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

[Vladimir Kozlov]

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
>>>