Document when each of the vectorization approaches should be used

[stephentoub]

We now have:

• Vector<T>
• Vector64<T>
• Vector128<T>
• Vector256<T>
• Each of the low-level intrinsics namespaces, e.g. Avx2

As part of shipping .NET 7 which includes several of these systems a new, it will be important to provide clear guidance on which should be used when. Should a developer start with Vector<T> and only fall back to other things if there are operations required it can't support? If not, why not? If a developer is using Vector128<T>, when should they also add a "duplicated" path for Vector256<T>? When should they also add a "duplicated" path for Vector64<T>? What are the most likely cases where VectorN<T> are insufficient and a developer will want to target the individual instruction set namespaces directly? Are there situations where they might be used together, e.g. a Vector128<T> path and then separate paths for the individual instruction sets? Etc.

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Issue Details

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We now have:

• Vector<T>
• Vector64<T>
• Vector128<T>
• Vector256<T>
• Each of the low-level intrinsics namespaces, e.g. Avx2

As part of shipping .NET 7 which includes several of these systems a new, it will be important to provide clear guidance on which should be used when. Should a developer start with Vector<T> and only fall back to other things if there are operations required it can't support? If not, why not? If a developer is using Vector128<T>, when should they also add a "duplicated" path for Vector256<T>? When should they also add a "duplicated" path for Vector64<T>? What are the most likely cases where VectorN<T> are insufficient and a developer will want to target the individual instruction set namespaces directly? Are there situations where they might be used together, e.g. a Vector128<T> path and then separate paths for the individual instruction sets? Etc.

Author:	stephentoub
Assignees:	-
Labels:	documentation, area-System.Runtime.Intrinsics
Milestone:	7.0.0

[tannergooding]

@adamsitnik started a doc on this topic, we can continue working on that in .NET 9+