overflows in SLDivNode

[christian.humer at gmail.com]

I've pushed a fix to SL yesterday.
http://hg.openjdk.java.net/graal/graal/rev/b625b459cf45

@Andreas feel free to update it if you decide to add an intrinsic.

- Christian Humer




------ Original Message ------
From: "Raffaello Giulietti" <raffaello.giulietti at supsi.ch>
To: graal-dev at openjdk.java.net
Sent: 27.04.2015 16:23:40
Subject: Re: overflows in SLDivNode

>Hi Andreas,
>
>yes, I just posted a comment on the idiv instruction a couple of 
>minutes ago ;-)
>
>OK, the important point is that SL behaves correctly, since its 
>structure will be surely copied by other Truffle language 
>implementations, including the division AST node.
>
>
>
>
>
>On 2015-04-27 16:17, Andreas Woess wrote:
>>Thanks for pointing out the bug, Raffaello. We'll fix SL.
>>However, I don't think we should add a divExact method. We cannot
>>intrinsify it (the idiv instruction does not set the overflow flag); 
>>for
>>the same reason Java 8 does not have a divideExact. Also, the utility
>>method cannot specialize (whereas the SLDivNode can), so having it 
>>does
>>not buy us anything.
>>
>>- andreas
>>
>>On 27/04/15 15:26, christian.humer at gmail.com wrote:
>>>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
>>>>
>>>
>>
>