Revert "Remove unnecessary check in RegExpExecStub."

src/x64/code-stubs-x64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/bootstrapper.h"
 #include "src/code-stubs.h"
@@ skipping 1556 matching lines @@
                        RegExpImpl::kFirstCaptureOffset),
           rdi);
   __ jmp(&next_capture);
   __ bind(&done);
 
   // Return last match info.
   __ movp(rax, r15);
   __ ret(REG_EXP_EXEC_ARGUMENT_COUNT * kPointerSize);
 
   __ bind(&exception);
-  // Result must now be exception.
+  // Result must now be exception. If there is no pending exception already a
+  // stack overflow (on the backtrack stack) was detected in RegExp code but
+  // haven't created the exception yet. Handle that in the runtime system.
+  // TODO(592): Rerunning the RegExp to get the stack overflow exception.
   ExternalReference pending_exception_address(
       Isolate::kPendingExceptionAddress, isolate());
   Operand pending_exception_operand =
       masm->ExternalOperand(pending_exception_address, rbx);
+  __ movp(rax, pending_exception_operand);
   __ LoadRoot(rdx, Heap::kTheHoleValueRootIndex);
-  __ movp(rax, pending_exception_operand);
-  __ movp(pending_exception_operand, rdx);  // Clear pending exception.
+  __ cmpp(rax, rdx);
+  __ j(equal, &runtime);
+  __ movp(pending_exception_operand, rdx);
 
   __ CompareRoot(rax, Heap::kTerminationExceptionRootIndex);
   Label termination_exception;
   __ j(equal, &termination_exception, Label::kNear);
   __ Throw(rax);
 
   __ bind(&termination_exception);
   __ ThrowUncatchable(rax);
 
   // Do the runtime call to execute the regexp.
@@ skipping 3341 matching lines @@
                               return_value_operand,
                               NULL);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64