SIMD problems when mixing llvm and non-llvm code

[vargaz]

Description

Currently on mono, SIMD is only supported when using LLVM compiled code. This can lead to various problems when mixing llvm and non-llvm compiled code. This mixing can happen for various reasons:

• profiled AOT
• LLVM doesn't support some IL constructs like filter clauses

1. Different calling conventions
   LLVM code passes SIMD types in SIMD registers, while JITted code passes them on the stack.
   Testcase:

using System;
using System.Numerics;

public class Tests
{
    public static void Main () {
        foo (new Vector<int> (1));
    }

    static void foo (Vector<int> v) {
        try {
            Console.WriteLine (v);
        } catch (Exception ex) when (ex is OverflowException) {
            }
    }
}

2. The IsSupported methods return different values in LLVM and non-LLVM methods, leading to subtle logic problems:

using System;
using System.Numerics;
using System.Runtime.Intrinsics;
using System.Runtime.Intrinsics.Arm;

public class Tests
{
    public static void Main () {
        if (AdvSimd.Arm64.IsSupported)
            foo (new Vector128<double> ());
    }

    static void foo (Vector128<double> v) {
        try {
        } catch (Exception ex) when (ex is OverflowException) {
            }
        AdvSimd.Arm64.Abs (v);
    }
}

This will fail when AOT-ed using --aot=llvm

Reproduction Steps

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Expected behavior

.

Actual behavior

.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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[vargaz]

@fanyang-mono

[lambdageek]

This is another instance of the same problem #73003

Basically as long as we have a fallback execution engine (JIT or interpreter) that doesn't support some intrinsic (or returns IsSupported == false) we can't really support that intrinsic in an AOT mode (or return IsSupported == true)

[SamMonoRT]

@lambdageek @fanyang-mono -- with #74357 merged(backported), I think we can move this to 8.0.0 ?