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<H1>[MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</H1>
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<LI><em>To</em>: <A HREF="mailto:mud-dev#kanga,nu">mud-dev#kanga,nu</A></LI>
<LI><em>Subject</em>: [MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</LI>
<LI><em>From</em>: Chris Gray <<A HREF="mailto:cg#ami-cg,GraySage.Edmonton.AB.CA">cg#ami-cg,GraySage.Edmonton.AB.CA</A>></LI>
<LI><em>Date</em>: Mon, 8 Feb 1999 20:29:46 -0700</LI>
<LI><em>Reply-To</em>: <A HREF="mailto:mud-dev#kanga,nu">mud-dev#kanga,nu</A></LI>
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<PRE>
[Richard Woolcock (KaVir):]
>I believe that switch statements are compiled down to the equivilent
>of if statements, ie:
>
> switch ( x )
> {
> case 1: ...
> case 2: ...
> case 3: ...
> default: ...
> }
>
>Is no more efficient than:
>
> if ( x == 1 ) ...
> else if ( x == 2 ) ...
> else if ( x == 3 ) ...
> else ...
>Some more modern compilers may treat switch statements better than
>the ones I'm used to - if so, I'd appreciate it if someone told me :)
Any compiler that still does that should be taken out and shot. Even
toy compilers do better. I very much doubt any commercial compiler will
do it. There are generally a small number of schemes that a compiler
will use to handle switch statements, depending on the density and
number of the index values. Some methods:
- emit an inline table containing offsets to the code to be executed.
Table is directly indexed by a multiple of the switch value,
usually with a range-test to handle the default. A variant is
to have branch instructions in the table, and to branch to
the appropriate one which then branches to the final code.
- emit an inline table containing index values and code offsets.
Either call a run-time routine, or emit an in-line binary
search to search the table. Similar alternate.
- emit a sequence of instructions that implement an inline binary
search of the index space directly, using inline constants.
- emit a sequence of instructions to do a simple linear search, much
as KaVir suggests. If the compiler does this for more than about
4 or 5 tests, however, it should be shot. This would be done when
the index values are such that, on average, this simple linear
search is the fastest method.
I'm sure I'm missing some - I found lots when I was writing disassemblers
about 10 years ago. I used the first and second in my Draco compiler,
and my AmigaMUD bytecode has those as well.
--
Don't design inefficiency in - it'll happen in the implementation. - me
Chris Gray cg#ami-cg,GraySage.Edmonton.AB.CA
<A HREF="http://www.GraySage.Edmonton.AB.CA/cg/">http://www.GraySage.Edmonton.AB.CA/cg/</A>
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<li><strong><A NAME="00369" HREF="msg00369.html">[MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</A></strong>
<ul compact><li><em>From:</em> "Adam J. Thornton" <adam#phoenix,Princeton.EDU></li></ul>
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<ul><li>Thread context:
<BLOCKQUOTE><UL>
<LI><strong><A NAME="00375" HREF="msg00375.html">[MUD-Dev] Re: optimizing code</A></strong>,
Chris Gray <a href="mailto:cg#ami-cg,GraySage.Edmonton.AB.CA">cg#ami-cg,GraySage.Edmonton.AB.CA</a>, Wed 10 Feb 1999, 02:41 GMT
<LI><strong><A NAME="00371" HREF="msg00371.html">[MUD-Dev] code profiling</A></strong>,
Chris Gray <a href="mailto:cg#ami-cg,GraySage.Edmonton.AB.CA">cg#ami-cg,GraySage.Edmonton.AB.CA</a>, Tue 09 Feb 1999, 06:00 GMT
<UL>
<LI><strong><A NAME="00373" HREF="msg00373.html">[MUD-Dev] optimizing code</A></strong>,
diablo <a href="mailto:diablo#best,com">diablo#best,com</a>, Tue 09 Feb 1999, 06:31 GMT
<UL>
<LI><strong><A NAME="00374" HREF="msg00374.html">[MUD-Dev] Re: optimizing code</A></strong>,
Hans-Henrik Staerfeldt <a href="mailto:hhs#cbs,dtu.dk">hhs#cbs,dtu.dk</a>, Tue 09 Feb 1999, 14:58 GMT
</LI>
</UL>
</LI>
</UL>
</LI>
<LI><strong><A NAME="00366" HREF="msg00366.html">[MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</A></strong>,
Chris Gray <a href="mailto:cg#ami-cg,GraySage.Edmonton.AB.CA">cg#ami-cg,GraySage.Edmonton.AB.CA</a>, Tue 09 Feb 1999, 03:31 GMT
<UL>
<LI><strong><A NAME="00369" HREF="msg00369.html">[MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</A></strong>,
Adam J. Thornton <a href="mailto:adam#phoenix,Princeton.EDU">adam#phoenix,Princeton.EDU</a>, Tue 09 Feb 1999, 04:34 GMT
<UL>
<LI><strong><A NAME="00372" HREF="msg00372.html">[MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</A></strong>,
Ben Greear <a href="mailto:greear#cyberhighway,net">greear#cyberhighway,net</a>, Tue 09 Feb 1999, 06:01 GMT
</LI>
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<li><Possible follow-up(s)><br>
<LI><strong><A NAME="00368" HREF="msg00368.html">[MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</A></strong>,
Chris Gray <a href="mailto:cg#ami-cg,GraySage.Edmonton.AB.CA">cg#ami-cg,GraySage.Edmonton.AB.CA</a>, Tue 09 Feb 1999, 03:54 GMT
<UL>
<LI><strong><A NAME="00370" HREF="msg00370.html">[MUD-Dev] Re: Question on c++ switch optimization, and parsers in general.</A></strong>,
T. Alexander Popiel <a href="mailto:popiel#snugharbor,com">popiel#snugharbor,com</a>, Tue 09 Feb 1999, 04:41 GMT
</LI>
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