Code cleanup in GenerateRecordCallTarget.

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 49904297febf74a2e61c0d712fd560c29ffce803..97804508fd4bfd220cbea733231eb454932a6f59 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -1785,7 +1785,7 @@ static void CallStubInRecordCallTarget(MacroAssembler* masm, CodeStub* stub) {
   // edi : the function to call
   FrameScope scope(masm, StackFrame::INTERNAL);
 
-  // Arguments register must be smi-tagged to call out.
+  // Number-of-arguments register must be smi-tagged to call out.
   __ Integer32ToSmi(rax, rax);
   __ Push(rax);
   __ Push(rdi);
@@ -1822,17 +1822,19 @@ static void GenerateRecordCallTarget(MacroAssembler* masm) {
 
   // A monomorphic cache hit or an already megamorphic state: invoke the
   // function without changing the state.
+  // We don't know if rcx is a WeakCell or a Symbol, but it's harmless to read
+  // at this position in a symbol (see static asserts in
+  // type-feedback-vector.h).
   Label check_allocation_site;
   __ cmpp(rdi, FieldOperand(rcx, WeakCell::kValueOffset));
   __ j(equal, &done, Label::kFar);
   __ CompareRoot(rcx, Heap::kmegamorphic_symbolRootIndex);
   __ j(equal, &done, Label::kFar);
-  __ CompareRoot(FieldOperand(rcx, 0), Heap::kWeakCellMapRootIndex);
+  __ CompareRoot(FieldOperand(rcx, HeapObject::kMapOffset),
+                 Heap::kWeakCellMapRootIndex);
   __ j(not_equal, FLAG_pretenuring_call_new ? &miss : &check_allocation_site);
 
-  // If edi is not equal to the weak cell value, and the weak cell value is
-  // cleared, we have a new chance to become monomorphic. Otherwise, we
-  // need to go megamorphic.
+  // If the weak cell is cleared, we have a new chance to become monomorphic.
   __ CheckSmi(FieldOperand(rcx, WeakCell::kValueOffset));
   __ j(equal, &initialize);
   __ jmp(&megamorphic);
@@ -1842,7 +1844,7 @@ static void GenerateRecordCallTarget(MacroAssembler* masm) {
     // If we came here, we need to see if we are the array function.
     // If we didn't have a matching function, and we didn't find the megamorph
     // sentinel, then we have in the slot either some other function or an
-    // AllocationSite. Do a map check on the object in rcx.
+    // AllocationSite.
     __ CompareRoot(FieldOperand(rcx, 0), Heap::kAllocationSiteMapRootIndex);
     __ j(not_equal, &miss);