[Tfug] GCC optimizations

[Bexley Hall]

Hi,

--- Robert Hunter <hunter at tfug.org> wrote:

> It seems that GCC generates either binary search or
> jump table code based on a hueristic which considers
> the number of cases and the range of case values.

Makes sense...

> For very small numbers of cases, or for very
> large ranges of values, gcc generates binary search
> code.

Hmmm... for small numbers of cases, it seems like the
extra tests for a binary search would just get in
the way.  And, for pathological scenarios, probably
be counterproductive  :<  (of course, expecting a
machine to get *all* of these things right is
asking a lot -- easier for the application designer
to put some smarts into the structure of the code)

> Otherwise it generates a jump table.

>  Previously I ran across some literature
> that discussed using different branching techniques,
> such as perfect hashes, but I don't have a
reference.
> 
> The article linked below addresses gcc for the cell,
> but I think it is generally relevant.
> 
>
http://www.cellperformance.com/articles/2006/04/programming_with_branches_patt.html#rule_3

*Excellent* reference!  I will bookmark this for
future reference.  Thank you!

I think my best bet is to coax the compiler into
generating a *small* jump table and ensure that
all cases have contiguous values (no holes).
It would be hard to do much better by hand -- unless
I statistically modeled the relative frequencies of
each branch (case) and tuned the code to that
distribution...  :<

Thanks!
--don

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