[15] RFR(S): 8237950: C2 compilation fails with "Live Node limit exceeded limit" during ConvI2L::Ideal optimization

[Tobias Hartmann]

Thanks Vladimir! I'll run our regular performance testing.

Best regards,
Tobias

On 15.06.20 20:02, Vladimir Kozlov wrote:
> +1
> 
> I would suggest to do our regular performance testing to make sure there is no regression.
> 
> Thanks,
> Vladimir
> 
> On 6/15/20 6:31 AM, Nils Eliasson wrote:
>> Hi Tobias,
>>
>> The change looks reasonable.
>>
>> Reviewed.
>>
>> Best regards,
>> Nils
>>
>> On 2020-06-15 13:22, Tobias Hartmann wrote:
>>> Hi,
>>>
>>> please review the following patch:
>>> https://bugs.openjdk.java.net/browse/JDK-8237950
>>> http://cr.openjdk.java.net/~thartmann/8237950/webrev.00/
>>>
>>> A long chain of StringBuffer.append calls is optimized by C2's string concatenation optimization
>>> which emits direct stores into the String internal byte array. GraphKit::array_element_address emits
>>> ConvI2L nodes for the array index (see Compile::conv_I2X_index) without any range check dependent
>>> CastII nodes because the bounds are known. As a result, the ConvI2L ideal optimization jumps in and
>>> creates over 34000 new ConvI2L nodes while pushing them through the long chain of AddNodes. We hit
>>> the node limit because during GVN, dead nodes are not removed.
>>>
>>> I propose to simply postpone that optimization to IGVN. This only affects array accesses emitted for
>>> the string concat optimizations because "normal" array accesses have a range check dependent CastII
>>> which blocks that ConvI2L optimization during parsing.
>>>
>>> Thanks,
>>> Tobias
>>