Switch expression in C# 8 behaves differently on x86 vs x64 android

[mkhamoyan]

Description

Switch expression in C# 8 behaves differently on x86 vs x64 Android. After third call it behaves differently for float type.

Reproduction Steps

Copy paste the below code in mono/samples/Android and try to run on x86 android emulator.
GetNumberAsString after third call starts returning NAN, even if it is called for the same value (float.MaxValue).

public static int Main(string[] args)
    {
        var random = new Random(42);

        List<float> Floats = new List<float>
        {
                // 0.000f,
                //1.1234e1f,
                //-1.1234e1f,
                float.MaxValue,
                float.MinValue
        };

        for (int i = 0; i < 10; i++)
        {
            float value = NextFloat(random);
            Floats.Add(value);
        }
        Console.WriteLine ("GetNumberAsString conversion is {0}", GetNumberAsString(float.MaxValue));
        Console.WriteLine ("GetNumberAsString conversion is {0}", GetNumberAsString(Floats[3]));
        Console.WriteLine ("GetNumberAsString conversion is {0}", GetNumberAsString(Floats[4]));
        Console.WriteLine ("GetNumberAsString conversion is {0}", GetNumberAsString(Floats[5]));
        Console.WriteLine ("GetNumberAsString conversion is {0}", GetNumberAsString(float.MaxValue));


        //RunAsRootTypeTest(Floats);
        return 42;
    }

    public static float NextFloat(Random random)
    {
        double mantissa = (random.NextDouble() * 2.0) - 1.0;
        double exponent = Math.Pow(2.0, random.Next(-126, 128));
        float value = (float)(mantissa * exponent);
        return value;
    }

    private static void RunAsRootTypeTest<T>(List<T> Floats)
    {
        foreach(T f in Floats)
        {
            Console.WriteLine ("GetNumberAsString from foreach conversion is {0}", GetNumberAsString(f));
        }
    }
    
    private static string GetNumberAsString<T>(T number)
    {            
        /*switch (Type.GetTypeCode(typeof(T)))
        {
            case TypeCode.Double:
            return Convert.ToDouble(number).ToString("G9", System.Globalization.CultureInfo.InvariantCulture);
            case TypeCode.Single:
            return Convert.ToSingle(number).ToString("G9", System.Globalization.CultureInfo.InvariantCulture);
            case TypeCode.Decimal:
            return Convert.ToDecimal(number).ToString(System.Globalization.CultureInfo.InvariantCulture);
            default:
            return number.ToString();
            }*/
            
            return number switch
            {
                double @double => @double.ToString("G9", System.Globalization.CultureInfo.InvariantCulture),
                float @float => @float.ToString("G9", System.Globalization.CultureInfo.InvariantCulture),
                decimal @decimal => @decimal.ToString(System.Globalization.CultureInfo.InvariantCulture),
                _ => number.ToString()
            };

    }

Expected behavior

GetNumberAsString should convert correctly after third call also.
To get expected behaviour please uncomment first switch in GetNumberAsString and comment last one.
TypeCode enum switch works correct on both x64 and x86, so my guess is switch expression in C# 8 behaves differently on x86 vs x64 Android.

Actual behavior

First 3 times GetNumberAsString returns correct value. After that it starts to return NAN.

Regression?

No response

Known Workarounds

Using TypeCode enum switch works fine on both x64 and x86.

Configuration

Try on Android x86.

Other information

return number switch
            {
                //double @double => @double.ToString("G9", System.Globalization.CultureInfo.InvariantCulture),
                float @float => @float.ToString("G9", System.Globalization.CultureInfo.InvariantCulture),
                //decimal @decimal => @decimal.ToString(System.Globalization.CultureInfo.InvariantCulture),
                _ => number.ToString()
            };

When I comment other cases in switch seems it works.

[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.

[steveisok]

/cc @SamMonoRT

[SamMonoRT]

@lambdageek - please correct me if I'm wrong. This is low priority for us to fix any x86 issues. Majority of devices are x64 or arm64 based. Does it make sense to fix these or can we disable the relevant tests on android x86 libraries lane ?

[akoeplinger]

This looks like a codegen bug so who knows where else it might show up, I don't think this is low priority.

[lambdageek]

I agree with Alexander, we need to do some investigation before we can judge if this is low impact.

[fanyang-mono]

I will start the investigation early next week.

[fanyang-mono]

I tried to reproduce today and wasn't able to reproduce the wrong behavior on Android x86. This is what I get

08-11 16:29:30.775  4183  4205 I DOTNET  : GetNumberAsString conversion is 3.40282347E+38
08-11 16:29:30.779  4183  4205 I DOTNET  : GetNumberAsString conversion is -47.9336586
08-11 16:29:30.779  4183  4205 I DOTNET  : GetNumberAsString conversion is -5.75174921E-19
08-11 16:29:30.779  4183  4205 I DOTNET  : GetNumberAsString conversion is 7.18106747
08-11 16:29:30.779  4183  4205 I DOTNET  : GetNumberAsString conversion is 3.40282347E+38

I am closing this issue. If it is reproducible again, please feel free to re-open it.

[akoeplinger]

I verified this doesn't happen in main anymore, it was fixed by #65723 since I can repro if I revert that change (you need to build in Release config to see it).