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

[Tobias Hartmann]

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