RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v14]

[Vladimir Ivanov]

On Mon, 5 Jun 2023 21:10:22 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

>> A couple of suggestions about the output:
>> * `merge`: it's clearer to call it `merge_obj`
>> * `obj` vs `merge` output: obj output is duplicated in ScopeDesc entries and Objects sections; before it was a short version printed in Locals/Expressions and all the details were included in Objects; I like to see field locations in the short version, but including everything looks way too much IMO;
>> * it makes sense to include selector and merge_pointer info in short version, but `is_root` can be omitted
>
> Thanks @iwanowww . Does the output below look good to you? It prints ObjectValue in the same format as it was before this PR and only print details of the merge in the "Objects" section. Is there other output section that you think needs to be adjusted?
> 
> 
> Compiled method (c2)     436   24             TestMultiSFO::test (48 bytes)
>  total in heap  [0x00007f1df5155590,0x00007f1df5155850] = 704
>  relocation     [0x00007f1df5155700,0x00007f1df5155718] = 24
>  main code      [0x00007f1df5155720,0x00007f1df5155788] = 104
>  stub code      [0x00007f1df5155788,0x00007f1df51557a0] = 24
>  oops           [0x00007f1df51557a0,0x00007f1df51557b0] = 16
>  metadata       [0x00007f1df51557b0,0x00007f1df51557b8] = 8
>  scopes data    [0x00007f1df51557b8,0x00007f1df51557f8] = 64
>  scopes pcs     [0x00007f1df51557f8,0x00007f1df5155848] = 80
>  dependencies   [0x00007f1df5155848,0x00007f1df5155850] = 8
> scopes:
> ScopeDesc(pc=0x00007f1df515573a offset=1a):
>    TestMultiSFO::test at -1 (line 12)
> ScopeDesc(pc=0x00007f1df515575c offset=3c):
>    TestMultiSFO::test at 28 (line 19)
>    Locals
>     - l0: empty
>     - l1: empty
>     - l2: empty
>     - l3: merge_obj[14]
>     - l4: obj[15]
> 
>    Objects
>     - merge_obj[14], selector="reg rbp [10],int", merge_pointer="nullptr", candidate_objs=[15, 16]
>     - obj[15], is_root=1, klass: TestMultiSFO$Point 
> 	  Fields: stack[12], stack[8]
>     - obj[16], is_root=0, klass: TestMultiSFO$Point 
> 	  Fields: stack[8], stack[12]

Thanks, it looks much better now (except the position in Objects array is missing). 

It makes sense to mention `is_root` for merge_obj case even though it's always equals to '1`. Also, make merge_pointer optional and omit it when its value is null.

BTW instead of printing `is_root=0/1`, you can introduce a more compact notation for and mark relevant lines with a single symbol:

    - 0: R merge_obj[14], selector="reg rbp [10],int" candidates=[15, 16]
    - 1: R obj[15], klass: TestMultiSFO$Point 
 	        Fields: stack[12], stack[8]
    - 2:   obj[16], klass: TestMultiSFO$Point 
	        Fields: stack[8], stack[12]

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PR Review Comment: https://git.openjdk.org/jdk/pull/12897#discussion_r1218642386