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

[Evgeny Astigeevich]

On Thu, 17 Feb 2022 15:40:07 GMT, Boris Ulasevich <bulasevich at openjdk.org> 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;
}

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.

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

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