RFR: JDK-8241503: C2: Share MacroAssembler between mach nodes during code emission [v6]

[Andrew Haley]

On Thu, 14 Dec 2023 19:44:09 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

>> # Description
>> 
>> Please review this PR with a patch to re-use the same C2_MacroAssembler object to emit all instructions in the same compilation unit.
>> 
>> Overall, the change is pretty simple. However, due to the renaming of the variable to access C2_MacroAssembler, from `_masm.` to `masm->`, and also some method prototype changes, the patch became quite large.
>> 
>> # Help Needed for Testing
>> 
>> I don't have access to all platforms necessary to test this. I hope some other folks can help with testing on `S390`, `RISC-V` and `PPC`.
>> 
>> # Testing status
>> 
>> ## tier1
>> 
>> |          |   Win   |   Mac   |  Linux  |
>> |----------|---------|---------|---------|
>> | ARM64    |         |    ✔     |         |
>> | ARM32    |         |         |         |
>> | x86      |         |         |    ✔     |
>> | x64      |         |         |     ✔    |
>> | PPC64    |         |         |         |
>> | S390x    |         |         |         |
>> | RiscV    |    n/a     |    n/a     |    ✔     |
>
> Cesar Soares Lucas has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:
> 
>  - Merge with origin/master
>  - Fix build, copyright dates, m4 files.
>  - Fix merge
>  - Catch up with master branch.
>    
>    Merge remote-tracking branch 'origin/master' into reuse-macroasm
>  - Some inst_mark fixes; Catch up with master.
>  - Catch up with changes on master
>  - Reuse same C2_MacroAssembler object to emit instructions.

It seems odd to me that this substantial and complex patch lacks any justification. As far as I can tell, the decision to make class MacroAssembler very lightweight so that new instances could be created as needed was deliberate. Why change now? Is it performance, or something else?

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PR Comment: https://git.openjdk.org/jdk/pull/16484#issuecomment-1858810100