Add a separate storage class for PI nodes

This allows us to add pi value to restore target support for PI nodes without
increasing the memory usage for other nodes.

Right now the PI node has a separate header that's used for allocated bit;
this allows us to reduce header bitcount in the future.

src/pugixml.cpp

@@ -972,6 +972,19 @@ namespace pugi
 
 		impl::compact_pointer<xml_attribute_struct, 11, 0>	first_attribute;		///< First attribute
 	};
+
+	struct xml_node_pi_struct: xml_node_struct
+	{
+		xml_node_pi_struct(impl::xml_memory_page* page): xml_node_struct(page, node_pi), pi_header(page, 0)
+		{
+			PUGI__STATIC_ASSERT(sizeof(xml_node_pi_struct) == 20);
+		}
+
+		impl::compact_header pi_header;
+		impl::compact_string<3> pi_value;
+
+		unsigned char padding[2];
+	};
 }
 #else
 namespace pugi
@@ -1015,6 +1028,16 @@ namespace pugi
 		
 		xml_attribute_struct*	first_attribute;		///< First attribute
 	};
+
+	struct xml_node_pi_struct: xml_node_struct
+	{
+		xml_node_pi_struct(impl::xml_memory_page* page): xml_node_struct(page, node_pi), pi_header(reinterpret_cast<uintptr_t>(page)), pi_value(0)
+		{
+		}
+
+		uintptr_t pi_header;
+		char_t* pi_value;
+	};
 }
 #endif
 
@@ -1069,12 +1092,22 @@ PUGI__NS_BEGIN
 	}
 
 	inline xml_node_struct* allocate_node(xml_allocator& alloc, xml_node_type type)
+	{
+		if (type != node_pi)
 		{
 			xml_memory_page* page;
 			void* memory = alloc.allocate_memory(sizeof(xml_node_struct), page);
 
 			return new (memory) xml_node_struct(page, type);
 		}
+		else
+		{
+			xml_memory_page* page;
+			void* memory = alloc.allocate_memory(sizeof(xml_node_pi_struct), page);
+
+			return new (memory) xml_node_pi_struct(page);
+		}
+	}
 
 	inline void destroy_attribute(xml_attribute_struct* a, xml_allocator& alloc)
 	{
@@ -3097,7 +3130,8 @@ PUGI__NS_BEGIN
 					else
 					{
 						// store value and step over >
-						// TODO: node_pi value:cursor->value = value;
+						static_cast<xml_node_pi_struct*>(cursor)->pi_value = value;
+
 						PUGI__POPNODE();
 
 						PUGI__ENDSEG();
@@ -4060,10 +4094,10 @@ PUGI__NS_BEGIN
 				writer.write('<', '?');
 				writer.write_string(node->contents ? node->contents : default_name);
 
-				if (node->contents)
+				if (static_cast<xml_node_pi_struct*>(node)->pi_value)
 				{
 					writer.write(' ');
-					writer.write_string(node->contents);
+					writer.write_string(static_cast<xml_node_pi_struct*>(node)->pi_value);
 				}
 
 				writer.write('?', '>');
@@ -4235,6 +4269,14 @@ PUGI__NS_BEGIN
 	{
 		node_copy_string(dn->contents, dn->header, xml_memory_page_contents_allocated_mask, sn->contents, sn->header, shared_alloc);
 
+		if (PUGI__NODETYPE(dn) == node_pi)
+		{
+			xml_node_pi_struct* dnp = static_cast<xml_node_pi_struct*>(dn);
+			xml_node_pi_struct* snp = static_cast<xml_node_pi_struct*>(sn);
+
+			node_copy_string(dnp->pi_value, dnp->pi_header, xml_memory_page_contents_allocated_mask, snp->pi_value, snp->pi_header, shared_alloc);
+		}
+
 		for (xml_attribute_struct* sa = sn->first_attribute; sa; sa = sa->next_attribute)
 		{
 			xml_attribute_struct* da = append_new_attribute(dn, get_allocator(dn));
@@ -5251,7 +5293,16 @@ namespace pugi
 	
 	PUGI__FN const char_t* xml_node::value() const
 	{
-		return (_root && impl::has_value(_root) && _root->contents) ? _root->contents + 0 : PUGIXML_TEXT("");
+		if (_root)
+		{
+			if (impl::has_value(_root) && _root->contents)
+				return _root->contents;
+
+			if (PUGI__NODETYPE(_root) == node_pi && static_cast<xml_node_pi_struct*>(_root)->pi_value)
+				return static_cast<xml_node_pi_struct*>(_root)->pi_value;
+		}
+
+		return PUGIXML_TEXT("");
 	}
 	
 	PUGI__FN xml_node xml_node::child(const char_t* name_) const
@@ -5382,6 +5433,12 @@ namespace pugi
 		switch (type())
 		{
 		case node_pi:
+		{
+			xml_node_pi_struct* pn = static_cast<xml_node_pi_struct*>(_root);
+
+			return impl::strcpy_insitu(pn->pi_value, pn->pi_header, impl::xml_memory_page_contents_allocated_mask, rhs);
+		}
+
 		case node_cdata:
 		case node_pcdata:
 		case node_comment: