RFR: 8324751: C2 SuperWord: Aliasing Analysis runtime check [v8]

[Emanuel Peter]

On Tue, 12 Aug 2025 17:00:54 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Let me try to find an answer here...
>> At least there is an assert in the constructor of `MemPointer`:
>> 
>>     for (int i = 0; i < summands.length(); i++) {
>>       const MemPointerSummand& s = summands.at(i);
>>       assert(s.variable() != nullptr, "variable cannot be null");
>>       assert(!s.scale().is_NaN(), "non-NaN scale");
>>     }
>>     for (int i = 0; i < raw_summands.length(); i++) {
>>       const MemPointerRawSummand& s = raw_summands.at(i);
>>       assert(!s.scaleI().is_NaN(), "non-NaN scale");
>>       assert(!s.scaleL().is_NaN(), "non-NaN scale");
>>     }
>
> And I think there is already some filtering in `canonicalize_raw_summands`:
> 
>     // Keep summands with non-zero scale.
>     if (!scaleI.is_zero() && !scaleL.is_NaN()) {
>       _raw_summands.at_put(pos_put++, MemPointerRawSummand(variable, scaleI, scaleL, int_group));
>     }

Ah, but the real work gets done here, in `MemPointer::make`:

    if (raw_summands.length() <= RAW_SUMMANDS_SIZE &&
        summands.length() <= SUMMANDS_SIZE &&
        has_no_NaN_in_con_and_summands(con, summands)) {
      return MemPointer(pointer, raw_summands, summands, con, size NOT_PRODUCT(COMMA trace));
    } else {
      return MemPointer::make_trivial(pointer, size NOT_PRODUCT(COMMA trace));
    }

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PR Review Comment: https://git.openjdk.org/jdk/pull/24278#discussion_r2270573790