Re: Most efficient way to check for null string?

[kenner@lab.ultra.nyu.edu (Richard Kenner)]

In article <dewar.866897158@merv> dewar@merv.cs.nyu.edu (Robert Dewar) writes:
>>...If you really want to
>>shave cycles, the following will likely be the most efficient:
>>
>>     if Str'Last < Str'First then ...
>
>Bletch.  That's the least readable of all, but Tucker's right that it
>might be more efficient.  Tucker didn't say why.  It's because Ada
>(foolishly, IMHO) allows Str's range to be something like
>1_000 .. -1_000_000.  Which means that calculating the 'Length involves
>a conditional jump in the general case.

That's true, but the condition on the conditional jump is precisely the
same as the condition of the string being null, so the tests should be
equivalent.  Out of curiousity, I checked to see what GNAT will do.  It
neither optimized it to the above test, nor did the general computation of
'Length (involving conditional jumps) and tested its resul, but instead
simply did the above test twice in a row!

That redundant test should have been eliminated by the GCC optimizer
and I just put on my list to find out one of these days why it wasn't.