RFR: 8341781: Improve Min/Max node identities

[Jasmine Karthikeyan]

On Thu, 10 Oct 2024 06:06:19 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Hi all,
>> This patch implements some missing identities for Min/Max nodes. It adds static type-based operand choosing for MinI/MaxI, such as the ones that MinL/MaxL use. In addition, it adds simplification for patterns such as `Max(A, Max(A, B))` to `Max(A, B)` and `Max(A, Min(A, B))` to `A`. These simplifications stem from the [lattice identity rules](https://en.wikipedia.org/wiki/Lattice_(order)#As_algebraic_structure). The main place I've seen this pattern is with MinL/MaxL nodes created during loop optimizations. Some examples of where this occurs include BigInteger addition/subtraction, and regex code. I've run some of the existing benchmarks and found some nice improvements:
>> 
>>                                                                 Baseline                    Patch
>> Benchmark                                 Mode  Cnt       Score       Error  Units    Score       Error  Units  Improvement
>> BigIntegers.testAdd                       avgt   15      25.096 ±     3.936  ns/op   19.214  ±    0.521  ns/op  (+ 26.5%)
>> PatternBench.charPatternCompile           avgt    8     453.727 ±   117.265  ns/op   370.054 ±   26.106  ns/op  (+ 20.3%)
>> PatternBench.charPatternMatch             avgt    8     917.604 ±   121.766  ns/op   810.560 ±   38.437  ns/op  (+ 12.3%)
>> PatternBench.charPatternMatchWithCompile  avgt    8    1477.703 ±   255.783  ns/op  1224.460 ±   28.220  ns/op  (+ 18.7%)
>> PatternBench.longStringGraphemeMatches    avgt    8     860.909 ±   124.661  ns/op   743.729 ±   22.877  ns/op  (+ 14.6%)
>> PatternBench.splitFlags                   avgt    8     420.506 ±    76.252  ns/op   321.911 ±   11.661  ns/op  (+ 26.6%)
>> 
>> I've added some IR tests, and tier 1 testing passes on my linux machine. Reviews would be appreciated!
>
> test/hotspot/jtreg/compiler/vectorization/runner/BasicShortOpTest.java line 220:
> 
>> 218:         short[] res = new short[SIZE];
>> 219:         for (int i = 0; i < SIZE; i++) {
>> 220:             res[i] = (short) Math.min(a[i], b[i]);
> 
> I guess without this change, this collapses to a constant which enables vectorization which was not expected before?

Yeah, exactly - since `65536` is larger than the maximum short value, with this patch it can optimize the MinI node away entirely. I changed it to use the `b` array, which is what the `vectorMax` test case below uses.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21439#discussion_r1796410477