RFR: 8374889: C2 VectorAPI: must handle impossible combination of signed cast from float

[Quan Anh Mai]

On Tue, 13 Jan 2026 07:43:23 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> src/hotspot/share/opto/vectorIntrinsics.cpp line 2338:
>> 
>>> 2336:     // expected to violate this at runtime, but we may compile unreachable code
>>> 2337:     // where such impossible combinations arise.
>>> 2338:     if (is_ucast && (!is_integral_type(elem_bt_from) || elem_bt_from == T_LONG)) {
>> 
>> I would suggest eagerly killing this path to be extra sure that it is indeed a dead path and we are not encountering some other issue here.
>
> Hmm, indeed, we could try to put a `Halt` node here, right?
> @merykitty How exactly would you do that? Are there places we already do that?

If you look at the end of `GraphKit::uncommon_trap`, the procedure would look like this:

    HaltNode* halt = new HaltNode(control(), frameptr(), "uncommon trap returned which should never happen"
                                                         PRODUCT_ONLY(COMMA /*reachable*/false));
    _gvn.set_type_bottom(halt);
    root()->add_req(halt);

    stop_and_kill_map();

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

PR Review Comment: https://git.openjdk.org/jdk/pull/29169#discussion_r2685366719