MCRegisterInfo: Add MCSubRegIndexIterator.

This iterator iterates over subregister and their associated subregister indices
at the same time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223893 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCRegisterInfo.h

@@ -233,6 +233,7 @@ public:
   // These iterators are allowed to sub-class DiffListIterator and access
   // internal list pointers.
   friend class MCSubRegIterator;
+  friend class MCSubRegIndexIterator;
   friend class MCSuperRegIterator;
   friend class MCRegUnitIterator;
   friend class MCRegUnitMaskIterator;
@@ -461,6 +462,38 @@ public:
   }
 };
 
+/// Iterator that enumerates the sub-registers of a Reg and the associated
+/// sub-register indices.
+class MCSubRegIndexIterator {
+  MCSubRegIterator SRIter;
+  const uint16_t *SRIndex;
+public:
+  /// Constructs an iterator that traverses subregisters and their
+  /// associated subregister indices.
+  MCSubRegIndexIterator(unsigned Reg, const MCRegisterInfo *MCRI)
+    : SRIter(Reg, MCRI) {
+    SRIndex = MCRI->SubRegIndices + MCRI->get(Reg).SubRegIndices;
+  }
+
+  /// Returns current sub-register.
+  unsigned getSubReg() const {
+    return *SRIter;
+  }
+  /// Returns sub-register index of the current sub-register.
+  unsigned getSubRegIndex() const {
+    return *SRIndex;
+  }
+
+  /// Returns true if this iterator is not yet at the end.
+  bool isValid() const { return SRIter.isValid(); }
+
+  /// Moves to the next position.
+  void operator++() {
+    ++SRIter;
+    ++SRIndex;
+  }
+};
+
 /// MCSuperRegIterator enumerates all super-registers of Reg.
 /// If IncludeSelf is set, Reg itself is included in the list.
 class MCSuperRegIterator : public MCRegisterInfo::DiffListIterator {