Re: Arithmetic overflow checking
On 7/8/11 6:17 PM, markspace wrote:
On 7/8/2011 5:40 PM, Eric Sosman wrote:
Not a single instruction to AddWithTrap, but a it does have a global
state register, and a test/branch instrution, so you just pair up an ADD
followed by a JO (Jump if Overflow) and that's it. Two easy instructions
paired together, and it works the same for MUL and SUB too. (DIV can't
overflow; think about it).
I Am Curious, Ultraviolet: Even for Integer.MIN_VALUE / -1?
Hmm, that was the description I read in the docs, or thought I read. I'm
not really sure what the hardware will do in that case. One way to find
out would be to try it. :)
It had better trigger the same overflow as for other arithmetic
operations. And in C# (which is using the hardware features), it does:
using System;
namespace TestDivOverflow
{
class Program
{
static void Main(string[] args)
{
try
{
checked
{
int i = int.MinValue, j = i / -1;
}
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
}
}
}
Output:
Arithmetic operation resulted in an overflow.
Note that .NET is taking advantage of the hardware exception; it doesn't
have to check the overflow bit, because the CPU generates an interrupt
when the overflow occurs, when set up properly. Here's the
(unoptimized) assembly for the line of code in the "checked" block:
00000041 mov dword ptr [ebp-40h],80000000h
00000048 mov eax,dword ptr [ebp-40h]
0000004b or ecx,0FFFFFFFFh
0000004e cdq
0000004f idiv eax,ecx
00000051 mov dword ptr [ebp-44h],eax
Java _could_ do the same, but of course you'd have to, one way or the
other, wrap "checked" areas of code with the toggle of the
interrupt-trapping so that only those areas of code you want to generate
the overflow check actually get it.
Pete