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From: Lawrence D'Oliveiro <ldo@nz.invalid>
Newsgroups: comp.arch
Subject: Re: Short Vectors Versus Long Vectors
Date: Tue, 23 Apr 2024 08:31:21 -0000 (UTC)
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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.

> 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.