Optimising std::find on x86 and PPC

[Matt Austern]

On Dec 14, 2004, at 6:32 AM, Chris Jefferson wrote:

> Hello,
>
> I recently tried changing the std::find random_access overload to 
> change the main loop from:
>
> difference_type __trip_count = (__last - __first) >> 2;
> for(; __trip_count > 0 ; --__trip_count) { if(*__first = __val) return 
> __first; ++__first; (4 times)  }
>
> to:
>
> Iterator __newlast = __last - (__last - __first) % 4;
> for( ; __first < __newlast;){ if(*__first = __val) return __first; 
> ++__first; (4 times)  }
>
> This knocked about 30% off the time taken on x86 (Note that in a final 
> version I'd change the %4 into some kind of &ing and/or shifting).
>
> Unfortunatly, a quick test on Mac OS X by Andrew Pinski (thank you!) 
> found that this slightly decreased both performance in terms of both 
> space and time on the, as this new version will no longer use the 
> specialised "count" operator.

What kind of iterators did you use for your timing test?  My guess 
would be int* or char*, but you should probably say.

			--Matt