Integrated: JDK-8316991: Reduce nullable allocation merges

[Cesar Soares Lucas]

On Tue, 19 Sep 2023 18:54:34 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       |
> | TestLoadAfterTrap                    |      28.370      |   ...

This pull request has now been integrated.

Changeset: a887fd21
Author:    Cesar Soares Lucas <cslucas at openjdk.org>
Committer: Vladimir Kozlov <kvn at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/a887fd2144ce067844f18a514afb5078255601ff
Stats:     2417 lines in 13 files changed: 2154 ins; 91 del; 172 mod

8316991: Reduce nullable allocation merges

Reviewed-by: kvn, vlivanov

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PR: https://git.openjdk.org/jdk/pull/15825