performance of gather/scatter with different types of indexes

[Kay F. Jahnke]

Hi all!

Given a simdized type ST, I noticed recently that, apart from using 
ST::IndexType (which uses int) for gather/scatter operations, I can use 
any type offering operator[] and holding the required number of indexes. 
Initially I filed this under nice-to-have, but since the code I'm 
working on produces indexes initially as ptrdiff_t (which is long on my 
system), I experimentally coded passing vigra::TinyVector<ptrdiff_t,N> 
as index type, vigra::TinyVector being a fixed size container type. To 
my surprise, the resulting code was faster than the code using 
ST::IndexType, so it looks like a win-win situation: I don't need to 
safeguard against index overflow (as unlikely as that may be) and I get 
faster code. Since this is just a single observation on a single system 
(though it occurs both with g++ and clang++) I wonder if anyone else has 
looked into this - and what causes the effect.

Kay