66 lines
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66 lines
4.3 KiB
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>A.3. Optimization</TITLE>
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<meta name="description" value=" Optimization">
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<meta name="resource-type" value="document">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html6150 HREF="node357.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6148 HREF="node347.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6142 HREF="node355.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6152 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6153 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6151 HREF="node357.html"> Basic Restrictions</A>
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<B>Up:</B> <A NAME=tex2html6149 HREF="node347.html"> Series</A>
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<B> Previous:</B> <A NAME=tex2html6143 HREF="node355.html"> Alteration of Series</A>
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<HR> <P>
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<H1><A NAME=SECTION003430000000000000000>A.3. Optimization</A></H1>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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<A NAME=SERIESESECTION>Series</A>
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expressions are transformed into loops by pipelining them-the
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computation is converted from a form where entire series are computed one
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after the other to a form where the series are incrementally computed in
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parallel. In the resulting loop, each individual element is computed just
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once, used, and then discarded before the next element is computed. For
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this pipelining to be possible, a number of restrictions have to be
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satisfied. Before these restrictions are explained, it will be useful to consider
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a related issue.
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<P>
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The composition of two series functions cannot be pipelined unless the
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destination function consumes series elements in the same order that the source
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function produces them. Taken together, the series functions guarantee
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that this will always be true, because they all follow the same fixed
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processing order. In particular, they are all <i>preorder</i>
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functions-they process the elements of their series inputs and outputs in
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ascending order starting with the first element. Further, while it is easy
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for users to define new series functions, it is impossible to define one
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that is not a preorder function.
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<P>
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It turns out that most series operations can easily be implemented in a
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preorder fashion, the most notable exceptions being reversal and sorting. As
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a result, little is lost by outlawing non-preorder series functions. If some
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non-preorder operation has to be applied to a series, the series can be
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collected into a list or vector and the operation applied to this new data
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structure. (This is inefficient, but no less efficient than what would be
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required if non-preorder series functions were supported.)
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<HR>
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<UL>
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<LI> <A NAME=tex2html6154 HREF="node357.html#SECTION003431000000000000000"> Basic Restrictions</A>
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<LI> <A NAME=tex2html6155 HREF="node358.html#SECTION003432000000000000000"> Constraint Cycles</A>
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<LI> <A NAME=tex2html6156 HREF="node359.html#SECTION003433000000000000000"> Defining New Series Functions</A>
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<LI> <A NAME=tex2html6157 HREF="node360.html#SECTION003434000000000000000"> Declarations</A>
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</UL>
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<BR> <HR><A NAME=tex2html6150 HREF="node357.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6148 HREF="node347.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6142 HREF="node355.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6152 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6153 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6151 HREF="node357.html"> Basic Restrictions</A>
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<B>Up:</B> <A NAME=tex2html6149 HREF="node347.html"> Series</A>
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<B> Previous:</B> <A NAME=tex2html6143 HREF="node355.html"> Alteration of Series</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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