RFR: 8347555: [REDO] C2: implement optimization for series of Add of unique value [v9]

[Emanuel Peter]

On Mon, 17 Mar 2025 21:01:23 GMT, Kangcheng Xu <kxu at openjdk.org> wrote:

>> [JDK-8347555](https://bugs.openjdk.org/browse/JDK-8347555) is a redo of [JDK-8325495](https://bugs.openjdk.org/browse/JDK-8325495) was [first merged](https://git.openjdk.org/jdk/pull/20754) then backed out due to a regression. This patch redos the feature and fixes the bit shift overflow problem. For more information please refer to the previous PR.
>> 
>> When constanlizing multiplications (possibly in forms on `lshifts`), the multiplier is upgraded to long and then later narrowed to int if needed. However, when a `lshift` operand is exactly `32`, overflowing an int, using long has an unexpected result. (i.e., `(1 << 32) = 1` and `(int) (1L << 32) = 0`)
>> 
>> The following was implemented to address this issue.
>> 
>> if (UseNewCode2) {
>>     *multiplier = bt == T_INT
>>         ? (jlong) (1 << con->get_int()) // loss of precision is expected for int as it overflows
>>         : ((jlong) 1) << con->get_int();
>> } else {
>>     *multiplier = ((jlong) 1 << con->get_int());
>> }
>> 
>> 
>> Two new bitshift overflow tests were added.
>
> Kangcheng Xu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix ((x<<y) << z) + (x<<y) patterns

You match cases like `(a * CON) + a`.
- What do we do with patterns `a + (a * CON)`?
- What about `(a << CON1) + (a * CON2)` and other generalizations that have mixed operations on either side? You mentioned that things like `(a*b) + (a*c)` -> `a*(b + c)` are already handled by `AddNode::IdealIL`.

It would just be a shame to have all the complexity of matching specific cases, but not take the chance to make it a bit more general.

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PR Comment: https://git.openjdk.org/jdk/pull/23506#issuecomment-2772097646