Re: Short Vectors Versus Long Vectors

BGB wrote:

On 4/23/2024 1:22 AM, Anton Ertl wrote:

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

On Tue, 23 Apr 2024 02:14:32 +0000, MitchAlsup1 wrote:
big snip>

As can be noted, SIMD is easy to implement.

ADD/SUB is, MUL and DIV and SHIFTs and CMPs are not; especially when
MUL does 2n = n × n and DIV does 2n / n -> n (quotient) + n (remainder)

Main obvious drawback is the potential for combinatorial explosions of instructions. One needs to keep a fairly careful watch over this.

Like, if one is faced with an NxN or NxM grid of possibilities, naive strategy is to be like "I will define an instruction for every possibility in the grid.", but this is bad. More reasonable to devise a minimal set of instructions that will allow the operation to be done within in a reasonable number of instructions.

But, then again, I can also note that I axed things like packed-byte operations and saturating arithmetic, which are pretty much de-facto in packed-integer SIMD.

MANY SIMD algorithms need saturating arithmetic because they cannot do
b + b -> h and avoid the overflow. And they cannot do B + b -> h because
that would consume vast amounts of encoding space.

Likewise, a lot of the gaps are filled in with specialized converter and helper ops. Even here, some conversion chains will require multiple instructions.

Well, and if there is no practical difference between a scalar and SIMD version of an instruction, may well just use the SIMD version for scalar.

....

- anton