Add missing verified write on x64; extend test coverage

runtime/vm/flow_graph_compiler_x64.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"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/flow_graph_compiler.h"
 
 #include "vm/ast_printer.h"
@@ skipping 1253 matching lines @@
   // optimization/deoptimization cycles we will attempt.
   const Array& counter = Array::ZoneHandle(Array::New(1, Heap::kOld));
   counter.SetAt(0, Smi::Handle(Smi::New(0)));
   __ Comment("Edge counter");
   __ LoadObject(RAX, counter, PP);
 #if defined(DEBUG)
   intptr_t increment_start = assembler_->CodeSize();
 #endif  // DEBUG
   __ IncrementSmiField(FieldAddress(RAX, Array::element_offset(0)), 1);
 #if defined(DEBUG)
+  // If the assertion below fails, update EdgeCounterIncrementSizeInBytes.
   intptr_t expected = EdgeCounterIncrementSizeInBytes();
   intptr_t actual = assembler_->CodeSize() - increment_start;
   if (actual != expected) {
     FATAL2("Edge counter increment length: %" Pd ", expected %" Pd "\n",
            actual,
            expected);
   }
 #endif  // DEBUG
 }
 
 
 int32_t FlowGraphCompiler::EdgeCounterIncrementSizeInBytes() {
+  // Used by CodePatcher; so must be constant across all code in an isolate.
   return VerifiedMemory::enabled() ? 73 : 5;
 }
 
 
 void FlowGraphCompiler::EmitOptimizedInstanceCall(
     ExternalLabel* target_label,
     const ICData& ic_data,
     intptr_t argument_count,
     intptr_t deopt_id,
     intptr_t token_pos,
@@ skipping 488 matching lines @@
   __ movups(reg, Address(RSP, 0));
   __ AddImmediate(RSP, Immediate(kFpuRegisterSize), PP);
 }
 
 
 #undef __
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64