RFR: JDK-8316991: Reduce nullable allocation merges [v4]

[Andrey Turbanov]

On Mon, 30 Oct 2023 19:59:47 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

>> ### Description
>> 
>> Many, if not most, allocation merges (Phis) are nullable because they join object allocations with "NULL", or objects returned from method calls, etc. Please review this Pull Request that improves Reduce Allocation Merge implementation so that it can reduce at least some of these allocation merges.
>> 
>> Overall, the improvements are related to 1) making rematerialization of merges able to represent "NULL" objects, and 2) being able to reduce merges used by CmpP/N and CastPP. 
>> 
>> The approach to reducing CmpP/N and CastPP is pretty similar to that used in the `MemNode::split_through_phi` method: a clone of the node being split is added on each input of the Phi. I make use of `optimize_ptr_compare` and some type information to remove redundant CmpP and CastPP nodes. I added a bunch of ASCII diagrams illustrating what some of the more important methods are doing.
>> 
>> ### Benchmarking
>> 
>> **Note:** In some of these tests no reduction happens. I left them in to validate that no perf. regression happens in that case.
>> **Note 2:** Marging of error was negligible.
>> 
>> | Benchmark                            |  No RAM (ms/op)  |   Yes RAM (ms/op) |
>> |--------------------------------------|------------------|-------------------|
>> | TestTrapAfterMerge                   |      19.515      |      13.386       |
>> | TestArgEscape                        |      33.165      |      33.254       |
>> | TestCallTwoSide                      |      70.547      |      69.427       |
>> | TestCmpAfterMerge                    |      16.400      |       2.984       |
>> | TestCmpMergeWithNull_Second          |      27.204      |      27.293       |
>> | TestCmpMergeWithNull                 |       8.248      |       4.920       |
>> | TestCondAfterMergeWithAllocate       |      12.890      |       5.252       |
>> | TestCondAfterMergeWithNull           |       6.265      |       5.078       |
>> | TestCondLoadAfterMerge               |      12.713      |       5.163       |
>> | TestConsecutiveSimpleMerge           |      30.863      |       4.068       |
>> | TestDoubleIfElseMerge                |      16.069      |       2.444       |
>> | TestEscapeInCallAfterMerge           |      23.111      |      22.924       |
>> | TestGlobalEscape                     |      14.459      |      14.425       |
>> | TestIfElseInLoop                     |     246.061      |      42.786       |
>> | TestLoadAfterLoopAlias               |      45.808      |      45.812       |
>> ...
>
> Cesar Soares Lucas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Ammend previous fix & add repro tests.

test/micro/org/openjdk/bench/vm/compiler/AllocationMerges.java line 1079:

> 1077:         Load p1 = new Load(x, y);
> 1078:         Load p2 = new Load(x, y);
> 1079:         int val  = 0;

Suggestion:

        int val = 0;

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

PR Review Comment: https://git.openjdk.org/jdk/pull/15825#discussion_r1390973273