RFR: 8280872: Reorder code cache segments to improve code density [v2]

[Boris Ulasevich]

On Wed, 2 Mar 2022 15:28:29 GMT, Evgeny Astigeevich <duke at openjdk.java.net> wrote:

>> With this patch I do not change trampoline calls. I change far_jump and far_call procedures only.
>> Instead of far_branches() function we have two functions:
>> - codecache_branch_needs_far_jump to find if we need a far jump for intra-codecache branches
>> - codestub_branch_needs_far_jump to find if we need a far branch for codecache-to-nonmethodEntrypoint branch
>> So in this place I leave codecache_branch_needs_far_jump as exact equivalent of former far_branches() call.
>
> I understand the changes. My comment is about names. `MacroAssembler` only needs to know if it needs a far jump. Details "why" are not needed here.
> We ask `MacroAssembler`. `MacroAssembler` gets `CodeCache` configuration info and checks whether a far jump is needed.

Ok. For the sake of formal logic here, let me introduce trampoline_needs_far_jump() function:

  if (a.trampoline_needs_far_jump() && ..) {
    stub = a.emit_trampoline_stub(dest);
  }
  if (stub == NULL) {
    // If we generated no stub, patch this call directly to dest.
    set_destination(dest);

>>> max_distance_to_non_nmethod_heap?
>>> As this is public API, it sounds strange without the start point.
>> 
>> Start point is any point in the CodeCache. Will the comment below help?
>> // maximum distance from any point in the CodeCache to any entry point in the non_nmethod CodeCache segment
>> This is really too many words for a self-explanatory function name.
>> 
>>> If someone changes positions of the heap, would it work as expected?
>> 
>> Sure
>
> Can we moved the code to `codestub_branch_needs_far_jump`? It is the only place where the code is used.
> We might need to make either `get_code_heap` public or `MacroAssembler` a friend of `CodeCache`.

Yes, it is the only place. Though I don't do this for encapsulation reasons: I think it's the internal details of the code cache if it is segmented and what the segment boundaries are. Right?

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

PR: https://git.openjdk.java.net/jdk/pull/7517