Arithmetic and conditional errors with (byte) casts

[Ruberik]

Summary

Casting uints to bytes sometimes causes unexpected behaviour in simple arithmetic and conditional operations. This appears to happen only when the uint in question has its 128 bit set. The output depends on whether it's being built in Debug or Release mode. This has been tested on at least a T4, and with both rc1 and older ILGPU code.

Code

public class MiscClean {
    public static void KernelByteBugs(Index1D index, ArrayView1D<uint, Stride1D.Dense> input, ArrayView1D<uint, Stride1D.Dense> output) {
        for (int i = 0; i < input.Length; i++) {
            byte b = (byte)input[i];
            output[0] += b;
            output[1] += ((byte)input[i]) & 255u;
            output[2] += ((byte)input[i]);
            output[3] += input[i] & 255;
            byte b0 = (byte)(input[i]);
            if (b0 < 128) output[4]++;
            if (((input[i]) & 255) < 128) output[5]++;
        }
    }

    static void Main(string[] args) {
        using var context = Context.Create().Cuda().ToContext();
        using var acc = context.CreateCudaAccelerator(0);
        var input = new uint[1] { 130u };
        var output = new uint[6];
        var gpuInput = acc.Allocate1D(input);
        var gpuOutput = acc.Allocate1D(output);
        acc.LaunchAutoGrouped(KernelByteBugs, new Index1D(1), gpuInput.View, gpuOutput.View);
        gpuOutput.CopyToCPU(output);
        for (int i = 0; i < output.Length; i++) {
            Console.WriteLine($"output[{i}] = {output[i]}");
        }
    }
}

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
    <ApplicationIcon />
    <OutputType>Exe</OutputType>
    <StartupObject />
    <RootNamespace>FurrowUtilities</RootNamespace>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="ILGPU" Version="1.1.0-rc1" />
    <PackageReference Include="ILGPU.Algorithms" Version="1.1.0-rc1" />
    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
  </ItemGroup>
</Project>

Output

Expected:

output[0] = 130
output[1] = 130
output[2] = 130
output[3] = 130
output[4] = 0
output[5] = 0

Actual, in debug mode:

output[0] = 130
output[1] = 130
output[2] = 4294967170
output[3] = 130
output[4] = 0
output[5] = 0

Actual, in release mode:

output[0] = 130
output[1] = 130
output[2] = 4294967170
output[3] = 130
output[4] = 1
output[5] = 0

Note the difference on output[4].

Notes

1. 4294967170 = 0xFFFFFF82 (where 0x82 = 130).
2. The hard-coded value=130 is important: this issue apparently happens iff (value & 128) != 0. Behaviour is as expected if that bit isn't set.

[MoFtZ]

@Ruberik I have raised a PR that fixes this issue.

@m4rs-mt The issue appears to be that nested conversions are ignoring the unsigned flag of the inner conversion. However, I could only replicate this on byte to uint. Is there something that I am missing?

[Ruberik]

Thanks, @MoFtZ. I tested on my machine and can confirm this apparently fixes the problem.