Better single stepping in VM debugger

runtime/vm/stub_code_x64.cc

Index: runtime/vm/stub_code_x64.cc
===================================================================
--- runtime/vm/stub_code_x64.cc	(revision 24627)
+++ runtime/vm/stub_code_x64.cc	(working copy)
@@ -1416,6 +1416,19 @@
   }
 #endif  // DEBUG
 
+  // Check single stepping.
+  Label not_stepping;
+  __ movq(RAX, FieldAddress(CTX, Context::isolate_offset()));
+  __ movzxb(RAX, Address(RAX, Isolate::single_step_offset()));
+  __ cmpq(RAX, Immediate(0));
+  __ j(EQUAL, &not_stepping, Assembler::kNearJump);
+  __ EnterStubFrame();
+  __ pushq(RBX);
+  __ CallRuntime(kSingleStepHandlerRuntimeEntry);
+  __ popq(RBX);
+  __ LeaveFrame();
+  __ Bind(&not_stepping);
+
   // Load arguments descriptor into R10.
   __ movq(R10, FieldAddress(RBX, ICData::arguments_descriptor_offset()));
   // Loop that checks if there is an IC data match.
@@ -1636,6 +1649,19 @@
   }
 #endif  // DEBUG
 
+  // Check single stepping.
+  Label not_stepping;
+  __ movq(RAX, FieldAddress(CTX, Context::isolate_offset()));
+  __ movzxb(RAX, Address(RAX, Isolate::single_step_offset()));
+  __ cmpq(RAX, Immediate(0));
+  __ j(EQUAL, &not_stepping, Assembler::kNearJump);
+  __ EnterStubFrame();
+  __ pushq(RBX);  // Preserve IC data object.
+  __ CallRuntime(kSingleStepHandlerRuntimeEntry);
+  __ popq(RBX);
+  __ LeaveFrame();
+  __ Bind(&not_stepping);
+
   // RBX: IC data object (preserved).
   __ movq(R12, FieldAddress(RBX, ICData::ic_data_offset()));
   // R12: ic_data_array with entries: target functions and count.
@@ -2098,6 +2124,17 @@
 // Returns ZF set.
 void StubCode::GenerateUnoptimizedIdenticalWithNumberCheckStub(
     Assembler* assembler) {
+  // Check single stepping.
+  Label not_stepping;
+  __ movq(RAX, FieldAddress(CTX, Context::isolate_offset()));
+  __ movzxb(RAX, Address(RAX, Isolate::single_step_offset()));
+  __ cmpq(RAX, Immediate(0));
+  __ j(EQUAL, &not_stepping, Assembler::kNearJump);
+  __ EnterStubFrame();
+  __ CallRuntime(kSingleStepHandlerRuntimeEntry);
+  __ LeaveFrame();
+  __ Bind(&not_stepping);
+
   const Register left = RAX;
   const Register right = RDX;