VM: Remove obsolete workaround for a GC issue in the deoptimization stub.

runtime/vm/stub_code_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"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/compiler.h"
 #include "vm/dart_entry.h"
@@ skipping 445 matching lines @@
   // Code above cannot cause GC.
   // There is a Dart Frame on the stack. We must restore PP and leave frame.
   __ LeaveDartFrame();
 
   // Frame is fully rewritten at this point and it is safe to perform a GC.
   // Materialize any objects that were deferred by FillFrame because they
   // require allocation.
   // Enter stub frame with loading PP. The caller's PP is not materialized yet.
   __ EnterStubFrame();
   if (preserve_result) {
-    __ pushq(Immediate(0));  // Workaround for dropped stack slot during GC.
     __ pushq(RBX);  // Preserve result, it will be GC-d here.
   }
   __ pushq(Immediate(Smi::RawValue(0)));  // Space for the result.
   __ CallRuntime(kDeoptimizeMaterializeRuntimeEntry, 0);
   // Result tells stub how many bytes to remove from the expression stack
   // of the bottom-most frame. They were used as materialization arguments.
   __ popq(RBX);
   __ SmiUntag(RBX);
   if (preserve_result) {
     __ popq(RAX);  // Restore result.
-    __ Drop(1);  // Workaround for dropped stack slot during GC.
   }
   __ LeaveStubFrame();
 
   __ popq(RCX);  // Pop return address.
   __ addq(RSP, RBX);  // Remove materialization arguments.
   __ pushq(RCX);  // Push return address.
   __ ret();
 }
 
 
@@ skipping 1660 matching lines @@
 // Result:
 //  RCX: entry point.
 void StubCode::GenerateMegamorphicLookupStub(Assembler* assembler) {
   EmitMegamorphicLookup(assembler, RDI, RBX, RCX);
   __ ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64