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

[Tobias Hartmann]

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
>>>>