Sophie: sdcc-3.0.0-0.fc14 x86

sdcc-3.0.0-0.fc14.x86_64.rpm

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">

<!--Converted with LaTeX2HTML 2008 (1.71)
original version by:  Nikos Drakos, CBLU, University of Leeds
* revised and updated by:  Marcus Hennecke, Ross Moore, Herb Swan
* with significant contributions from:
  Jens Lippmann, Marek Rouchal, Martin Wilck and others -->
<HTML>
<HEAD>
<TITLE>8.1.1 Sub-expression Elimination</TITLE>
<META NAME="description" CONTENT="8.1.1 Sub-expression Elimination">
<META NAME="keywords" CONTENT="sdccman">
<META NAME="resource-type" CONTENT="document">
<META NAME="distribution" CONTENT="global">

<META NAME="Generator" CONTENT="LaTeX2HTML v2008">
<META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css">

<LINK REL="STYLESHEET" HREF="sdccman.css">

<LINK REL="next" HREF="node171.html">
<LINK REL="previous" HREF="node169.html">
<LINK REL="up" HREF="node169.html">
<LINK REL="next" HREF="node171.html">
</HEAD>

<BODY >
<!--Navigation Panel-->
<A NAME="tex2html3371"
  HREF="node171.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next.png"></A> 
<A NAME="tex2html3365"
  HREF="node169.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up.png"></A> 
<A NAME="tex2html3359"
  HREF="node169.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="prev.png"></A> 
<A NAME="tex2html3367"
  HREF="node1.html">
<IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="contents.png"></A> 
<A NAME="tex2html3369"
  HREF="node191.html">
<IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="index.png"></A> 
<BR>
<B> Next:</B> <A NAME="tex2html3372"
  HREF="node171.html">8.1.2 Dead-Code Elimination</A>
<B> Up:</B> <A NAME="tex2html3366"
  HREF="node169.html">8.1 Optimizations</A>
<B> Previous:</B> <A NAME="tex2html3360"
  HREF="node169.html">8.1 Optimizations</A>
 &nbsp; <B>  <A NAME="tex2html3368"
  HREF="node1.html">Contents</A></B> 
 &nbsp; <B>  <A NAME="tex2html3370"
  HREF="node191.html">Index</A></B> 
<BR>
<BR>
<!--End of Navigation Panel-->

<H2><A NAME="SECTION00911000000000000000"></A><A NAME="3969"></A>
<BR>
8.1.1 Sub-expression Elimination
</H2>

<P>
The compiler does local and <I>g</I>lobal <I>c</I>ommon <I>s</I>ubexpression
<I>e</I>limination, e.g.: 
<BLOCKQUOTE>
<TT>i = x + y + 1; </TT>&nbsp;
<BR><TT>j = x + y;</TT>

</BLOCKQUOTE>
will be translated to
<BLOCKQUOTE>
<TT>iTemp = x + y; </TT>&nbsp;
<BR><TT>i = iTemp + 1; </TT>&nbsp;
<BR><TT>j = iTemp;</TT>

</BLOCKQUOTE>
Some subexpressions are not as obvious as the above example, e.g.:
<BLOCKQUOTE>
<TT>a-&gt;b[i].c = 10; </TT>&nbsp;
<BR><TT>a-&gt;b[i].d = 11;</TT>

</BLOCKQUOTE>
In this case the address arithmetic a-&gt;b[i] will be computed only
once; the equivalent code in C would be.
<BLOCKQUOTE>
<TT>iTemp = a-&gt;b[i]; </TT>&nbsp;
<BR><TT>iTemp.c = 10; </TT>&nbsp;
<BR><TT>iTemp.d = 11;</TT>

</BLOCKQUOTE>
The compiler will try to keep these temporary variables in registers.

<P>
<BR><HR>
<ADDRESS>

2011-03-20
</ADDRESS>
</BODY>
</HTML>