RFR: 8333177: Invalid value used for enum Cell in ciTypeFlow::get_start_state

[Tobias Hartmann]

Ubsan detected undefined behavior in `ciTypeFlow::get_start_state` because an invalid value of `4294967295` is assigned to enum `Cell`:
https://github.com/openjdk/jdk/blob/ac7119f0d5319a3fb44dc67a938c3e1eb21b9202/src/hotspot/share/ci/ciTypeFlow.hpp#L150-L152

The problem is that if the C++ compiler decides to encode `Cell` with an unsigned int, casting a negative integer value will lead to an underflow and therefore a value > `Cell_max = INT_MAX`. Here, `state->tos()` returns a value < 0:
https://github.com/openjdk/jdk/blob/ac7119f0d5319a3fb44dc67a938c3e1eb21b9202/src/hotspot/share/ci/ciTypeFlow.cpp#L407

which is casted to a `Cell`:
https://github.com/openjdk/jdk/blob/ac7119f0d5319a3fb44dc67a938c3e1eb21b9202/src/hotspot/share/ci/ciTypeFlow.hpp#L211

I simply re-wrote the code to not require a negative `Cell` value to iterate over the locals and setting them to bottom type.

Thanks,
Tobias

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Commit messages:
 - 8333177: Invalid value used for enum Cell in ciTypeFlow::get_start_state

Changes: https://git.openjdk.org/jdk/pull/19520/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19520&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333177
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19520.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19520/head:pull/19520

PR: https://git.openjdk.org/jdk/pull/19520