using a prefetchw helps avoid later RTS->RTO upgrades and cache probes
        if ((EmitSync & 2048) && VM_Version::supports_3dnow_prefetch() && os::is_MP()) {

@@ -1812,11 +1813,14 @@
        if (os::is_MP()) {
          lock();
        }
        cmpxchgptr(scrReg, Address(boxReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(owner)));
        movptr(Address(scrReg, 0), 3);          // box->_displaced_header = 3
+       // If we weren't able to swing _owner from NULL to the BasicLock
+       // then take the slow path.
        jccb  (Assembler::notZero, DONE_LABEL);
+       // update _owner from BasicLock to thread
        get_thread (scrReg);                    // beware: clobbers ICCs
        movptr(Address(boxReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(owner)), scrReg);
        xorptr(boxReg, boxReg);                 // set icc.ZFlag = 1 to indicate success
 
        // If the CAS fails we can either retry or pass control to the slow-path.

@@ -2081,10 +2085,13 @@
        jccb  (Assembler::notZero, LSuccess);
 
        xorptr(boxReg, boxReg);                  // box is really EAX
        if (os::is_MP()) { lock(); }
        cmpxchgptr(rsp, Address(tmpReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(owner)));
+       // There's no successor so we tried to regrab the lock with the
+       // placeholder value. If that didn't work, then another thread
+       // grabbed the lock so we're done (and exit was a success).
        jccb  (Assembler::notEqual, LSuccess);
        // Since we're low on registers we installed rsp as a placeholding in _owner.
        // Now install Self over rsp.  This is safe as we're transitioning from
        // non-null to non=null
        get_thread (boxReg);

@@ -2188,10 +2195,13 @@
       // box is really RAX -- the following CMPXCHG depends on that binding
       // cmpxchg R,[M] is equivalent to rax = CAS(M,rax,R)
       movptr(boxReg, (int32_t)NULL_WORD);
       if (os::is_MP()) { lock(); }
       cmpxchgptr(r15_thread, Address(tmpReg, OM_OFFSET_NO_MONITOR_VALUE_TAG(owner)));
+      // There's no successor so we tried to regrab the lock.
+      // If that didn't work, then another thread grabbed the
+      // lock so we're done (and exit was a success).
       jccb  (Assembler::notEqual, LSuccess);
       // Intentional fall-through into slow-path
 
       bind  (LGoSlowPath);
       orl   (boxReg, 1);                      // set ICC.ZF=0 to indicate failure