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

[Tobias Hartmann]

Hi Nils,

thanks for the review!

Best regards,
Tobias

On 15.06.20 15:31, 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
>