Optimising std::find on x86 and PPC

[Chris Jefferson]

Matt Austern wrote:

> 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.
>
I used int*. I also tried it with vector<int>::iterator, which is not an 
actual int*, but the optimiser manages to "pick" an int* out of it and 
just use that in the loop.

Chris