Improve support for 512-bit Vector<T>

[saucecontrol]

Fixes #104978
Contributes to #108869

• Fixes tests that had bogus failures (e.g. not enough test data)
• Adds tests to cover some existing broken paths
• Fixes JIT and CoreLib paths that did not support 512-bit Vector<T> or assumed AVX2 support meant Vector<T> was 256-bit
• Enables support for --max-vectort-bitwidth:512 in AOT and adds a smoke test for it
• Ensures Vector<T> is no larger than PreferredVectorBitWidth

Tests pass locally (AVX-512 machine) with DOTNET_MaxVectorTBitWidth=512 and DOTNET_MaxVectorTBitWidth=128

I believe this also addresses any problems that prevented opportunistic enablement of AVX2 on AOT when AVX is enabled, but I'll do some more testing around that and make a separate PR for that change if all is well.

[saucecontrol]

CC @tannergooding. This is ready for review.

Would it be all right to add the alt-sized Vector<T> configs to one of the jitstress-isas pipelines?

[saucecontrol]

The local copy of these tests had been bit rotting for a while. I could bring it up to date, but it seems reasonable to just share it with JIT.

[saucecontrol]

I kept changes to the regression tests minimal -- this is handled more robustly in the CpuId tests.

[tannergooding]

/azp run runtime-coreclr jitstress-isas-x86

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[xtqqczze]

We do not guarantee alignment of writes in SpanHelpers.Fill<T>, which means using an AVX-512 path could potentially regress performance due to stores that cross a cache line boundary (64 bytes). Since AVX-512 operates with a width of 64 bytes, any misalignment of writes ensures that a cache line split will occur.

[tannergooding]

which means using an AVX-512 path could potentially regress performance due to stores that cross a cache line boundary (64 bytes)

That's an issue with the code using Vector<T> without either pinning or attempting opportunistic alignment. The issue already exists due to the variable sized nature of Vector<T> and us not guaranteeing a maximize size.

It's not a blocker for this PR which enables opt-in usage of Vector<T> as Vector512<T>, nor would it be an issue for say Arm64 SVE where Vector<T> can be up to 2048 bits (although no such hardware exists today).

[BruceForstall]

@saucecontrol Are you waiting for review, or are you still working on this? @tannergooding You've looked at this before; can you look again?

[BruceForstall]

(@saucecontrol you should probably merge up to main)

[saucecontrol]

@saucecontrol Are you waiting for review, or are you still working on this?

It's ready on my end. Never did get a clean CI run, but no failures appeared to be related. Let's see how it does after merging back to main.

[tannergooding]

This needs to either be handled or have a tracking issue that is picked up shortly after merge in a follow up PR.

[saucecontrol]

I worded that comment oddly. The issue is not specific to Vector<T> but rather Vector512 in general isn't covered by these interop tests. Do we have something tracking that already?

[tannergooding]

Don't believe so. These tests are really centered around Vector and most of the V128/256/512 tests are elsewhere and newer/different

[saucecontrol]

These are under tests/Interop/PInvoke/Generics, which AFAICT cover scenarios involving all the generic types that have specific ABI handling. What I mean is that we don't currently have any tests for Vector512 ABI handling. Do we even have support for that?

[tannergooding]

Do we even have support for that?

Yesn't. It's expected to throw today, but the tests are still setup since there's only a single known issue actually blocking the support (which is that the default calling convention for returning vectors on Windows should be in register, not via a return buffer).

[saucecontrol]

the tests are still setup

This is the part I'm confused about. Vector512 is conspicuously absent from the list of types covered by the tests under https://github.com/dotnet/runtime/tree/main/src/tests/Interop/PInvoke/Generics.

I'm happy to open a tracking issue for adding them, but I would have assumed that would be an implicit part of enabling the ABI support.

I'm now concerned we may be missing some coverage, though. If Vector512 used in P/Invoke is supposed to throw, we'd need to be sure the same happens for 512-bit Vector<T>, but I don't see where we're testing either.

[tannergooding]

CC. @dotnet/jit-contrib, @EgorBo, @BruceForstall for secondary review