X86: Only reset npc to reflect instruction length once.

When redirecting fetch to handle branches, the npc of the current pc state
needs to be left alone. This change makes the pc state record whether or not
the npc already reflects a real value by making it keep track of the current
instruction size, or if no size has been set.

src/arch/x86/predecoder.hh

@@ -225,7 +225,11 @@ namespace X86ISA
         {
             assert(emiIsReady);
             emiIsReady = false;
-            nextPC.npc(nextPC.pc() + getInstSize());
+            if (!nextPC.size()) {
+                Addr size = getInstSize();
+                nextPC.size(size);
+                nextPC.npc(nextPC.pc() + size);
+            }
             return emi;
         }
     };

src/arch/x86/types.hh

@@ -222,7 +222,55 @@ namespace X86ISA
         return true;
     }
 
-    typedef GenericISA::UPCState<MachInst> PCState;
+    class PCState : public GenericISA::UPCState<MachInst>
+    {
+      protected:
+        typedef GenericISA::UPCState<MachInst> Base;
+
+        uint8_t _size;
+
+      public:
+        void
+        set(Addr val)
+        {
+            Base::set(val);
+            _size = 0;
+        }
+
+        PCState() {}
+        PCState(Addr val) { set(val); }
+
+        uint8_t size() const { return _size; }
+        void size(uint8_t newSize) { _size = newSize; }
+
+        void
+        advance()
+        {
+            Base::advance();
+            _size = 0;
+        }
+
+        void
+        uEnd()
+        {
+            Base::uEnd();
+            _size = 0;
+        }
+
+        void
+        serialize(std::ostream &os)
+        {
+            Base::serialize(os);
+            SERIALIZE_SCALAR(_size);
+        }
+
+        void
+        unserialize(Checkpoint *cp, const std::string &section)
+        {
+            Base::unserialize(cp, section);
+            UNSERIALIZE_SCALAR(_size);
+        }
+    };
 
     struct CoreSpecific {
         int core_type;