[11] RFR(XS) 8202948: C2: assert(init_offset >= 0) failed: positive offset from object start

[Vladimir Kozlov]

Thank you, Tobias

Vladimir

On 6/14/18 11:35 PM, Tobias Hartmann wrote:
> Hi Vladimir,
> 
> looks good, thanks for adding the test.
> 
> Best regards,
> Tobias
> 
> On 14.06.2018 22:51, Vladimir Kozlov wrote:
>> I added test and ran it on all platforms:
>>
>> http://cr.openjdk.java.net/~kvn/8202948/webrev.01/
>>
>> Thanks,
>> Vladimir
>>
>> On 6/14/18 8:41 AM, Vladimir Kozlov wrote:
>>> Thank you, Tobias
>>>
>>> Yes, I need to add test.
>>>
>>> Thanks,
>>> Vladimir
>>>
>>> On 6/14/18 12:07 AM, Tobias Hartmann wrote:
>>>> Hi Vladimir,
>>>>
>>>> looks good to me but shouldn't we add the test (looks like you came up with a very simple version)?
>>>>
>>>> Thanks,
>>>> Tobias
>>>>
>>>> On 13.06.2018 19:14, Vladimir Kozlov wrote:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8202948
>>>>>
>>>>> Predicates and Range checks before main loop will not help since they are runtime checks and limit
>>>>> is not constant. The problem happens during compilation because vectorization is attempted for
>>>>> over-unrolling loop which will not be executed in runtime (dead loop). Stride is negative (-3)
>>>>> value
>>>>> and initial is 2 - loop will be executed at least once (or never) because of other conditions in
>>>>> code. C2 did not recognize this because the test fails only with -Xcomp when profiling data is no
>>>>> available (it passed without -Xcomp).
>>>>>
>>>>> Simplest fix is to convert the assert into compilation check which will skip superword optimization
>>>>> in such case.
>>>>>
>>>>> diff -r 0d47e89382ed src/hotspot/share/opto/superword.cpp
>>>>> --- a/src/hotspot/share/opto/superword.cpp
>>>>> +++ b/src/hotspot/share/opto/superword.cpp
>>>>> @@ -887,7 +887,9 @@
>>>>>      if (init_nd->is_Con() && p.invar() == NULL) {
>>>>>        int init = init_nd->bottom_type()->is_int()->get_con();
>>>>>        int init_offset = init * p.scale_in_bytes() + offset;
>>>>> -    assert(init_offset >= 0, "positive offset from object start");
>>>>> +    if (init_offset < 0) { // negative offset from object start?
>>>>> +      return false;        // may happen in dead loop
>>>>> +    }
>>>>>        if (vw % span == 0) {
>>>>>          // If vm is a multiple of span, we use formula (1).
>>>>>          if (span > 0) {
>>>>>
>>>>> Test passed with and without Predicates with this fix.
>>>>>
>>>>> Tested with tier1, tier2, precheckin-xcomp
>>>>>