Use Roslyn support for RuntimeHelpers.CreateSpan

[stephentoub]

As of dotnet/roslyn#61414, the optimization that previously extended only to byte/sbyte/bool will now also support char/ushort/short/uint/int/ulong/long/float/double. This PR uses it throughout dotnet/runtime. However, because of endianness issues, this requires the RuntimeHelpers.CreateSpan method that only exists as of .NET 7, so trying to use this optimization in builds targeting < .NET 7 will be a significant deoptimization. Until we have dedicated syntax (ala dotnet/csharplang#5295) and/or an analyzer (#69577) to avoid such issues, we need to be very careful where this applied.

I'm putting this up as a draft now for feedback, but we must not merge this until we've picked up a Roslyn compiler that has the aforementioned improvement.

Closes #60948
cc: @VSadov, @jcouv, @davidwrighton, @GrabYourPitchforks

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[huoyaoyuan]

Current implementation throws hardly for big endian. It still allows the option to copy the field and reverse the endianness in memory, right? Just make sure this feature is not impossible or extremely inefficient to implement for big endian.

[stephentoub]

Current implementation throws hardly for big endian. It still allows the option to copy the field and reverse the endianness in memory, right? Just make sure this feature is not impossible or extremely inefficient to implement for big endian.

The purpose of the CreateSpan API is to address endianness. coreclr doesn't support big endian at all today (well beyond this particular method), but mono does, including with this method.

runtime/src/mono/mono/metadata/icall.c

Line 960 in 5adac4a

#if G_BYTE_ORDER != G_LITTLE_ENDIAN

[ghost]

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Issue Details

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As of dotnet/roslyn#61414, the optimization that previously extended only to byte/sbyte/bool will now also support char/ushort/short/uint/int/ulong/long/float/double. This PR uses it throughout dotnet/runtime. However, because of endianness issues, this requires the RuntimeHelpers.CreateSpan method that only exists as of .NET 7, so trying to use this optimization in builds targeting < .NET 7 will be a significant deoptimization. Until we have dedicated syntax (ala dotnet/csharplang#5295) and/or an analyzer (#69577) to avoid such issues, we need to be very careful where this applied.

I'm putting this up as a draft now for feedback, but we must not merge this until we've picked up a Roslyn compiler that has the aforementioned improvement.

Closes #60948
cc: @VSadov, @jcouv, @davidwrighton, @GrabYourPitchforks

Author:	stephentoub
Assignees:	stephentoub
Labels:	NO-MERGE, area-Meta, tenet-performance
Milestone:	7.0.0

[marek-safar]

Wondering if there is analyzer for such patterns

[stephentoub]

It looks like the compiler dependency won't land in time for the compiler change to be merged. Closing for .NET 7.