C2: String concatenation optimization fails with "Hit MemLimit". If we disable this limit, it hits another error: "assert(req < Compile::current()->max_node_limit() - NodeLimitFudgeFactor) failed: Input limit exceeded"

In product, the test seems to hang.

Code pattern:
[...]
        s = new StringBuilder().append(s).append(s).toString();
        s = new StringBuilder().append(s).append(s).toString();
        s = new StringBuilder().append(s).append(s).toString();
        s = new StringBuilder().append(s).append(s).toString();
[..]

===

In TestStackedConcatsMany.java, a string of large size 2^24 is built. But even if half of those concats are removed resulting in a more reasonably sized 2^12 string, there's a later crash:

78 assert(C->live_nodes() <= C->max_node_limit(), "Live Node limit exceeded limit");

#0 src/hotspot/share/opto/node.cpp:78 in Node::verify_construction
[...]
# 7 src/hotspot/share/opto/stringopts.cpp:1558 in PhaseStringOpts::copy_string
# 8 src/hotspot/share/opto/stringopts.cpp:1612 in PhaseStringOpts::replace_string_concat

Less than 1000 live nodes before the call to replace_string_concat turns into 80000+ after.

===

Maybe the optimization needs to be bounded somehow.