Guard the debug check at the start of every JS function with...

src/x64/virtual-frame-x64.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 45 matching lines @@
 void VirtualFrame::Enter() {
   // Registers live on entry to a JS frame:
   //   rsp: stack pointer, points to return address from this function.
   //   rbp: base pointer, points to previous JS, ArgumentsAdaptor, or
   //        Trampoline frame.
   //   rsi: context of this function call.
   //   rdi: pointer to this function object.
   Comment cmnt(masm(), "[ Enter JS frame");
 
 #ifdef DEBUG
-  // Verify that rdi contains a JS function.  The following code
-  // relies on rax being available for use.
-  Condition not_smi = masm()->CheckNotSmi(rdi);
-  __ Check(not_smi,
-           "VirtualFrame::Enter - rdi is not a function (smi check).");
-  __ CmpObjectType(rdi, JS_FUNCTION_TYPE, rax);
-  __ Check(equal,
-           "VirtualFrame::Enter - rdi is not a function (map check).");
+  if (FLAG_debug_code) {
+    // Verify that rdi contains a JS function.  The following code
+    // relies on rax being available for use.
+    Condition not_smi = masm()->CheckNotSmi(rdi);
+    __ Check(not_smi,
+             "VirtualFrame::Enter - rdi is not a function (smi check).");
+    __ CmpObjectType(rdi, JS_FUNCTION_TYPE, rax);
+    __ Check(equal,
+             "VirtualFrame::Enter - rdi is not a function (map check).");
+  }
 #endif
 
   EmitPush(rbp);
 
   __ movq(rbp, rsp);
 
   // Store the context in the frame.  The context is kept in rsi and a
   // copy is stored in the frame.  The external reference to rsi
   // remains.
   EmitPush(rsi);
@@ skipping 969 matching lines @@
   // Grow the expression stack by handler size less one (the return
   // address is already pushed by a call instruction).
   Adjust(kHandlerSize - 1);
   __ PushTryHandler(IN_JAVASCRIPT, type);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal