Re: switch or if?
From: Dennis Myrén (dennis_at_oslokb.no)
Date: 03/19/04
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Date: Fri, 19 Mar 2004 09:30:46 +0100
Thank you for very good answer.
"Michael Sparks" <michael.sparks@remove.this.sbcglobal.net> wrote in message
news:P%j6c.94$1P6.93@newssvr23.news.prodigy.com...
> "Dennis Myrén" <dennis@oslokb.no> wrote in message
> news:NZg6c.1495$zf6.19073@news4.e.nsc.no...
> > I have to ask this question once for all;
> > Is there a good rule for when to use a switch statement rather than an
if
> > statement?
> > I know if statements is faster in small tests,
> > but how many if-elseif's are considered a maximum before it is
recommended
> > to use a switch statement?
>
> After seeing your question, I became curious myself, so I did some tests.
>
> I tested two scenarios, dense switches, and sparse switches. With dense
> switches, we are selecting from some set of values without skipping any
> values in the middle. With sparse switches, we are selecting from values
> that may skip around a lot.
>
> In all cases, the machine code generated for switch() will out-perform
that
> generated for if().
>
> The machine code generated for if() ... else if() ... is a pretty faithful
> translation of the source code. There are machine jump instructions for
> every value tested against, and they even appear in the same order as your
> source code. There are O(n) jump instructions, and it isn't affected by
> whether we're doing dense or sparse tests.
>
> The machine code generated for switch() depends on whether we do a dense
or
> sparse test.
>
> The dense switch() is extremely efficient, doing pointer arithmetic to
> compute the destination address of it's *single* jump instruction. This
> optimization is probably only possible thanks to the special IL
instruction
> "switch", which the C# compiler outputs in this situation. Here, we have
> O(1) jump instructions.
>
> The sparse switch() is also very efficient, it does a series of
comparisons
> paired with conditional jump instructions, and only jumps one time, to the
> desired case. Here, we also have O(1) jump instructions (that are
actually
> executed).
>
> In addition to this, the sparse switch does all comparisons and
conditional
> jumps in one big stretch of machine code, which is really good for CPU
> caches. The machine code for if() jumps all over the place to arrive at
its
> destination, so if you do a lot of work in each if(), there will be a
bunch
> of stuff to jump over - and therefore the locality of reference goes way
> down. That translates to CPU cache misses, which are performance killers.
>
> Another thing to consider with the dense switch() is that IL is expressive
> enough to capture the compiler's intent in this situation, so the JIT for
> other potential platforms may be able to optimize it even better (though
> they'll have a hard time beating O(1)).
>
> Conclusion: switch always out-performs if. That said, when there are a
> small number of tests, you may choose if() just for readability if it
makes
> sense in a particular situation.
>
> I only tested switch on integer types.. not strings. I suspect that
> switch() would still outperform if() in this case due to locality of
> reference.
>
> Mike
>
>
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