overflows in SLDivNode

[christian.humer at gmail.com]

Hi Raffaello,

Thanks for pointing this out.

I am a little unsure how to name the divExact method in ExactMath. The 
term "exact" might be misleading here as it actually rounds towards 0 
without throwing the exception. What if we want to add a divExact that 
throws if the result looses information? I am open to suggestions. Will 
push a fix just for SL in the meantime.

- Christian Humer




------ Original Message ------
From: "Raffaello Giulietti" <raffaello.giulietti at supsi.ch>
To: graal-dev at openjdk.java.net
Sent: 27.04.2015 14:17:53
Subject: Re: overflows in SLDivNode

>The following method (and the equivalent for int) might be added to 
>ExactMath to help implementing the division correctly.
>
>
>     public static long divExact(long x, long y) {
>         long r = x / y;
>         if ((x & y & r) < 0) {
>             throw new ArithmeticException("long overflow");
>         }
>         return r;
>     }
>
>
>Since SL is meant to be an exemplary Truffle language, I feel it will 
>be surely imitated, so it is important to keep it as correct as 
>possible.
>
>
>Greetings
>Raffaello
>
>
>
>
>On 2015-04-26 17:51, Raffaello Giulietti wrote:
>>Hi,
>>
>>I'm just studying the SL implementation in the last Graal/Truffle 
>>update.
>>
>>The long specialization of the division wrongly assumes that
>>/* No overflow is possible on a division. */
>>
>>When left is Long.MIN_VALUE and right is -1 the division overflows.
>>
>>OK, OK, OK, this is the only case and has a ridiculous
>>(cryptography-like) probability of 2^-128 to happen randomly. And, no,
>>neither java.lang.Math nor ExactMath cover this rare case of overflow.
>>
>>Greetings
>>Raffaello
>