Vector256.Create() randomly throws PlatformNotSupportedException

[dubiousconst282]

Description

Calling Vector256.Create() on a machine with no support for AVX2 will throw PlatformNotSupportedException after a short amount of time.

Reproduction Steps

using System.Runtime.Intrinsics;

int i = 0;
while (true) {
    Vector256.Create(0f);
    if (i++ % 10000 == 0) Console.WriteLine("OK " + i);
}

Expected behavior

The program above should run indefinitely.

Actual behavior

The program crashes after a short amount of time:

...
OK 3390001
OK 3400001
Unhandled exception. System.PlatformNotSupportedException: Operation is not supported on this platform.
   at System.Runtime.Intrinsics.Vector256.Create(Single value)
   at Program.<Main>$(String[] args) in ...\Program.cs:line 4

Regression?

I'm pretty sure it is. Changing the target to .NET 6 works as expected, and I've only found this issue after upgrading to .NET 7 preview 6.

Known Workarounds

No response

Configuration

.NET SDK: 7.0.100-preview.6.22352.1 (commit 492644e08e)
OS: Windows 10 (6.3.9600, x64)
CPU: Intel i3 Gen2 (Sandy Bridge, with no AVX2 support)

Other information

No response

[dotnet-issue-labeler]

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

[huoyaoyuan]

The behavior looks related to tiered compilation. Or maybe dynamic PGO?

[dubiousconst282]

I can confirm that dotnet run -p:TieredCompilation=false works as expected.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-runtime-intrinsics
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

Calling Vector256.Create() on a machine with no support for AVX2 will throw PlatformNotSupportedException after a short amount of time.

Reproduction Steps

using System.Runtime.Intrinsics;

int i = 0;
while (true) {
    Vector256.Create(0f);
    if (i++ % 10000 == 0) Console.WriteLine("OK " + i);
}

Expected behavior

The program above should run indefinitely.

Actual behavior

The program crashes after a short amount of time:

...
OK 3390001
OK 3400001
Unhandled exception. System.PlatformNotSupportedException: Operation is not supported on this platform.
   at System.Runtime.Intrinsics.Vector256.Create(Single value)
   at Program.<Main>$(String[] args) in ...\Program.cs:line 4

Regression?

I'm pretty sure it is. Changing the target to .NET 6 works as expected, and I've only found this issue after upgrading to .NET 7 preview 6.

Known Workarounds

No response

Configuration

.NET SDK: 7.0.100-preview.6.22352.1 (commit 492644e08e)
OS: Windows 10 (6.3.9600, x64)
CPU: Intel i3 Gen2 (Sandy Bridge, with no AVX2 support)

Other information

No response

Author:	dubiousconst282
Assignees:	-
Labels:	area-System.Runtime.Intrinsics, untriaged
Milestone:	-

[EgorBo]

There are two problems here:

1. We treat Vector256 "ISA" as not-supported on AVX1 only CPUs so Vector256.IsHardwareAccelerated returns false on e.g. Ivy Bridge

2. Inside Vector256.Create we still check for AVX1 https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector256.cs#L979-L984 and if it's true we think that Vector256.Create is an intrinsic (but it's actually not since Vector256 ISA is disabled).

Solution:

1. Replace Avx.IsSupported checks with Vector256.IsHardwareAccelerated for consistency so we will always be using software fallbacks on Avx1
2. Enable some Vector256 helper instrinsics on AVX1 (makes more sense)

[tannergooding]

This is the same issue that ImageSharp hit that I was looking into.