Re: Short Vectors Versus Long Vectors

Lawrence D'Oliveiro wrote:

On Tue, 23 Apr 2024 06:22:38 GMT, Anton Ertl wrote:

So what's the benefit of using vector/SIMD instructions at all rather
than doing it with scalar code?

On the original Cray machines, I read somewhere the benefit of using the vector versions over the scalar ones was a net positive for a vector length as low as 2.

Somewhere in the neighborhood of 4-5 length vectors. There was a 3 cycle
decode delay as pipeline scheduling slots were reserved for the vector
writebacks.

If your OoO capabilities are limited (and I think
they are on the Cray machines), you cannot start the second iteration of
the doall loop before the processing step of the first iteration has
finished with the register.

How would out-of-order execution help, anyway, given all the operations on the vector elements are supposed to be identical? Unless it’s just greater parallelism.

Out of order makes it easier to "run into" undiscovered dynamic dependency
free operations.