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

[Nils Eliasson]

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