Re: [OT] Switch question

From: Alan Balmer (albalmer_at_att.net)
Date: 02/23/04 

Date: Mon, 23 Feb 2004 11:28:37 -0700

On Sat, 21 Feb 2004 11:30:13 +0000 (UTC), "Sean Kenwrick"
<skenwrick@hotmail.com> wrote:

>I have a large switch statement (~50 cases ) inside a critical inner loop of
>a byte code interpreter I am working on, and I am wondering if I should take
>any steps to optimise this loop.

How would we know ? <G>. Is it running too slow? Have you profiled the
program and determined that the switch statement in this loop is using
a significant amount of time? If you haven't done this, you're not
ready to start optimizing.
>
>Are switch statements usually compiled as if...else if..else if...else type
>statements?
Very unlikely.

> Are there other ways that the compiler could implement switch
>statements that are more efficient?
Yes - several ways. The compiler may choose different ways depending
on its analysis of your code.
>
>With regards to my code, would it be better to put the most common cases
>near the top of the switch statement
Probably a waste of time, and may make the code less readable.

> or should I consider making a lookup
>table of function pointers instead (but the function call overhead might
>kill it) - this is the first time that I wish I could put labels into arrays
>and do 'goto label[x]' :-) )
No. This would be restricting the compiler's choices. As for "goto
label[x]', the compiler may choose to do it that way.
>
>I've tried posting to comp.compilers but nothing has appeared in this group
>for iver a week and it seems to be dead ....
>
>Any advice, hints etc would be appreciated...
>
>Sean
>
Profile first. Profile after each change. Look for better algorithms
before worrying about code details. Study the compiler's output - you
may be surprised. 

-- 
Al Balmer
Balmer Consulting
removebalmerconsultingthis@att.net

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