arch: Promote instReady to the base InstDecoder class.

Move the instDone flag, and the instReady function which was
consistently implemented just to return it, to the base InstDecoder
class. This flag can still be accessed easily from the subclasses, but
now it can be retrieved with just an InstDecoder pointer without a
virtual function call.

Change-Id: I8c662aa01da8fe33ffe679071c701e0aadc1a795
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52072
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/generic/decoder.hh

@@ -43,6 +43,8 @@ class InstDecoder
     size_t _moreBytesSize;
     Addr _pcMask;
 
+    bool instDone = false;
+
   public:
     template <typename MoreBytesType>
     InstDecoder(MoreBytesType *mb_buf) :
@@ -56,6 +58,16 @@ class InstDecoder
     void *moreBytesPtr() const { return _moreBytesPtr; }
     size_t moreBytesSize() const { return _moreBytesSize; }
     Addr pcMask() const { return _pcMask; }
+
+    /**
+     * Is an instruction ready to be decoded?
+     *
+     * CPU models call this method to determine if decode() will
+     * return a new instruction on the next call. It typically only
+     * returns false if the decoder hasn't received enough data to
+     * decode a full instruction.
+     */
+    bool instReady() const { return instDone; }
 };
 
 } // namespace gem5