Re: Copying string slices before calling subroutines?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 04 May 2007 08:07:59 -0400, Jeffrey Creem wrote:

> Dmitry A. Kazakov wrote:
>> On Fri, 04 May 2007 11:44:14 +0200, Jacob Sparre Andersen wrote:
>> 
>>> Jacob Sparre Andersen wrote:
>>>> Dmitry A. Kazakov wrote:
>>>>> On Fri, 04 May 2007 08:53:59 +0200, Jacob Sparre Andersen wrote:
>>>>>>      for Lines in 1 .. 10 loop
>>>>>>         Last := Index (Source  => Text (Text'First .. Last - 1),
>>>>> Did you check if GNAT did not copy the string slice before calling to
>>>>> Index?
>>>> No.  Do you have any suggestion for a way to check it.  The
>>>> reference manual (section 6.2) seems to indicate that it is up to
>>>> the compiler to decide.
>>> I have found a strong indication that GNAT copies the whole string
>>> slice onto the stack before calling Index:
>> 
>> So, Index was innocent.
> 
> I don't know that I'd say that... Though it might be innocent in this 
> case, a quick look at the source code (which then remaps index to a 
> common string search package) looks to me like there are other 
> opportunities where some versions of the Index call will make another 
> copy in cases when it need not and further this does not appear to match 
> the comments in the package.

Devastating. I take my words back.
 
> Hmm...I don't see the optimization for identity mappings anyplace...

It looks that they call "optimization" copying Source into Mapped_Source
with remapping. Probably they hoped that comparing Pattern with a remapping
slice could translate into a processor-specific string comparison
instruction. Anyway...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de