[arm64][simulator] Support --stop-sim-at flag.

Every simulator supports this flag except for Arm64 so let's add it. The
ExecuteInstruction() loop being performance sensitive, we avoid updating a
counter on a every loop unless the flag is set.

Change-Id: I5d40e9f3f03b743b4f354e31e9bfda32789ec098
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2061555Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#66368}

src/execution/arm64/simulator-arm64.cc

@@ -303,6 +303,7 @@ Simulator::Simulator(Decoder<DispatchingDecoderVisitor>* decoder,
       guard_pages_(false),
       last_debugger_input_(nullptr),
       log_parameters_(NO_PARAM),
+      icount_for_stop_sim_at_(0),
       isolate_(isolate) {
   // Setup the decoder.
   decoder_->AppendVisitor(this);
@@ -384,9 +385,25 @@ void Simulator::Run() {
   LogAllWrittenRegisters();
 
   pc_modified_ = false;
+
+  if (::v8::internal::FLAG_stop_sim_at == 0) {
+    // Fast version of the dispatch loop without checking whether the simulator
+    // should be stopping at a particular executed instruction.
+    while (pc_ != kEndOfSimAddress) {
+      ExecuteInstruction();
+    }
+  } else {
+    // FLAG_stop_sim_at is at the non-default value. Stop in the debugger when
+    // we reach the particular instruction count.
     while (pc_ != kEndOfSimAddress) {
+      icount_for_stop_sim_at_ =
+          base::AddWithWraparound(icount_for_stop_sim_at_, 1);
+      if (icount_for_stop_sim_at_ == ::v8::internal::FLAG_stop_sim_at) {
+        Debug();
+      }
       ExecuteInstruction();
     }
+  }
 }
 
 void Simulator::RunFrom(Instruction* start) {

src/execution/arm64/simulator-arm64.h

@@ -2492,6 +2492,8 @@ class Simulator : public DecoderVisitor, public SimulatorBase {
   }
 
   int log_parameters_;
+  // Instruction counter only valid if FLAG_stop_sim_at isn't 0.
+  int icount_for_stop_sim_at_;
   Isolate* isolate_;
 };