[Mono-devel-list] mcs code for switches on strings

[meddochat (Matthijs ter Woord)]

Running on microsoft .net framework v1.1.4322, the results are:

3
Time using if-statement: 70,1008
Time using switch-statement: 630,9072

so... i think it's something in MSIL

Matthijs
----- Original Message ----- 
From: "Jonathan Pryor" <jonpryor at vt.edu>
To: "Richard Torkar" <richard.torkar at htu.se>
Cc: "Mono Development List" <mono-devel-list at lists.ximian.com>
Sent: Saturday, March 20, 2004 1:44 PM
Subject: Re: [Mono-devel-list] mcs code for switches on strings


> Below...
>
> On Sat, 2004-03-20 at 04:49, Richard Torkar wrote:
> > On Sat, 2004-03-20 at 03:17 +0000, Marcus wrote:
> <snip/>
> > Hmm, is it just me or isn't switch/case supposed to execute faster when
> > comparing several strings like above? I learnt that many years ago, thus
> > I consequently use switch/case in stead of nested if-statements if it
> > looks like the above.
> >
> > Is this not true anymore as a general rule?
>
> Nothing is true as a general rule.  Everything has an exception. :-)
>
> Except the general rule of "test first, optimize later."  Except when
> you're creating the design, in which case you need to make sure the
> design will meet your performance requirements.  Guess there's an
> exception here to...
>
> There are numerous ways to implement the switch statement for strings,
> from using an internal array of strings and binary-searching the array,
> using the array to get the code offset, then jumping, to using a
> Dictionary of strings to lookup the jump point.  Many techniques are
> possible.
>
> The issue is that all of those techniques have some amount of overhead
> associated with them.  So switching on 2 strings + default case is not
> enough strings lookups to overcome the overhead of many implementation
> techniques, at least in this case.  I'm not sure what the break-even
> point is, and that would depend upon the length of those strings as well
> as the number of strings in the switch.
>
> Now, it might be possible to add intelligence to the compiler so that it
> knows what the break-even point is, and have it emit an "if" tree if
> some threshold isn't reached.  This might not be possible, though,
> especially if you have case statements that don't end in "break", but
> instead end with "goto case FOO".
>
> In any event, this would be something to look into implementing. :-)
>
>  - Jon
>
>
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