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<H1><A NAME="SECTION00700000000000000000">
6. TIPS</A>
</H1>
<P>
Here are a few guidelines that will help the compiler generate more
efficient code, some of the tips are specific to this compiler others
are generally good programming practice.
<UL>
<LI>Use the smallest data type to represent your data-value. If it is
known in advance that the value is going to be less than 256 then
use an 'unsigned char' instead of a 'short' or 'int'. Please note,
that ANSI C requires both signed and unsigned chars to be promoted
to 'signed int'<A NAME="3621"></A>
before doing any operation. This promotion<A NAME="3623"></A><A NAME="type_promotion"></A>can be omitted, if the result is the same. The effect of the promotion
rules together with the sign-extension is often surprising:
<P>
<BLOCKQUOTE>
<TT>unsigned char uc = 0xfe;</TT>
<BR><TT>if (uc * uc < 0) /* this is true! */</TT>
<BR><TT>{</TT>
<BR><TT> ....</TT>
<BR><TT>}</TT>
</BLOCKQUOTE>
<P>
<TT>uc * uc</TT> is evaluated as <TT>(int) uc * (int) uc =
(int) 0xfe * (int) 0xfe = (int) 0xfc04 = -1024</TT>.
<BR>
Another one:
<BLOCKQUOTE>
<TT>(unsigned char) -12 / (signed char) -3 = ...</TT>
</BLOCKQUOTE>
<P>
No, the result is not 4:
<BLOCKQUOTE>
<TT>(int) (unsigned char) -12 / (int) (signed char) -3 =</TT>
<BR><TT>(int) (unsigned char) 0xf4 / (int) (signed char) 0xfd =</TT>
<BR><TT>(int) 0x00f4 / (int) 0xfffd =</TT>
<BR><TT>(int) 0x00f4 / (int) 0xfffd =</TT>
<BR><TT>(int) 244 / (int) -3 =</TT>
<BR><TT>(int) -81 = (int) 0xffaf;</TT>
</BLOCKQUOTE>
<P>
Don't complain, that gcc gives you a different result. gcc uses 32
bit ints, while SDCC uses 16 bit ints. Therefore the results are different.
<BR>
From ''comp.lang.c FAQ'':
<BLOCKQUOTE>
<I>If well-defined overflow characteristics are important and negative
values are not, or if you want to steer clear of sign-extension problems
when manipulating bits or bytes, use one of the corresponding unsigned
types. (Beware when mixing signed and unsigned values in expressions,
though.)</I>
<BR><I>Although character types (especially unsigned char) can be
used as tiny integers, doing so is sometimes
more trouble than it's worth, due to unpredictable sign extension
and increased code size.</I>
</BLOCKQUOTE>
</LI>
<LI>Use unsigned when it is known in advance that the value is not going
to be negative. This helps especially if you are doing division or
multiplication, bit-shifting or are using an array index.
</LI>
<LI>NEVER jump into a LOOP.
</LI>
<LI>Declare the variables to be local<A NAME="3653"></A> whenever
possible, especially loop control variables (induction).
</LI>
<LI>Have a look at the assembly listing to get a ''feeling'' for the
code generation.
</LI>
</UL>
<P>
<BR><HR>
<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
<UL>
<LI><A NAME="tex2html3021"
HREF="node149.html">6.1 Porting code from or to other compilers</A>
<LI><A NAME="tex2html3022"
HREF="node150.html">6.2 Tools included in the distribution</A>
<LI><A NAME="tex2html3023"
HREF="node151.html">6.3 Documentation included in the distribution</A>
<LI><A NAME="tex2html3024"
HREF="node152.html">6.4 Communication online at SourceForge</A>
<LI><A NAME="tex2html3025"
HREF="node153.html">6.5 Related open source tools</A>
<LI><A NAME="tex2html3026"
HREF="node154.html">6.6 Related documentation / recommended reading</A>
<LI><A NAME="tex2html3027"
HREF="node155.html">6.7 Application notes specifically for SDCC</A>
<LI><A NAME="tex2html3028"
HREF="node156.html">6.8 Some Questions</A>
</UL>
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2011-03-20
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