RFR: 8345580: Remove const from Node::_idx which is modified

[Emanuel Peter]

On Mon, 9 Dec 2024 15:16:32 GMT, Yagmur Eren <duke at openjdk.org> wrote:

> `Node::_idx` is declared as `const`, however, it is modified by `Node::set_idx`. Please see: https://github.com/openjdk/jdk/blob/166c12771d9d8c466e73a9490c4eb1fc9a5f6c24/src/hotspot/share/opto/node.hpp#L588
> 
> As already stated in [JDK-8345580](https://bugs.openjdk.org/browse/JDK-8345580) issue, this behavior is counterintuitive because a const variable is expected to remain unmodified throughout its lifetime. Additionally, C++ International Standard states that "_...any attempt to modify a `const` object during its lifetime results in undefined behavior._ ". 
> To address this, `const` should be removed from the declaration of `Node::_idx` to align with its intended use and avoid violating the C++ standard. Tested with tier1,2,3,4,5.

@nelanbu You should probably then remove the hack with casting away the const-ness:

  void set_idx(uint new_idx) {
    const node_idx_t* ref = &_idx;
    *(node_idx_t*)ref = new_idx;
  }

Could simply become

  void set_idx(uint new_idx) {
    _idx = new_idx;
  }

Please check if there is any similar trick anywhere else.

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PR Comment: https://git.openjdk.org/jdk/pull/22646#issuecomment-2549125658