SPC: Set PlatformType property to arm64 instead of AnyCPU for arm64 builds

[akoeplinger]

We're setting AnyCPU as the PlatformTarget for corelib here (in all runtimes):

runtime/src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj

Lines 75 to 76 in 657865f

	<PropertyGroup Condition="'$(Platform)' == 'arm64'">
	<PlatformTarget>AnyCPU</PlatformTarget>

Roslyn gained support for setting arm64 as a platform in dotnet/roslyn@e65a6c5 about 5 years ago. I traced git history and it looks like the AnyCPU value for arm64 has been there from the beginning of coreclr being open-sourced 9 years ago so it was never updated.

I don't know exactly what the impact of changing this would be but thought I'd bring it up for discussion.

/cc @jkotas @vitek-karas

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Issue Details

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We're setting AnyCPU as the PlatformTarget for corelib here (in all runtimes):

runtime/src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj

Lines 75 to 76 in 657865f

	<PropertyGroup Condition="'$(Platform)' == 'arm64'">
	<PlatformTarget>AnyCPU</PlatformTarget>

Roslyn gained support for setting arm64 as a platform in dotnet/roslyn@e65a6c5 about 5 years ago. I traced git history and it looks like the AnyCPU value for arm64 has been there from the beginning of coreclr being open-sourced 9 years ago so it was never updated.

I don't know exactly what the impact of changing this would be but thought I'd bring it up for discussion.

/cc @jkotas @vitek-karas

Author:	akoeplinger
Assignees:	-
Labels:	arch-arm64, area-Meta
Milestone:	-

[jkotas]

Processor architecture on assembly identities is obsolete concept. Properties returning this enum are marked obsolete. We are not adding new architectures to ProcessorArchitecture enum. We cannot really do that even if we wanted to - the enum encoding in metadata does not have enough bits to hold all processor architectures supported by .NET.

The processor architecture on CoreLib is set just for compatibility with .NET Framework. There are (obsolete) APIs that return it. If we were to set processor architecture on CoreLib for Arm64, we would need to revisit the whole thing - add the Arm64 member to the obsolete ProcessorArchitecture, fix all obsolete APIs that deal with it to handle it, etc. Not worth it. It can easily break more things than it would fix.

[jkotas]

Duplicate of #58970

[akoeplinger]

I wasn't thinking about changing the ProcessorArchitecture enum, but I see this comment now:

runtime/src/coreclr/System.Private.CoreLib/src/System/Reflection/AssemblyName.CoreCLR.cs

Lines 142 to 144 in 657865f

	// ProcessorArchitecture is a legacy API and does not cover other Machine kinds.
	// For example ARM64 is not expressible
	return ProcessorArchitecture.None;

I stumbled over this due to dotnet/cecil#60 which uses code like this:

var pe64 = module.Architecture == TargetArchitecture.AMD64 ||
module.Architecture == TargetArchitecture.IA64 ||
module.Architecture == TargetArchitecture.ARM64;

Vitek said it looks weird that we don't set PlatformTarget for ARM64 so I thought I'd better ask 😄

[jkotas]

I stumbled over this due to dotnet/cecil#60 which uses code like this:

This code does not look right to me: dotnet/cecil#60 (comment)