CInt(Long) in VB.NET behaving differently in 32- and 64-bit environments - vb.net

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.

Answer 2

I don't know of any real reference as such, but if you go to this page:
http://msdn.microsoft.com/en-us/library/system.int32.aspx
You can see in the sample where they use CInt they do wrap it in a OverflowException handler (try searching for CInt on that page to find it). So at least they say implicitly that CInt can throw that in certain circumstances.
If you do not want the exceptions being thrown you can change the Remove integer overflow checks setting on the Advanced Compile Options page.

Answer 3

Try to change build platform target from “Any CPU” to "x86".

Answer 4

Just to complete the documentation of this issue I made this:
Imports System.Runtime.InteropServices
Module Module1
<DllImport("KERNEL32.DLL", EntryPoint:="DebugBreak", _
SetLastError:=False, CharSet:=CharSet.Unicode, _
ExactSpelling:=True, _
CallingConvention:=CallingConvention.StdCall)> _
Public Sub DebugBreak()
End Sub
Sub Main()
Dim alpha As Long = -1
Dim delta As Integer
DebugBreak() ' To call OllyDbg
' Needed to prevent the jitter from raising the overflow exception in the second CInt without really doing the convertion first
alpha = alpha Xor Environment.TickCount
Console.WriteLine(alpha)
delta = CInt(alpha And UInteger.MaxValue)
Console.WriteLine(delta)
alpha = alpha And UInteger.MaxValue
delta = CInt(alpha)
Console.WriteLine(delta)
Console.ReadLine()
End Sub
End Module
Using OllyDbg I got this:
CPU Disasm
Address Hex dump Command Comments
00D10070 55 PUSH EBP
00D10071 8BEC MOV EBP,ESP
00D10073 57 PUSH EDI
00D10074 56 PUSH ESI
00D10075 53 PUSH EBX
00D10076 E8 A1BFC7FF CALL 0098C01C
00D1007B E8 A18C1879 CALL <JMP.&KERNEL32.GetTickCount> ; Jump to KERNEL32.GetTickCount
00D10080 99 CDQ
00D10081 F7D0 NOT EAX
00D10083 F7D2 NOT EDX
00D10085 8BF0 MOV ESI,EAX
00D10087 8BFA MOV EDI,EDX
00D10089 E8 62D25D78 CALL 792ED2F0 ; Called everytime Console is referenced here
00D1008E 57 PUSH EDI
00D1008F 56 PUSH ESI
00D10090 8BC8 MOV ECX,EAX
00D10092 8B01 MOV EAX,DWORD PTR DS:[ECX]
00D10094 FF90 C4000000 CALL DWORD PTR DS:[EAX+0C4] ; Console.WriteLine(Int64)
00D1009A 8BDE MOV EBX,ESI ; Note: EDI:ESI holds alpha variable
00D1009C 83E3 FF AND EBX,FFFFFFFF ; delta = CInt(alpha And UInteger.MaxValue)
00D1009F E8 4CD25D78 CALL 792ED2F0
00D100A4 8BC8 MOV ECX,EAX
00D100A6 8BD3 MOV EDX,EBX
00D100A8 8B01 MOV EAX,DWORD PTR DS:[ECX]
00D100AA FF90 BC000000 CALL DWORD PTR DS:[EAX+0BC] ; Console.WriteLine(Int32)
00D100B0 33FF XOR EDI,EDI ; alpha = alpha And UInteger.MaxValue
00D100B2 85F6 TEST ESI,ESI ; delta = CInt(alpha) [Begins here]
00D100B4 7C 06 JL SHORT 00D100BC
00D100B6 85FF TEST EDI,EDI
00D100B8 75 2B JNE SHORT 00D100E5
00D100BA EB 05 JMP SHORT 00D100C1
00D100BC 83FF FF CMP EDI,-1
00D100BF 75 24 JNE SHORT 00D100E5
00D100C1 8BDE MOV EBX,ESI ; delta = CInt(alpha) [Ends here]
00D100C3 E8 28D25D78 CALL 792ED2F0
00D100C8 8BC8 MOV ECX,EAX
00D100CA 8BD3 MOV EDX,EBX
00D100CC 8B01 MOV EAX,DWORD PTR DS:[ECX]
00D100CE FF90 BC000000 CALL DWORD PTR DS:[EAX+0BC] ; Console.WriteLine(Int32)
00D100D4 E8 1B1AA878 CALL 79791AF4
00D100D9 8BC8 MOV ECX,EAX
00D100DB 8B01 MOV EAX,DWORD PTR DS:[ECX]
00D100DD FF50 64 CALL DWORD PTR DS:[EAX+64]
00D100E0 5B POP EBX
00D100E1 5E POP ESI
00D100E2 5F POP EDI
00D100E3 5D POP EBP
00D100E4 C3 RETN
As you can see the second CInt sentence is much more complex than just ANDing (which it could actually be suppressed as EBX won't change and the EFLAGS are not consumed anywhere). The probable origin of this problem can be seen in Hans' answer