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

runtime/vm/stub_code_arm64.cc

 // Copyright (c) 2014, 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_ARM64)
 
 #include "vm/assembler.h"
 #include "vm/code_generator.h"
 #include "vm/compiler.h"
@@ skipping 500 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) {
-    __ Push(ZR);  // Workaround for dropped stack slot during GC.
     __ Push(R1);  // Preserve result, it will be GC-d here.
   }
   __ Push(ZR);  // 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.
   __ Pop(R1);
   __ SmiUntag(R1);
   if (preserve_result) {
     __ Pop(R0);  // Restore result.
-    __ Drop(1);  // Workaround for dropped stack slot during GC.
   }
   __ LeaveStubFrame();
   // Remove materialization arguments.
   __ add(SP, SP, Operand(R1));
   __ ret();
 }
 
 
 void StubCode::GenerateDeoptimizeLazyStub(Assembler* assembler) {
   // Correct return address to point just after the call that is being
@@ skipping 1610 matching lines @@
 // Result:
 //  R1: entry point.
 void StubCode::GenerateMegamorphicLookupStub(Assembler* assembler) {
   EmitMegamorphicLookup(assembler, R0, R1, R1);
   __ ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64