--- a/hotspot/src/cpu/aarch64/vm/aarch64.ad	Thu Sep 10 17:56:43 2015 -0700
+++ b/hotspot/src/cpu/aarch64/vm/aarch64.ad	Tue Sep 08 14:08:58 2015 +0100
@@ -3803,81 +3803,37 @@
 
   enc_class aarch64_enc_cmpxchg(memory mem, iRegLNoSp oldval, iRegLNoSp newval) %{
     MacroAssembler _masm(&cbuf);
-    Register old_reg = as_Register($oldval$$reg);
-    Register new_reg = as_Register($newval$$reg);
-    Register base = as_Register($mem$$base);
-    Register addr_reg;
-    int index = $mem$$index;
-    int scale = $mem$$scale;
-    int disp = $mem$$disp;
-    if (index == -1) {
-       if (disp != 0) {
-        __ lea(rscratch2, Address(base, disp));
-        addr_reg = rscratch2;
-      } else {
-        // TODO
-        // should we ever get anything other than this case?
-        addr_reg = base;
-      }
-    } else {
-      Register index_reg = as_Register(index);
-      if (disp == 0) {
-        __ lea(rscratch2, Address(base, index_reg, Address::lsl(scale)));
-        addr_reg = rscratch2;
-      } else {
-        __ lea(rscratch2, Address(base, disp));
-        __ lea(rscratch2, Address(rscratch2, index_reg, Address::lsl(scale)));
-        addr_reg = rscratch2;
-      }
-    }
-    Label retry_load, done;
-    __ bind(retry_load);
-    __ ldxr(rscratch1, addr_reg);
-    __ cmp(rscratch1, old_reg);
-    __ br(Assembler::NE, done);
-    __ stlxr(rscratch1, new_reg, addr_reg);
-    __ cbnzw(rscratch1, retry_load);
-    __ bind(done);
+    guarantee($mem$$index == -1 && $mem$$disp == 0, "impossible encoding");
+    __ cmpxchg($mem$$base$$Register, $oldval$$Register, $newval$$Register,
+               &Assembler::ldxr, &MacroAssembler::cmp, &Assembler::stlxr);
   %}
 
   enc_class aarch64_enc_cmpxchgw(memory mem, iRegINoSp oldval, iRegINoSp newval) %{
     MacroAssembler _masm(&cbuf);
-    Register old_reg = as_Register($oldval$$reg);
-    Register new_reg = as_Register($newval$$reg);
-    Register base = as_Register($mem$$base);
-    Register addr_reg;
-    int index = $mem$$index;
-    int scale = $mem$$scale;
-    int disp = $mem$$disp;
-    if (index == -1) {
-       if (disp != 0) {
-        __ lea(rscratch2, Address(base, disp));
-        addr_reg = rscratch2;
-      } else {
-        // TODO
-        // should we ever get anything other than this case?
-        addr_reg = base;
-      }
-    } else {
-      Register index_reg = as_Register(index);
-      if (disp == 0) {
-        __ lea(rscratch2, Address(base, index_reg, Address::lsl(scale)));
-        addr_reg = rscratch2;
-      } else {
-        __ lea(rscratch2, Address(base, disp));
-        __ lea(rscratch2, Address(rscratch2, index_reg, Address::lsl(scale)));
-        addr_reg = rscratch2;
-      }
-    }
-    Label retry_load, done;
-    __ bind(retry_load);
-    __ ldxrw(rscratch1, addr_reg);
-    __ cmpw(rscratch1, old_reg);
-    __ br(Assembler::NE, done);
-    __ stlxrw(rscratch1, new_reg, addr_reg);
-    __ cbnzw(rscratch1, retry_load);
-    __ bind(done);
-  %}
+    guarantee($mem$$index == -1 && $mem$$disp == 0, "impossible encoding");
+    __ cmpxchg($mem$$base$$Register, $oldval$$Register, $newval$$Register,
+               &Assembler::ldxrw, &MacroAssembler::cmpw, &Assembler::stlxrw);
+  %}
+
+
+  // The only difference between aarch64_enc_cmpxchg and
+  // aarch64_enc_cmpxchg_acq is that we use load-acquire in the
+  // CompareAndSwap sequence to serve as a barrier on acquiring a
+  // lock.
+  enc_class aarch64_enc_cmpxchg_acq(memory mem, iRegLNoSp oldval, iRegLNoSp newval) %{
+    MacroAssembler _masm(&cbuf);
+    guarantee($mem$$index == -1 && $mem$$disp == 0, "impossible encoding");
+    __ cmpxchg($mem$$base$$Register, $oldval$$Register, $newval$$Register,
+               &Assembler::ldaxr, &MacroAssembler::cmp, &Assembler::stlxr);
+  %}
+
+  enc_class aarch64_enc_cmpxchgw_acq(memory mem, iRegINoSp oldval, iRegINoSp newval) %{
+    MacroAssembler _masm(&cbuf);
+    guarantee($mem$$index == -1 && $mem$$disp == 0, "impossible encoding");
+    __ cmpxchg($mem$$base$$Register, $oldval$$Register, $newval$$Register,
+               &Assembler::ldaxrw, &MacroAssembler::cmpw, &Assembler::stlxrw);
+  %}
+
 
   // auxiliary used for CompareAndSwapX to set result register
   enc_class aarch64_enc_cset_eq(iRegINoSp res) %{
@@ -4398,13 +4354,10 @@
 
     // Compare object markOop with mark and if equal exchange scratch1
     // with object markOop.
-    // Note that this is simply a CAS: it does not generate any
-    // barriers.  These are separately generated by
-    // membar_acquire_lock().
     {
       Label retry_load;
       __ bind(retry_load);
-      __ ldxr(tmp, oop);
+      __ ldaxr(tmp, oop);
       __ cmp(tmp, disp_hdr);
       __ br(Assembler::NE, cas_failed);
       // use stlxr to ensure update is immediately visible
@@ -4454,7 +4407,7 @@
       {
         Label retry_load, fail;
         __ bind(retry_load);
-        __ ldxr(rscratch1, tmp);
+        __ ldaxr(rscratch1, tmp);
         __ cmp(disp_hdr, rscratch1);
         __ br(Assembler::NE, fail);
         // use stlxr to ensure update is immediately visible
@@ -8017,10 +7970,10 @@
   match(MemBarAcquireLock);
   ins_cost(VOLATILE_REF_COST);
 
-  format %{ "membar_acquire_lock" %}
-
-  ins_encode %{
-    __ membar(Assembler::LoadLoad|Assembler::LoadStore);
+  format %{ "membar_acquire_lock (elided)" %}
+
+  ins_encode %{
+    __ block_comment("membar_acquire_lock (elided)");
   %}
 
   ins_pipe(pipe_serial);
@@ -8080,10 +8033,10 @@
   match(MemBarReleaseLock);
   ins_cost(VOLATILE_REF_COST);
 
-  format %{ "membar_release_lock" %}
-
-  ins_encode %{
-    __ membar(Assembler::LoadStore|Assembler::StoreStore);
+  format %{ "membar_release_lock (elided)" %}
+
+  ins_encode %{
+    __ block_comment("membar_release_lock (elided)");
   %}
 
   ins_pipe(pipe_serial);
@@ -8369,7 +8322,11 @@
   ins_pipe(pipe_serial);
 %}
 
-// this has to be implemented as a CAS
+
+// storeLConditional is used by PhaseMacroExpand::expand_lock_node
+// when attempting to rebias a lock towards the current thread.  We
+// must use the acquire form of cmpxchg in order to guarantee acquire
+// semantics in this case.
 instruct storeLConditional(indirect mem, iRegLNoSp oldval, iRegLNoSp newval, rFlagsReg cr)
 %{
   match(Set cr (StoreLConditional mem (Binary oldval newval)));
@@ -8381,12 +8338,14 @@
     "cmpw rscratch1, zr\t# EQ on successful write"
   %}
 
-  ins_encode(aarch64_enc_cmpxchg(mem, oldval, newval));
+  ins_encode(aarch64_enc_cmpxchg_acq(mem, oldval, newval));
 
   ins_pipe(pipe_slow);
 %}
 
-// this has to be implemented as a CAS
+// storeIConditional also has acquire semantics, for no better reason
+// than matching storeLConditional.  At the time of writing this
+// comment storeIConditional was not used anywhere by AArch64.
 instruct storeIConditional(indirect mem, iRegINoSp oldval, iRegINoSp newval, rFlagsReg cr)
 %{
   match(Set cr (StoreIConditional mem (Binary oldval newval)));
@@ -8398,7 +8357,7 @@
     "cmpw rscratch1, zr\t# EQ on successful write"
   %}
 
-  ins_encode(aarch64_enc_cmpxchgw(mem, oldval, newval));
+  ins_encode(aarch64_enc_cmpxchgw_acq(mem, oldval, newval));
 
   ins_pipe(pipe_slow);
 %}

--- a/hotspot/src/cpu/aarch64/vm/macroAssembler_aarch64.hpp	Thu Sep 10 17:56:43 2015 -0700
+++ b/hotspot/src/cpu/aarch64/vm/macroAssembler_aarch64.hpp	Tue Sep 08 14:08:58 2015 +0100
@@ -917,6 +917,8 @@
 
   void cmpptr(Register src1, Address src2);
 
+  // Various forms of CAS
+
   void cmpxchgptr(Register oldv, Register newv, Register addr, Register tmp,
                   Label &suceed, Label *fail);
 
@@ -938,6 +940,23 @@
     str(rscratch2, adr);
   }
 
+  // A generic CAS; success or failure is in the EQ flag.
+  template <typename T1, typename T2>
+  void cmpxchg(Register addr, Register expected, Register new_val,
+               T1 load_insn,
+               void (MacroAssembler::*cmp_insn)(Register, Register),
+               T2 store_insn,
+               Register tmp = rscratch1) {
+    Label retry_load, done;
+    bind(retry_load);
+    (this->*load_insn)(tmp, addr);
+    (this->*cmp_insn)(tmp, expected);
+    br(Assembler::NE, done);
+    (this->*store_insn)(tmp, new_val, addr);
+    cbnzw(tmp, retry_load);
+    bind(done);
+  }
+
   // Calls
 
   address trampoline_call(Address entry, CodeBuffer *cbuf = NULL);