[xquery-talk] Re: Optimizations

Fri Oct 3 16:27:01 PDT 2008

This is a case where Saxon (even Saxon-SA) has stopped optimizing when it
could have achieved a little more. The relevant part of the final expression
tree is

     <return>
        <sequence>
          <choose>
            <when>
              <compareToInteger op="ge" value="4">
                <literal value="3" type="xs:integer"/>
              </compareToInteger>
            </when>
            <then>
              <directElement name="debug" validation="preserve">
                <variableReference name="dosmthg"/>
              </directElement>
            </then>
          </choose>
          <variableReference name="dosmthg"/>
        </sequence>
      </return>

and it hasn't noticed that the <when> condition is now constant.

Deciding when to stop optimizing is the trickiest part of the exercise - if
you go on too long, optimization can take longer than any run-time savings
it is delivering. In this occasion, two more passes would have paid
dividends - one to evaluate the <when> condition, the other to reduce the
<choose> to ().

Michael Kay
http://www.saxonica.com/

  _____  

From: talk-bounces at x-query.com [mailto:talk-bounces at x-query.com] On Behalf
Of Rémi Dewitte
Sent: 03 October 2008 14:24
To: talk at x-query.com
Subject: [xquery-talk] Re: Optimizations

Hi again,

In the same spirit, this is not at all optimized.
Maybe I need Saxon SA ?

declare variable $level as xs:integer := 3 ;

declare function local:debug($l as xs:integer,$what){
  if($l >=4) then
    <debug>{$what}</debug>
  else ()
};

let $dosmthg := <oki>test trace</oki>

return
(local:debug($level,$dosmthg),$dosmthg)

Rémi

On Fri, Oct 3, 2008 at 3:17 PM, Rémi Dewitte <remi at gide.net> wrote:

Hi,

I have made a little experiment after reading Michael (Kay) blog posts about
optimizations and recent James Fuller article.

I have a question quite specific to saxon still interesting in general.

Why in the following example even the function call is not skipped ?

XQuery is ::

declare variable $level := 3;

declare function local:debug($what){
  if($level >=4) then
    <debug>{$what}</debug>
  else ()
};

let $dosmthg := <oki>test trace</oki>

return
(local:debug($dosmthg),$dosmthg)

When I run saxon with -explain we get 

<query>
  <globalVariables>
    <declareVariable name="level">
      <literal value="3" type="xs:integer"/>
    </declareVariable>
  </globalVariables>
  <declareFunction name="local:debug" tailRecursive="false">
    <literal value="()"/>
  </declareFunction>
  <body>
    <let variable="dosmthg" as="element(oki,
{http://www.w3.org/2001/XMLSchema}untyped
<http://www.w3.org/2001/XMLSchema%7Duntyped> )">
      <be>
        <directElement name="oki" validation="skip">
          <valueOf>
            <literal value="test trace" type="xs:string"/>
          </valueOf>
        </directElement>
      </be>
      <return>
        <sequence>
          <functionCall name="local:debug" tailCall="false">
            <variableReference name="dosmthg"/>
          </functionCall>
          <variableReference name="dosmthg"/>
        </sequence>
      </return>
    </let>
  </body>
</query>

Cheers,
Rémi

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