Implement write barrier on ARM.

runtime/vm/stub_code_arm.cc

Index: runtime/vm/stub_code_arm.cc
===================================================================
--- runtime/vm/stub_code_arm.cc	(revision 21081)
+++ runtime/vm/stub_code_arm.cc	(working copy)
@@ -356,8 +356,52 @@
 }
 
 
+DECLARE_LEAF_RUNTIME_ENTRY(void, StoreBufferBlockProcess, Isolate* isolate);
+
+// Helper stub to implement Assembler::StoreIntoObject.
+// Input parameters:
+//   R0: Address (i.e. object) being stored into.
 void StubCode::GenerateUpdateStoreBufferStub(Assembler* assembler) {
-  __ Unimplemented("UpdateStoreBuffer stub");
+  // Save values being destroyed.
+  __ PushList((1 << R1) | (1 << R2) | (1 << R3));
+
+  // Load the isolate out of the context.
+  // Spilled: R1, R2, R3.
+  // R0: Address being stored.
+  __ ldr(R1, FieldAddress(CTX, Context::isolate_offset()));
+
+  // Load top_ out of the StoreBufferBlock and add the address to the pointers_.
+  // R1: Isolate.
+  intptr_t store_buffer_offset = Isolate::store_buffer_block_offset();
+  __ LoadFromOffset(kLoadWord, R2, R1,
+                    store_buffer_offset + StoreBufferBlock::top_offset());
+  __ add(R3, R1, ShifterOperand(R2, LSL, 2));
+  __ StoreToOffset(kStoreWord, R0, R3,
+                   store_buffer_offset + StoreBufferBlock::pointers_offset());
+
+  // Increment top_ and check for overflow.
+  // R2: top_
+  // R1: Isolate
+  Label L;
+  __ add(R2, R2, ShifterOperand(1));
+  __ StoreToOffset(kStoreWord, R2, R1,
+                   store_buffer_offset + StoreBufferBlock::top_offset());
+  __ CompareImmediate(R2, StoreBufferBlock::kSize);
+  // Restore values.
+  __ PopList((1 << R1) | (1 << R2) | (1 << R3));
+  __ b(&L, EQ);
+  __ Ret();
+
+  // Handle overflow: Call the runtime leaf function.
+  __ Bind(&L);
+  // Setup frame, push callee-saved registers.
+
+  __ EnterCallRuntimeFrame(0 * kWordSize);
+  __ ldr(R0, FieldAddress(CTX, Context::isolate_offset()));
+  __ CallRuntime(kStoreBufferBlockProcessRuntimeEntry);
+  // Restore callee-saved registers, tear down frame.
+  __ LeaveCallRuntimeFrame();
+  __ Ret();
 }