Inline instance object hash code into object header on 64 bit.

runtime/vm/intrinsifier_arm64.cc

Index: runtime/vm/intrinsifier_arm64.cc
diff --git a/runtime/vm/intrinsifier_arm64.cc b/runtime/vm/intrinsifier_arm64.cc
index 80d51eb3e4c0c564710339473e61c281d61f3275..3c5f4f6c341544c753153f0fa936edda62308192 100644
--- a/runtime/vm/intrinsifier_arm64.cc
+++ b/runtime/vm/intrinsifier_arm64.cc
@@ -1832,6 +1832,23 @@ void Intrinsifier::String_getHashCode(Assembler* assembler) {
 }
 
 
+void Intrinsifier::Object_getHash(Assembler* assembler) {
+  __ ldr(R0, Address(SP, 0 * kWordSize));
+  __ ldr(R0, FieldAddress(R0, String::hash_offset()), kUnsignedWord);
+  __ SmiTag(R0);
+  __ ret();
+}
+
+
+void Intrinsifier::Object_setHash(Assembler* assembler) {
+  __ ldr(R0, Address(SP, 1 * kWordSize));  // Object.
+  __ ldr(R1, Address(SP, 0 * kWordSize));  // Value.
+  __ SmiUntag(R1);
+  __ str(R1, FieldAddress(R0, String::hash_offset()), kUnsignedWord);

[rmacnak, 2017/06/13 00:17:52]

These intrinsics and their C++ counterparts need to use atomics since the
mutator can assign a hash at the same time the sweeper is trying to clear the
mark bit.  

I suspect the combination of a 4-byte access here and an overlapping 8-byte
access from the sweeper creates undefined behavior for the sweeper's atomic
access on ARM.

Alternatively, the declaration in RawObject could be split into two fields as
you did in become.h so the sweeper isn't touching the hash.

In any case, FreeListElement, ForwardingCorpse and RawObject should all
consistently use a one-field or two-field header.

[Vyacheslav Egorov (Google), 2017/06/13 06:12:25]

Nice catch! Alternatively to atomics we could probably use 4 byte accesses in
the sweeper instead - because it does not need to access hash part of the header
anyway.

[erikcorry, 2017/06/16 09:26:39]

I fixed all the places we update the tags bits to use 32 bit operations.  I
didn't change the ones that happen during allocation, since the allocator does
not race with the sweeper threads (you can't allocate on a page where sweeping
has not completed).

+  __ ret();
+}
+
+
 void GenerateSubstringMatchesSpecialization(Assembler* assembler,
                                             intptr_t receiver_cid,
                                             intptr_t other_cid,