Re: [xsl] tail recursion optimization (was How efficient is DVC?)

["David Rosenborg" <darolst@xxxxxxxxxx>]

Mike Brown wrote:

>   <xsl:template match="group">
>     <xsl:copy>
>       <xsl:copy-of select="@*"/>
>       <xsl:copy-of select="./city"/>
>     </xsl:copy>
>     <xsl:apply-templates select="./group"/>
>   </xsl:template>
>
> I don't know if any processors can optimize this. It may seem obvious to
you
> and me that the apply-templates instruction will result in this same
template
> being applied, but there might be some other template that also matches
> certain 'group' elements at a higher priority, so there's no easy way for
the
> processor to be sure (before runtime) that this template will be the one
that
> is invoked for every one of the selected nodes. Or at least, the analysis
> required to be certain that this is tail-recursive is not simple enough
that
> anyone has bothered to figure it out yet. (Suggestions welcome...)
>

>From a tail call perspecitve, there is no real difference between a named
template call and applying templates. Tail call opt. has more to do
with where the call appears than how and what is being called.
The key issue is to leave or reuse the current stack frame before you
make the call. The only difference in an apply-templates call is that
it actually issues one call per selected node, so you would need to reuse
the
stack frame for each of these calls. The fact that the actual template
applied
is decided dynamically is not a tail call issue.

Cheers,
David

-----------------------------------------------
David Rosenborg
Pantor Engineering AB


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