RFR: 8324890: C2 SuperWord: refactor out VLoop, make unrolling_analysis static, remove init/reset mechanism [v4]

[Emanuel Peter]

On Fri, 2 Feb 2024 18:33:29 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   timing code from JDK-8325159
>
> Thank you for running the timing testing.  What about memory fragmentation? Is this code will uses default chunks in Arena (they can be reused) or allocates new chunk (malloc) each time which may lead to fragmentation.

@vnkozlov is there a way to measure memory fragmentation? I don't know how to answer that question.
And is there really a difference to how it was done before? Before we just put everything on the `comp_arena`, and never recover the memory until that arena is given up. Now we have a new `autovectorization_arena` for each AutoVectorization pass over all loops. I guess there could be multiple such passes in a single compilation. Before this patch, this means that each such AutoVectorization pass creates its SuperWord object, and allocates memory on the `comp_arena`, and the memory usage of all these passes adds up. With this patch, we at least are able to give up the memory after every pass. Of course this is only helpful if the malloc/free'd chunks can properly be reused.

An alternative: we can put the `autovectorization_arena` at `Compile`. That way, the chunks are kept until the end of compilation, and can be reused between the different AutoVectorization passes/phases.

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PR Comment: https://git.openjdk.org/jdk/pull/17624#issuecomment-1925202347