Re: How do Visual C++ compiler optimize switch & cases?

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He Shiming wrote:
> Hi,
>
> I have this switch block in my program, and it has over 200 cases. I'm
> wondering if I should worry about its performance. Is iterating over
> all the case labels a time-consuming job?
>
> I know from others that if the compiler is "smart enough", it could
> sort the case labels to make label-addressing faster. Is that what
> Visual C++ compiler has been doing? In general, do I need to worry
> about the performance of such structured code?

In an optimized build, VC++ will generate a mixture of up to three different
techniques:

1. Sequential if/else logic (for switches with very few cases - e.g. 2).

2. Table lookup (for switches with one or more ranges of consequtive case
values).

3. Tree search (optimized if/else by sorting the tags and then generating a
binary search of the tag space).

The compiler will mix these techniques: a switch with mixed labels and
contiguous ranges will typically use the binary tree logic to select from
indivudal cases or table-based dispatch.

-cd


.

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