MemoryExtensions.IndexOfAnyExcept produces invalid results in AVX-512 enabled hardware

@tannergooding

Consider the following repro:

Console.WriteLine(Avx512F.IsSupported);
var arr = new int[16];
arr[1] = 1;
Console.WriteLine(arr.AsSpan().ContainsAnyExcept(0));

Using a recent build of runtime with DOTNET_EnableAVX512F set to 0 the above prints:

False
True

While setting it to 1 prints

True
False

This is causing ConcurrentDictionary to misbehave under certain circumstances because the following method

runtime/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs

Lines 1925 to 1926 in d7a52b2

    
           private bool AreAllBucketsEmpty() => 
        
               !_tables._countPerLock.AsSpan().ContainsAnyExcept(0);

can return invalid results.

cc @tannergooding @MihaZupan @stephentoub

Looks like a difference between ~Equals(a, b) != Zero and !EqualsAny(a, b).

Vector128<int> input = Vector128.Create(0, 0, 0, 1);
Vector128<int> value = Vector128<int>.Zero;

Console.WriteLine($"containsAnyExcept: {~Vector128.Equals(input, value) != Vector128<int>.Zero}");
Console.WriteLine($"!EqualsAny: {!Vector128.EqualsAny(input, value)}");
// containsAnyExcept: True
// !EqualsAny: False

where the latter is being used by the Vector512 code path.

Adding regression tests for dotnet#89512

Fix is up here: #89522, would appreciate a review/sign-off

Fixing the new V512 logic to properly negate the mask checks (#89522)

Navigation Menu

Repository navigation

MemoryExtensions.IndexOfAnyExcept produces invalid results in AVX-512 enabled hardware #89512

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

	private bool AreAllBucketsEmpty() =>
	!_tables._countPerLock.AsSpan().ContainsAnyExcept(0);

MemoryExtensions.IndexOfAnyExcept produces invalid results in AVX-512 enabled hardware #89512

Description

Activity

MihaZupan commented on Jul 26, 2023

tannergooding commented on Jul 27, 2023

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions