[release/8.0-staging] Ensure we consistently broadcast the result of simd dot product

[tannergooding]

Backport of #105888 to release/8.0-staging

/cc @tannergooding

Customer Impact

• Customer reported: Net8 - Vector2.Normalize and old(?) CPU - wrong result #99391
• Found internally

Users with hardware that supports SSE3-SSSE3 (2004-2007) and using Vector2 can experience incorrect results in some cases as the result is not correctly replicated to all elements of the vector. This is not an issue with hardware that only supports SSE2 (our baseline ISA) or with hardware that supports SSE4.1+ (2007).

Regression

• Yes
• No

The bug was introduced in #81335 when an optimization was added to avoid redundant broadcast. It was missed that for SSE3-SSSE3 and for Vector2 in particular (but not for Vector3/4 or Vector64/128/256/512<T>), a slightly different code path was gone down under which the broadcast was not redundant.

Testing

Explicit tests covering the bug were added. Additional tests covering the repro for the other vector types were also added.

Risk

Low. This impacts a single type and only on relatively old hardware. Users can also see the failure on new hardware if they are doing testing and set the DOTNET_EnableSSE41=0 environment variable.

The effective fix here was to execute the same code path as was already being used by SSE2 hardware (the same baseline that NAOT, Crossgen, and ReadyToRun default to).

[tannergooding]

/azp run runtime-coreclr jitstress-isas-x86

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[tannergooding]

@carlossanlop, looks like I can't run any outerloop CI legs due to a permissions issue. Is that expected?

This is impacting downlevel hardware, so running the stress job to explicitly validate those scenarios is important.

[carlossanlop]

looks like I can't run any outerloop CI legs due to a permissions issue. Is that expected?

What should I be looking at?

[tannergooding]

The runtime-coreclr jitstress-isas-x86 job.

All builds in it were cancelled with

##[error]Pipeline does not have permissions to use the referenced pool(s) NetCore-Svc-Public. For authorization details, refer to https://aka.ms/yamlauthz.

[carlossanlop]

I shared this problem to FR. I tagged you.

[tannergooding]

CC. @JulieLeeMSFT, @jeffschwMSFT for servicing approval

As per the template, this was a regression introduced in .NET 8 against a small subset of hardware that supports SSE3 but not SSE4.1

[JulieLeeMSFT]

Approved. cc @jeffschwMSFT for servicing consider.

[jeffschwMSFT]

lgtm. we will take for consideration in 8.0.x

[carlossanlop]

@tannergooding did this get approved by Tactics? I'm about to close the branch for the September Release. If this is not merged now, we will have to wait until October.

[tannergooding]

Hasn't been taken to tactics yet, to my knowledge.