Rewrite DIExpression::Verify() using an iterator. NFC.

Addresses review comments for r226627.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226747 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DebugInfo.h

@@ -59,7 +59,7 @@ class DIObjCProperty;
 typedef DenseMap<const MDString *, MDNode *> DITypeIdentifierMap;
 
 class DIHeaderFieldIterator
-    : public std::iterator<std::input_iterator_tag, StringRef, std::ptrdiff_t,
+    : public std::iterator<std::forward_iterator_tag, StringRef, std::ptrdiff_t,
                            const StringRef *, StringRef> {
   StringRef Header;
   StringRef Current;
@@ -834,6 +834,8 @@ public:
   void printExtendedName(raw_ostream &OS) const;
 };
 
+class DIExpressionIterator;
+
 /// \brief A complex location expression.
 class DIExpression : public DIDescriptor {
   friend class DIDescriptor;
@@ -862,8 +864,64 @@ public:
   uint64_t getPieceOffset() const;
   /// \brief Return the size of this piece in bytes.
   uint64_t getPieceSize() const;
+
+  DIExpressionIterator begin() const;
+  DIExpressionIterator end() const;
 };
 
+/// \brief An iterator for DIExpression elments.
+class DIExpressionIterator
+    : public std::iterator<std::forward_iterator_tag, StringRef, unsigned,
+                           const uint64_t *, uint64_t> {
+  DIHeaderFieldIterator I;
+  DIExpressionIterator(DIHeaderFieldIterator I) : I(I) {}
+public:
+  DIExpressionIterator() {}
+  DIExpressionIterator(const DIExpression Expr)
+    : I(Expr.getHeader()) { ++I; }
+  uint64_t operator*() const {
+    uint64_t UInt;
+    if (I->getAsInteger(0, UInt))
+      return 0;
+    return UInt;
+  }
+  DIExpressionIterator &operator++() {
+    increment();
+    return *this;
+  }
+  DIExpressionIterator operator++(int) {
+    DIExpressionIterator X(*this);
+    increment();
+    return X;
+  }
+  bool operator==(const DIExpressionIterator &X) const {
+    return I == X.I;
+  }
+  bool operator!=(const DIExpressionIterator &X) const {
+    return !(*this == X);
+  }
+
+  uint64_t getArg(unsigned N) const {
+    auto In = I;
+    std::advance(In, N);
+    return *DIExpressionIterator(In);
+  }
+
+  const DIHeaderFieldIterator& getBase() const { return I; }
+
+private:
+  void increment() {
+    switch (**this) {
+    case dwarf::DW_OP_piece: std::advance(I, 3); break;
+    case dwarf::DW_OP_plus:  std::advance(I, 2); break;
+    case dwarf::DW_OP_deref: std::advance(I, 1); break;
+    default:
+      assert("unsupported operand");
+    }
+  }
+};
+
+
 /// \brief This object holds location information.
 ///
 /// This object is not associated with any DWARF tag.