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NNTP-Posting-Date: Fri, 08 Jul 2011 21:49:11 -0500
Date: Fri, 08 Jul 2011 19:49:11 -0700
From: Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com>
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Subject: Re: Arithmetic overflow checking
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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