<!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>8.1.13 Peephole Optimizer</TITLE>
<META NAME="description" CONTENT="8.1.13 Peephole Optimizer">
<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="previous" HREF="node181.html">
<LINK REL="up" HREF="node169.html">
<LINK REL="next" HREF="node183.html">
</HEAD>
<BODY >
<!--Navigation Panel-->
<A NAME="tex2html3537"
HREF="node183.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next.png"></A>
<A NAME="tex2html3531"
HREF="node169.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up.png"></A>
<A NAME="tex2html3527"
HREF="node181.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="prev.png"></A>
<A NAME="tex2html3533"
HREF="node1.html">
<IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="contents.png"></A>
<A NAME="tex2html3535"
HREF="node191.html">
<IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="index.png"></A>
<BR>
<B> Next:</B> <A NAME="tex2html3538"
HREF="node183.html">8.2 ANSI-Compliance</A>
<B> Up:</B> <A NAME="tex2html3532"
HREF="node169.html">8.1 Optimizations</A>
<B> Previous:</B> <A NAME="tex2html3528"
HREF="node181.html">8.1.12 Higher Order Byte</A>
<B> <A NAME="tex2html3534"
HREF="node1.html">Contents</A></B>
<B> <A NAME="tex2html3536"
HREF="node191.html">Index</A></B>
<BR>
<BR>
<!--End of Navigation Panel-->
<H2><A NAME="SECTION009113000000000000000"></A><A NAME="sub:Peephole-Optimizer"></A><A NAME="4317"></A>
<BR>
8.1.13 Peephole Optimizer
</H2>
<P>
The compiler uses a rule based, pattern matching and re-writing mechanism
for peep-hole optimization. It is inspired by <I>copt</I> a peep-hole
optimizer by Christopher W. Fraser (cwfraser @ microsoft.com). A
default set of rules are compiled into the compiler, additional rules
may be added with the <I>--peep-file<A NAME="4319"></A>
<filename></I> option. The rule language is best illustrated with examples.
<BLOCKQUOTE>
<TT>replace { </TT>
<BR><TT> mov %1,a </TT>
<BR><TT> mov a,%1</TT>
<BR><TT>} by {</TT>
<BR><TT> mov %1,a</TT>
<BR><TT>}</TT>
</BLOCKQUOTE>
The above rule will change the following assembly<A NAME="4328"></A>
sequence:
<BLOCKQUOTE>
<TT>mov r1,a </TT>
<BR><TT>mov a,r1</TT>
</BLOCKQUOTE>
to
<BLOCKQUOTE>
<TT>mov r1,a</TT>
</BLOCKQUOTE>
Note: All occurrences of a <I>%n</I> (pattern variable) must denote
the same string. With the above rule, the assembly sequence:
<BLOCKQUOTE>
<TT>mov r1,a </TT>
<BR><TT>mov a,r2</TT>
</BLOCKQUOTE>
will remain unmodified.
<BR>
<BR>
Other special case optimizations may be added by the user (via
<I>--peep-file option</I>). E.g. some variants of the 8051 MCU<A NAME="4342"></A>
allow only <TT>ajmp</TT> and <TT>acall</TT>. The following two rules
will change all <TT>ljmp</TT> and <TT>lcall</TT> to <TT>ajmp</TT>
and <TT>acall</TT>
<BLOCKQUOTE>
<TT>replace { lcall %1 } by { acall %1 } </TT>
<BR><TT>replace { ljmp %1 } by { ajmp %1 }</TT>
</BLOCKQUOTE>
(NOTE: from version 2.7.3 on, you can use option --acall-ajmp<A NAME="4354"></A>,
which also takes care of aligning the interrupt vectors properly.)
<BR>
<P>
The <I>inline-assembler code</I> is also passed through the peep hole
optimizer, thus the peephole optimizer can also be used as an assembly
level macro expander. The rules themselves are MCU dependent whereas
the rule language infra-structure is MCU independent. Peephole optimization
rules for other MCU can be easily programmed using the rule language.
<BR>
<BR>
The syntax for a rule is as follows:
<BLOCKQUOTE>
<TT>rule := replace [ restart ] '{' <assembly sequence> '\n'
</TT>
<BR><TT> '}' by '{'
'\n' </TT>
<BR><TT> <assembly
sequence> '\n' </TT>
<BR><TT> '}' [if <functionName>
] '\n' </TT>
</BLOCKQUOTE>
<assembly sequence> := assembly instruction (each instruction including
labels must be on a separate line).
<BR>
<BR>
The optimizer will apply to the rules one by one from the top in
the sequence of their appearance, it will terminate when all rules
are exhausted. If the 'restart' option is specified, then the optimizer
will start matching the rules again from the top, this option for
a rule is expensive (performance), it is intended to be used in situations
where a transformation will trigger the same rule again. An example
of this (not a good one, it has side effects) is the following rule:
<BLOCKQUOTE>
<TT>replace restart { </TT>
<BR><TT> pop %1 </TT>
<BR><TT> push %1 } by { </TT>
<BR><TT> ; nop </TT>
<BR><TT>}</TT>
</BLOCKQUOTE>
Note that the replace pattern cannot be a blank, but can be a comment
line. Without the 'restart' option only the innermost 'pop' 'push'
pair would be eliminated, i.e.:
<BLOCKQUOTE>
<TT>pop ar1 </TT>
<BR><TT>pop ar2 </TT>
<BR><TT>push ar2 </TT>
<BR><TT>push ar1</TT>
</BLOCKQUOTE>
would result in:
<BLOCKQUOTE>
<TT>pop ar1 </TT>
<BR><TT>; nop </TT>
<BR><TT>push ar1</TT>
</BLOCKQUOTE>
<I>with</I> the restart option the rule will be applied again to the
resulting code and then all the pop-push pairs will be eliminated
to yield:
<BLOCKQUOTE>
<TT>; nop </TT>
<BR><TT>; nop</TT>
</BLOCKQUOTE>
A conditional function can be attached to a rule. Attaching rules
are somewhat more involved, let me illustrate this with an example.
<BLOCKQUOTE>
<TT>replace { </TT>
<BR><TT> ljmp %5 </TT>
<BR><TT>%2:</TT>
<BR><TT>} by { </TT>
<BR><TT> sjmp %5 </TT>
<BR><TT>%2:</TT>
<BR><TT>} if labelInRange</TT>
</BLOCKQUOTE>
The optimizer does a look-up of a function name table defined in function
<I>callFuncByName</I> in the source file SDCCpeeph.c, with the name
<I>labelInRange</I>. If it finds a corresponding entry the function
is called. Note there can be no parameters specified for these functions,
in this case the use of <I>%5</I> is crucial, since the function
<I>labelInRange</I> expects to find the label in that particular variable
(the hash table containing the variable bindings is passed as a parameter).
If you want to code more such functions, take a close look at the
function labelInRange and the calling mechanism in source file SDCCpeeph.c.
Currently implemented are <I>labelInRange, labelRefCount, labelIsReturnOnly,
operandsNotSame, xramMovcOption, 24bitMode, portIsDS390, 24bitModeAndPortDS390</I>
and <I>notVolatile</I>.
<P>
I know this whole thing is a little kludgey, but maybe some day we
will have some better means. If you are looking at this file, you
will see the default rules that are compiled into the compiler, you
can add your own rules in the default set there if you get tired of
specifying the --peep-file option.
<P>
<HR>
<!--Navigation Panel-->
<A NAME="tex2html3537"
HREF="node183.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next.png"></A>
<A NAME="tex2html3531"
HREF="node169.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up.png"></A>
<A NAME="tex2html3527"
HREF="node181.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="prev.png"></A>
<A NAME="tex2html3533"
HREF="node1.html">
<IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="contents.png"></A>
<A NAME="tex2html3535"
HREF="node191.html">
<IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="index.png"></A>
<BR>
<B> Next:</B> <A NAME="tex2html3538"
HREF="node183.html">8.2 ANSI-Compliance</A>
<B> Up:</B> <A NAME="tex2html3532"
HREF="node169.html">8.1 Optimizations</A>
<B> Previous:</B> <A NAME="tex2html3528"
HREF="node181.html">8.1.12 Higher Order Byte</A>
<B> <A NAME="tex2html3534"
HREF="node1.html">Contents</A></B>
<B> <A NAME="tex2html3536"
HREF="node191.html">Index</A></B>
<!--End of Navigation Panel-->
<ADDRESS>
2011-03-20
</ADDRESS>
</BODY>
</HTML>
distrib
>
Fedora
>
14
>
x86_64
>
media
>
updates
>
by-pkgid
>
e677bbbdff6d27fe001f15e0ef2bb4cc
>
files
>
168
