Atomic update of header word on ARM.

runtime/vm/stub_code_arm.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_ARM)
 
 #include "vm/assembler.h"
 #include "vm/code_generator.h"
 #include "vm/cpu.h"
@@ skipping 1027 matching lines @@
   // store buffer if the object is in the store buffer already.
   // Spilled: R1, R2, R3
   // R0: Address being stored
   __ ldr(R2, FieldAddress(R0, Object::tags_offset()));
   __ tst(R2, Operand(1 << RawObject::kRememberedBit));
   __ b(&add_to_buffer, EQ);
   __ PopList((1 << R1) | (1 << R2) | (1 << R3));
   __ Ret();
 
   __ Bind(&add_to_buffer);
-  __ orr(R2, R2, Operand(1 << RawObject::kRememberedBit));
-  __ str(R2, FieldAddress(R0, Object::tags_offset()));
+  // R2: Header word.
+  if (TargetCPUFeatures::arm_version() == ARMv5TE) {
+    // TODO(21263): Implement 'swp' and use it below.
+    ASSERT(OS::NumberOfAvailableProcessors() <= 1);
+    __ orr(R2, R2, Operand(1 << RawObject::kRememberedBit));
+    __ str(R2, FieldAddress(R0, Object::tags_offset()));
+  } else {
+    // Atomically set the remembered bit of the object header.
+    ASSERT(Object::tags_offset() == 0);
+    __ sub(R3, R0, Operand(kHeapObjectTag));
+    // R3: Untagged address of header word (ldrex/strex do not support offsets).
+    Label retry;
+    __ Bind(&retry);
+    __ ldrex(R2, R3);
+    __ orr(R2, R2, Operand(1 << RawObject::kRememberedBit));
+    __ strex(R1, R2, R3);
+    __ cmp(R1, Operand(1));
+    __ b(&retry, EQ);
+  }
 
   // Load the isolate.
   // Spilled: R1, R2, R3.
   // R0: address being stored.
   __ LoadIsolate(R1);
 
   // Load the StoreBuffer block out of the isolate. Then load top_ out of the
   // StoreBufferBlock and add the address to the pointers_.
   // R1: isolate.
   __ ldr(R1, Address(R1, Isolate::store_buffer_offset()));
@@ skipping 975 matching lines @@
   const Register right = R0;
   __ ldr(left, Address(SP, 1 * kWordSize));
   __ ldr(right, Address(SP, 0 * kWordSize));
   GenerateIdenticalWithNumberCheckStub(assembler, left, right, temp);
   __ Ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM