RFR(S): 8166742 : SIGFPE in C2 Loop IV elimination

[Chuck Rasbold]

Sorry for not being transparent enough.  Here's an external reference that
describes the problem
that is being encountered by the division:

https://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Signed-Integer-Division.html

That's why the original fix targeted a very specific case.  One can't
represent ratio_con as a 32 bit value in that case.
Worse, trying to compute it by division causes a SIGFPE.

Do you think the revised code below is as straightforward as the original?

-- Chuck

On Tue, Sep 27, 2016 at 9:48 AM, Vladimir Kozlov <vladimir.kozlov at oracle.com
> wrote:

> So why it is SIGFPE when both values are 'int'?
>
> I thought it is incorrect results cause SIGFPE that is why I suggested to
> check for integer overflow.
>
> Lets then go with your second suggested change here. But let check that
> ratio is small first and do cast to (jint) otherwise the long check is
> useless:
>
>   // The ratio of the two strides cannot be represented as an int
>   // if stride_con2 is min_int and stride_con is -1.
>   jlong ratio_conl = ((jlong)stride_con2 / stride_con);
>
>   if ((ratio_conl < 0x80000000L) &&
>       (jint)(ratio_conl * stride_con) == stride_con2) { // Check for exact
>      jint ratio_con = (jint)ratio_conl;
>
> Thanks,
> Vladimir
>
> On 9/27/16 7:56 AM, Chuck Rasbold wrote:
>
>>
>>
>> On Mon, Sep 26, 2016 at 5:35 PM, Vladimir Kozlov
>> <vladimir.kozlov at oracle.com <mailto:vladimir.kozlov at oracle.com>> wrote:
>>
>>     Slightly different (cast after /) and jlong type:
>>
>>       jlong ratio_conl = (jlong) (stride_con2 / stride_con);
>>
>>
>> The division above won't work (at least, it raises a SIGFPE on my Linux
>> x86 platform) when stride_con2 == min_jint   and stride_con == -1.
>>
>>
>>       if ((ratio_conl * stride_con) == (jlong)stride_con2) { // Check
>>     for exact
>>
>>
>> What would be the value of ratio_conl such that this test fails?  I
>> think I'm missing something...
>>
>> -- Chuck
>>
>>
>>     Vladimir
>>
>>     On 9/26/16 5:01 PM, Chuck Rasbold wrote:
>>
>>         Just to confirm, are you suggesting that the ratio be first
>>         computed as a 64 bit quantity, effectively along the lines of...
>>
>>            long ratio_conl = ((long) stride_con2) / stride_con;
>>
>>            if ((ratio_conl * stride_con) == stride_con2 &&
>>                ratio_conl < 0x8000000 ) { // Check for exact
>>              int ratio_con = (int) ratio_conl;
>>
>>
>>         On Mon, Sep 26, 2016 at 3:45 PM, Vladimir Kozlov
>>         <vladimir.kozlov at oracle.com <mailto:vladimir.kozlov at oracle.com>
>>         <mailto:vladimir.kozlov at oracle.com
>>         <mailto:vladimir.kozlov at oracle.com>>> wrote:
>>
>>             Hi Chuck
>>
>>             Can you do 'long' arithmetic in existing condition to catch
>>         integer overflow instead?
>>
>>             if ((ratio_con * stride_con) == stride_con2) { // Check for
>>         exact
>>
>>             thanks,
>>             Vladimir
>>
>>
>>
>>             On 9/26/16 3:18 PM, Chuck Rasbold wrote:
>>
>>                 A small fix for an edge case crash in C2...
>>
>>                 Bug: https://bugs.openjdk.java.net/browse/JDK-8166742
>>         <https://bugs.openjdk.java.net/browse/JDK-8166742>
>>         <https://bugs.openjdk.java.net/browse/JDK-8166742
>>         <https://bugs.openjdk.java.net/browse/JDK-8166742>>
>>                 Webrev:
>>         http://cr.openjdk.java.net/~rasbold/8166742/webrev.00/
>>         <http://cr.openjdk.java.net/~rasbold/8166742/webrev.00/>
>>         <http://cr.openjdk.java.net/~rasbold/8166742/webrev.00/
>>         <http://cr.openjdk.java.net/~rasbold/8166742/webrev.00/>>
>>
>>                 Requesting a sponsor and reviews.  Thanks.
>>
>>                 -- Chuck
>>
>>
>>
>>
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