RFR: 8280872: Reorder code cache segments to improve code density

[Boris Ulasevich]

On Tue, 22 Feb 2022 23:53:19 GMT, Evgeny Astigeevich <duke at openjdk.java.net> wrote:

>> Currently the codecache segment order is [non-nmethod, non-profiled, profiled]. With this change we move the non-nmethod segment between two code segments. It changes nothing for any platform besides AARCH.
>> 
>> In AARCH the offset limit for a branch instruction is 128MB. The bigger jumps are encoded with three instructions. Most of far branches are jumps into the non-nmethod blobs. With the non-nmethod segment in between code segments the jump distance from method to the stub becomes shorter. The result is a 4% reduction in generated code size for the CodeCache range from 128MB to 240MB.
>> 
>> As a side effect, the performance of some tests is slightly improved:
>> ``ArraysFill.testCharFill      10  thrpt   15  170235.720 -> 178477.212  ops/ms``
>> 
>> Testing: jdk/hotspot jtreg and microbenchmarks on AMD and AARCH
>
> src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 411:
> 
>> 409:   assert(CodeCache::find_blob(entry.target()) != NULL,
>> 410:          "destination of far call not found in code cache");
>> 411:   if (far_branches()) {
> 
> Can we write something like this:
> 
> if (is_target_far_from_heap(entry, cbuf->target_code_heap())) {
> ...
> }
> 
> 
> And the implementation:
> 
> static inline bool is_target_far_from_heap(Address addr, CodeHeap* heap = nullptr) {
>   if (!SegmentedCodeCache || heap == nullptr) {
>     return ReservedCodeCacheSize > branch_range;
>   }
> 
>   return max_dist_to_heap(addr, heap) > branch_range;
> }

Yes, inline expression is difficult to read. I added target_needs_far_branch, I hope it is better now.

> src/hotspot/share/code/codeCache.cpp line 893:
> 
>> 891: }
>> 892: 
>> 893: bool CodeCache::is_codestub(address addr) {
> 
> Should it be named `is_non_nmethod`? According to the comments, there can be buffers, adapters and stubs.

Ok. Thanks

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

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