[aarch64-port-dev ] Remove unnecessary memory barriers around CAS operations

[Andrew Haley]

After much careful reading of the AArch64 spec and many runs of jcstress,
I have decided that it is safe to remove the barriers around CAS.

A more thorough reworking of memory barriers is on my list of things
to do, but it needs some HotSpot changes from upstream that aren't yet
in our code base.

Andrew.


# HG changeset patch
# User aph
# Date 1396369343 14400
#      Tue Apr 01 12:22:23 2014 -0400
# Node ID 780ed75ea21a727949abcfe57ea5544f7a1ca22c
# Parent  e176eb39c5f53127fe18a7e528ca6bc6f0c23cea
Remove unnecessary memory barriers around CAS operations

diff -r e176eb39c5f5 -r 780ed75ea21a src/cpu/aarch64/vm/c1_LIRAssembler_aarch64.cpp
--- a/src/cpu/aarch64/vm/c1_LIRAssembler_aarch64.cpp	Mon Mar 31 10:20:26 2014 -0400
+++ b/src/cpu/aarch64/vm/c1_LIRAssembler_aarch64.cpp	Tue Apr 01 12:22:23 2014 -0400
@@ -1584,9 +1584,8 @@
   Label retry_load, nope;
   // flush and load exclusive from the memory location
   // and fail if it is not what we expect
-  __ membar(__ AnyAny);
   __ bind(retry_load);
-  __ ldxrw(rscratch1, addr);
+  __ ldaxrw(rscratch1, addr);
   __ cmpw(rscratch1, cmpval);
   __ cset(rscratch1, Assembler::NE);
   __ br(Assembler::NE, nope);
@@ -1603,9 +1602,8 @@
   Label retry_load, nope;
   // flush and load exclusive from the memory location
   // and fail if it is not what we expect
-  __ membar(__ AnyAny);
   __ bind(retry_load);
-  __ ldxr(rscratch1, addr);
+  __ ldaxr(rscratch1, addr);
   __ cmp(rscratch1, cmpval);
   __ cset(rscratch1, Assembler::NE);
   __ br(Assembler::NE, nope);
diff -r e176eb39c5f5 -r 780ed75ea21a src/cpu/aarch64/vm/macroAssembler_aarch64.cpp
--- a/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp	Mon Mar 31 10:20:26 2014 -0400
+++ b/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp	Tue Apr 01 12:22:23 2014 -0400
@@ -1817,13 +1817,11 @@
   bind(retry_load);
   // flush and load exclusive from the memory location
   // and fail if it is not what we expect
-  membar(AnyAny);
-  ldxr(tmp, addr);
+  ldaxr(tmp, addr);
   cmp(tmp, oldv);
   br(Assembler::NE, nope);
   // if we store+flush with no intervening write tmp wil be zero
-  stxr(tmp, newv, addr);
-  membar(AnyAny);
+  stlxr(tmp, newv, addr);
   cbzw(tmp, succeed);
   // retry so we only ever return after a load fails to compare
   // ensures we don't return a stale value after a failed write.
@@ -1847,13 +1845,11 @@
   bind(retry_load);
   // flush and load exclusive from the memory location
   // and fail if it is not what we expect
-  membar(AnyAny);
-  ldxrw(tmp, addr);
+  ldaxrw(tmp, addr);
   cmp(tmp, oldv);
   br(Assembler::NE, nope);
   // if we store+flush with no intervening write tmp wil be zero
-  stxrw(tmp, newv, addr);
-  membar(AnyAny);
+  stlxrw(tmp, newv, addr);
   cbzw(tmp, succeed);
   // retry so we only ever return after a load fails to compare
   // ensures we don't return a stale value after a failed write.