RFR: 8347555: [REDO] C2: implement optimization for series of Add of unique value [v4]

[Roland Westrelin]

On Thu, 27 Feb 2025 18:17:19 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:
> 
>   update license header year

Changes requested by roland (Reviewer).

src/hotspot/share/opto/addnode.cpp line 524:

> 522:       }
> 523: 
> 524:       lhs_multiplier = bt == T_INT

Why isn't it: `java_shift_left(1, con->get_int(), bt)` ?

src/hotspot/share/opto/addnode.cpp line 546:

> 544:         }
> 545: 
> 546:         *multiplier = lhs_multiplier + (bt == T_INT

Same here

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

PR Review: https://git.openjdk.org/jdk/pull/23506#pullrequestreview-2650277818
PR Review Comment: https://git.openjdk.org/jdk/pull/23506#discussion_r1975130092
PR Review Comment: https://git.openjdk.org/jdk/pull/23506#discussion_r1975130496