RFR: 8333177: Invalid value used for enum Cell in ciTypeFlow::get_start_state [v2]

[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

Tobias Hartmann has updated the pull request incrementally with one additional commit since the last revision:

  Update src/hotspot/share/ci/ciTypeFlow.cpp
  
  Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19520/files
  - new: https://git.openjdk.org/jdk/pull/19520/files/ead0e611..c4d45ca4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19520&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19520&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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