Prefer loading Smi 0 instead of null into a runtime call result slot.

runtime/vm/stub_code_arm64.cc

Index: runtime/vm/stub_code_arm64.cc
diff --git a/runtime/vm/stub_code_arm64.cc b/runtime/vm/stub_code_arm64.cc
index 09d973b1e4a8752030a613d6f927b7db895d7bbe..81dc2752e771017aba5ce04356535ddc2fa43703 100644
--- a/runtime/vm/stub_code_arm64.cc
+++ b/runtime/vm/stub_code_arm64.cc
@@ -324,7 +324,7 @@ void StubCode::GenerateCallStaticFunctionStub(Assembler* assembler) {
   __ EnterStubFrame();
   // Setup space on stack for return value and preserve arguments descriptor.
   __ Push(R4);
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   __ CallRuntime(kPatchStaticCallRuntimeEntry, 0);
   // Get Code object result and restore arguments descriptor array.
   __ Pop(CODE_REG);
@@ -350,7 +350,7 @@ void StubCode::GenerateFixCallersTargetStub(Assembler* assembler) {
   __ EnterStubFrame();
   // Setup space on stack for return value and preserve arguments descriptor.
   __ Push(R4);
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   __ CallRuntime(kFixCallersTargetRuntimeEntry, 0);
   // Get Code object result and restore arguments descriptor array.
   __ Pop(CODE_REG);
@@ -372,7 +372,7 @@ void StubCode::GenerateFixAllocationStubTargetStub(Assembler* assembler) {
   __ ldr(CODE_REG, Address(THR, Thread::fix_allocation_stub_code_offset()));
   __ EnterStubFrame();
   // Setup space on stack for return value.
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   __ CallRuntime(kFixAllocationStubTargetRuntimeEntry, 0);
   // Get Code object result.
   __ Pop(CODE_REG);
@@ -563,7 +563,7 @@ static void GenerateDispatcherCode(Assembler* assembler,
   __ LoadFieldFromOffset(R2, R4, ArgumentsDescriptor::count_offset());
   __ add(TMP, FP, Operand(R2, LSL, 2));  // R2 is Smi.
   __ LoadFromOffset(R6, TMP, kParamEndSlotFromFp * kWordSize);
-  __ PushObject(Object::null_object());
+  __ Push(ZR);  // Result slot.
   __ Push(R6);  // Receiver.
   __ Push(R5);  // ICData/MegamorphicCache.
   __ Push(R4);  // Arguments descriptor.
@@ -594,7 +594,7 @@ void StubCode::GenerateMegamorphicMissStub(Assembler* assembler) {
   // Push the receiver.
   // Push IC data object.
   // Push arguments descriptor array.
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   __ Push(R6);
   __ Push(R5);
   __ Push(R4);
@@ -756,7 +756,7 @@ void StubCode::GenerateAllocateArrayStub(Assembler* assembler) {
   __ EnterStubFrame();
   // Setup space on stack for return value.
   // Push array length as Smi and element type.
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   __ Push(R2);
   __ Push(R1);
   __ CallRuntime(kAllocateArrayRuntimeEntry, 2);
@@ -1189,8 +1189,7 @@ void StubCode::GenerateAllocationStubForClass(Assembler* assembler,
   // Create a stub frame as we are pushing some objects on the stack before
   // calling into the runtime.
   __ EnterStubFrame();  // Uses pool pointer to pass cls to runtime.
-  // Setup space on stack for return value.
-  __ PushObject(Object::null_object());
+  __ Push(ZR);  // Result slot.
   __ PushObject(cls);  // Push class of object to be allocated.
   if (is_cls_parameterized) {
     // Push type arguments.
@@ -1227,7 +1226,7 @@ void StubCode::GenerateCallClosureNoSuchMethodStub(Assembler* assembler) {
   // Push space for the return value.
   // Push the receiver.
   // Push arguments descriptor array.
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   __ Push(R6);
   __ Push(R4);
 
@@ -1482,7 +1481,7 @@ void StubCode::GenerateNArgsCheckInlineCacheStub(
   __ Push(R4);  // Preserve arguments descriptor array.
   __ Push(R5);  // Preserve IC Data.
   // Setup space on stack for the result (target code object).
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   // Push call arguments.
   for (intptr_t i = 0; i < num_args; i++) {
     __ LoadFromOffset(TMP, R7, -i * kWordSize);
@@ -1713,7 +1712,7 @@ void StubCode::GenerateLazyCompileStub(Assembler* assembler) {
 void StubCode::GenerateICCallBreakpointStub(Assembler* assembler) {
   __ EnterStubFrame();
   __ Push(R5);
-  __ PushObject(Object::null_object());  // Space for result.
+  __ Push(ZR);  // Space for result.
   __ CallRuntime(kBreakpointRuntimeHandlerRuntimeEntry, 0);
   __ Pop(CODE_REG);
   __ Pop(R5);
@@ -1725,7 +1724,7 @@ void StubCode::GenerateICCallBreakpointStub(Assembler* assembler) {
 
 void StubCode::GenerateRuntimeCallBreakpointStub(Assembler* assembler) {
   __ EnterStubFrame();
-  __ PushObject(Object::null_object());  // Space for result.
+  __ Push(ZR);  // Space for result.
   __ CallRuntime(kBreakpointRuntimeHandlerRuntimeEntry, 0);
   __ Pop(CODE_REG);
   __ LeaveStubFrame();
@@ -1906,7 +1905,7 @@ void StubCode::GenerateOptimizeFunctionStub(Assembler* assembler) {
   __ EnterStubFrame();
   __ Push(R4);
   // Setup space on stack for the return value.
-  __ PushObject(Object::null_object());
+  __ Push(ZR);
   __ Push(R6);
   __ CallRuntime(kOptimizeInvokedFunctionRuntimeEntry, 1);
   __ Pop(R0);  // Discard argument.
@@ -2192,9 +2191,9 @@ void StubCode::GenerateUnlinkedCallStub(Assembler* assembler) {
   __ EnterStubFrame();
   __ Push(R0);  // Preserve receiver.
 
-  __ PushObject(Object::null_object());  // Result.
-  __ Push(R0);                           // Arg0: Receiver
-  __ Push(R5);                           // Arg1: UnlinkedCall
+  __ Push(ZR);  // Result slot.
+  __ Push(R0);  // Arg0: Receiver
+  __ Push(R5);  // Arg1: UnlinkedCall
   __ CallRuntime(kUnlinkedCallRuntimeEntry, 2);
   __ Drop(2);
   __ Pop(R5);  // result = IC
@@ -2237,8 +2236,8 @@ void StubCode::GenerateSingleTargetCallStub(Assembler* assembler) {
   __ EnterStubFrame();
   __ Push(R0);  // Preserve receiver.
 
-  __ PushObject(Object::null_object());  // Result.
-  __ Push(R0);                           // Arg0: Receiver
+  __ Push(ZR);  // Result slot.
+  __ Push(R0);  // Arg0: Receiver
   __ CallRuntime(kSingleTargetMissRuntimeEntry, 1);
   __ Drop(1);
   __ Pop(R5);  // result = IC
@@ -2258,8 +2257,8 @@ void StubCode::GenerateMonomorphicMissStub(Assembler* assembler) {
   __ EnterStubFrame();
   __ Push(R0);  // Preserve receiver.
 
-  __ PushObject(Object::null_object());  // Result.
-  __ Push(R0);                           // Arg0: Receiver
+  __ Push(ZR);  // Result slot.
+  __ Push(R0);  // Arg0: Receiver
   __ CallRuntime(kMonomorphicMissRuntimeEntry, 1);
   __ Drop(1);
   __ Pop(R5);  // result = IC