Re: GNAT: Performance of String functions

["Tarjei T. Jensen" <tarjei@online.no>]

>> 
>>Tarjei T. Jensen writes:
>> 
>> I suspect bounded strings will perform badly against a counted string
>> implementation because assignment of strings involves copying the entire array
>> whether it contains valid data or not. This makes Ada less than wonderful in
>> applications that involve a lot of bounded string assignments.
>
>Larry Kilgallen wrote:
> No, it might affect particular _implementations_ of Ada, but I have
> read no indications that counted strings would not be a valid way of
> implementing bounded strings.  If the Varying String datatype made
> quite public by DEC Pascal were used in some Ada implementation to
> implement bounded strings (and for all I know, it or something quite
> close to it is) execution should be quite efficient for strings
> below a certain size (ultimately one gets into page faults, cache
> misses and other stack-vs-heap issues).
> 
> Larry Kilgallen

That would mean that the implementation supply a default maximum size.
If the implementation should be more effective than the usual bounded
string implementation the compiler whould have to know what the real
length of a string is for assignments. Otherwise the code generated
spends a lot of time copying junk data.

A "smart" compiler is paramount since we cannot do anything about the
assignment operator in Ada. We can write a custom procedure or function
to handle assignment, but it is not the same as ":=".

I would be delighted if someone could show that a smart compiler is not
neccessary to optimize counted or bounded string operations.


Greetings,


-- 
// Tarjei T. Jensen 
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