Fix slack tracking when instance prototype changes.

src/x64/stub-cache-x64.cc

Index: src/x64/stub-cache-x64.cc
diff --git a/src/x64/stub-cache-x64.cc b/src/x64/stub-cache-x64.cc
index cd71086eecf7a5a41733064d244df1e5815bdb46..b120efb376b6c5def906887d3dbf8f29ae53ecb0 100644
--- a/src/x64/stub-cache-x64.cc
+++ b/src/x64/stub-cache-x64.cc
@@ -3240,6 +3240,7 @@ Handle<Code> ConstructStubCompiler::CompileConstructStub(
 #endif
 
   // Load the initial map and verify that it is in fact a map.
+  // rdi: constructor
   __ movq(rbx, FieldOperand(rdi, JSFunction::kPrototypeOrInitialMapOffset));
   // Will both indicate a NULL and a Smi.
   STATIC_ASSERT(kSmiTag == 0);
@@ -3249,18 +3250,22 @@ Handle<Code> ConstructStubCompiler::CompileConstructStub(
 
 #ifdef DEBUG
   // Cannot construct functions this way.
-  // rdi: constructor
   // rbx: initial map
   __ CmpInstanceType(rbx, JS_FUNCTION_TYPE);
-  __ Assert(not_equal, "Function constructed by construct stub.");
+  __ Check(not_equal, "Function constructed by construct stub.");
 #endif
 
   // Now allocate the JSObject in new space.
-  // rdi: constructor
   // rbx: initial map
+  ASSERT(function->has_initial_map());
+  int instance_size = function->initial_map()->instance_size();
+#ifdef DEBUG
   __ movzxbq(rcx, FieldOperand(rbx, Map::kInstanceSizeOffset));
   __ shl(rcx, Immediate(kPointerSizeLog2));
-  __ AllocateInNewSpace(rcx, rdx, rcx, no_reg,
+  __ cmpq(rcx, Immediate(instance_size));
+  __ Check(equal, "Instance size of initial map changed.");
+#endif
+  __ AllocateInNewSpace(instance_size, rdx, rcx, no_reg,
                         &generic_stub_call, NO_ALLOCATION_FLAGS);
 
   // Allocated the JSObject, now initialize the fields and add the heap tag.
@@ -3306,7 +3311,6 @@ Handle<Code> ConstructStubCompiler::CompileConstructStub(
   }
 
   // Fill the unused in-object property fields with undefined.
-  ASSERT(function->has_initial_map());
   for (int i = shared->this_property_assignments_count();
        i < function->initial_map()->inobject_properties();
        i++) {