Another strange debugger/Math.Sqrt interaction

tomtheisen · December 2019

I'm seeing another strange behavior that seems independent from other stuff I've found today. It seems that the mere presence of a breakpoint on a line can affect the computation.

I'm using Linqpad 6.5.5 x64

.NET Core version (host): 3.0.0
.NET Core version (queries): 3.0.0
Roslyn Version: 3.3.1-beta4-19462-11
FSharp.Compiler.Service version: 32.0.0.0

Here's the reproduction.

Create a C# statements script

Console.WriteLine(Math.Sqrt(1));
Console.WriteLine(Math.Sqrt(1)); // breakpoint
Console.WriteLine(Math.Sqrt(1));
Console.WriteLine(Math.Sqrt(4));
Console.WriteLine(Math.Sqrt(4)); // breakpoint 
Console.WriteLine(Math.Sqrt(4));
Console.WriteLine(Math.Sqrt(9));
Console.WriteLine(Math.Sqrt(9)); // breakpoint
Console.WriteLine(Math.Sqrt(9));

Set breakpoints on the appropriate lines.
Ensure compiler optimizations are turned off.
Execute the script.
For each breakpoint, just continue the script.

The final output (on my machine) is

1
1.0000000000000278
1
2
2.0000000000000555
2
3
3.000000000000074
3

tomtheisen · December 2019

I've delved into this a little more. I don't understand it but I've scoped out the behavior a little more. It seems that in certain cases, a breakpoint causes the first byte of double to get overwritten.

http://share.linqpad.net/2t5kcw.linq

So far, I've observed 251, 184, and 185 overwriting the first byte. Curiously it seems that the double value 0 is immune from this behavior, although infinity and NaN are not.

const double F = 1;
double f;

f = F;
Console.WriteLine(BitConverter.GetBytes(f)[0]);

f = F; // breakpoint
Console.WriteLine(BitConverter.GetBytes(f)[0]);

f = F; // breakpoint
Console.WriteLine("{0}", BitConverter.GetBytes(f)[0]);

f = F; // breakpoint
Console.WriteLine("{1} {0}", f, BitConverter.GetBytes(f)[0]);

Output:

0
251
184
185 1.000000000000041

JoeAlbahari · December 2019

I'm seeing the same behavior in Visual Studio. This is very peculiar, and I'd suggest logging a bug report either on the Visual Studio site or here:
https://github.com/dotnet/coreclr/issues

If you mention that you're experiencing the same behavior in LINQPad's debugger, it should help narrow it down for them.

Another strange debugger/Math.Sqrt interaction

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