[Mono-dev] [PATCH] - optimization for System.Xml.XmlNode::SelectSingleNode

[tom hindle]

Hi,

While performance profiling our code, with mono's nice profiling
tools :), I noticed System.Xml.XmlNode::SelectSingleNode was taking 23ms
a call while the sum of the methods it was calling took < 5ms.
SelectSingleNode is a very simple method however it contains a (dynamic)
down cast. I replaced the (Cstyle/prefix) cast with a 'as' cast and this
seem to reduce the method time by about 7ms. 


// With 'Cstyle' cast
 39672.303    1717   23.106
System.Xml.XmlNode::SelectSingleNode(string,XmlNamespaceManager)
  Callers (with count) that contribute at least for 1%:
        1717  100 % System.Xml.XmlNode::SelectSingleNode(string)

// With 'as' cast
 29238.117    1880   15.552
System.Xml.XmlNode::SelectSingleNode(string,XmlNamespaceManager)
  Callers (with count) that contribute at least for 1%:
        1880  100 % System.Xml.XmlNode::SelectSingleNode(string)


I read that this is because 'as' cast generates IL instr 'isinst' while
'cstyle' cast generates 'castclass'.

The msdn documentation about XmlNode.SelectSingleNode doesn't state that
an InvalidCastException, can/could be thrown.  

Is this a sensible thing to do? If so could someone review/commit my
patch as I believe it will make a fairly major difference for
applications that make much use of SelectSingleNode.


Thanks
Tom
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