Question

Today I had a problem converting a Long (Int64) to an Integer (Int32). The problem is that my code was always working in 32-bit environments, but when I try THE SAME executable in a 64-bit computer it crashes with a System.OverflowException exception.

I’ve prepared this test code in Visual Studio 2008 in a new project with default settings:

Module Module1

    Sub Main()
      Dim alpha As Long = -1
      Dim delta As Integer

      Try
         delta = CInt(alpha And UInteger.MaxValue)
         Console.WriteLine("CINT OK")
         delta = Convert.ToInt32(alpha And UInteger.MaxValue)
         Console.WriteLine("Convert.ToInt32 OK")
      Catch ex As Exception
         Console.WriteLine(ex.GetType().ToString())
      Finally
         Console.ReadLine()
      End Try
   End Sub

End Module

On my 32-bit setups (Windows XP SP3 32-bit and Windows 7 32-bit) it prints up to “CINT OK”, but in the 64-bit computer (Windows 7 64-bit) that I’ve tested THE SAME executable it prints the exception name only.

Is this behavior documented? I tried to find a reference, but I failed miserably.

For reference I leave the CIL code too:

.method public static void  Main() cil managed
{
  .entrypoint
  .custom instance void [mscorlib]System.STAThreadAttribute::.ctor() = ( 01 00 00 00 )
  // Code size       88 (0x58)
  .maxstack  2
  .locals init ([0] int64 alpha,
           [1] int32 delta,
           [2] class [mscorlib]System.Exception ex)
  IL_0000:  nop
  IL_0001:  ldc.i4.m1
  IL_0002:  conv.i8
  IL_0003:  stloc.0
  IL_0004:  nop
  .try
  {
    .try
    {
      IL_0005:  ldloc.0
      IL_0006:  ldc.i4.m1
      IL_0007:  conv.u8
      IL_0008:  and
      IL_0009:  conv.ovf.i4
      IL_000a:  stloc.1
      IL_000b:  ldstr      "CINT OK"
      IL_0010:  call       void [mscorlib]System.Console::WriteLine(string)
      IL_0015:  nop
      IL_0016:  ldloc.0
      IL_0017:  ldc.i4.m1
      IL_0018:  conv.u8
      IL_0019:  and
      IL_001a:  call       int32 [mscorlib]System.Convert::ToInt32(int64)
      IL_001f:  stloc.1
      IL_0020:  ldstr      "Convert.ToInt32 OK"
      IL_0025:  call       void [mscorlib]System.Console::WriteLine(string)
      IL_002a:  nop
      IL_002b:  leave.s    IL_0055
    }  // End .try
    catch [mscorlib]System.Exception
    {
      IL_002d:  dup
      IL_002e:  call       void [Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.ProjectData::SetProjectError(class [mscorlib]System.Exception)
      IL_0033:  stloc.2
      IL_0034:  nop
      IL_0035:  ldloc.2
      IL_0036:  callvirt   instance class [mscorlib]System.Type [mscorlib]System.Exception::GetType()
      IL_003b:  callvirt   instance string [mscorlib]System.Type::ToString()
      IL_0040:  call       void [mscorlib]System.Console::WriteLine(string)
      IL_0045:  nop
      IL_0046:  call       void [Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.ProjectData::ClearProjectError()
      IL_004b:  leave.s    IL_0055
    }  // End handler
  }  // End .try
  finally
  {
    IL_004d:  nop
    IL_004e:  call       string [mscorlib]System.Console::ReadLine()
    IL_0053:  pop
    IL_0054:  endfinally
  }  // End handler
  IL_0055:  nop
  IL_0056:  nop
  IL_0057:  ret
} // End of method Module1::Main

I suspect that the instruction that is behaving differently is either conv.ovf.i4 or the ldc.i4.m1/conv.u8 pair.

What is going on?

Convert.ToInt32(long) fails in both environments. It is only CInt(Long) which is behaving differently.

Answer 1

Unfortunately, the 64-bit version is accurate. It really is an overflow, the result of the expression is a long with the value &hffffffff. The sign bit is AND-ed off the value, it is no longer negative. The resulting value cannot be converted to an integer, the maximum integer value is &h7fffffff. You can see this by adding this code to your snippet:

 Dim value As Long = alpha And UInteger.MaxValue
 Console.WriteLine(value)

Output: 4294967295

The x64 jitter uses an entirely different way to check for overflows, it doesn’t rely on the CPU overflow exception but explicitly compares the values to Integer.MaxValue and Integer.MinValue. The x86 jitter gets it wrong, it optimizes the code too much and ends up making an unsigned operation that doesn’t trip the CPU exception.

Filing a bug report at connect.microsoft.com is probably not worth the effort, fixing this for the x86 jitter would be a drastically breaking change. You’ll have to rework this logic. Not sure how, I don’t see what you are trying to do.