RFR: 8354727: CompilationPolicy creates too many compiler threads when code cache space is scarce

[Manuel Hässig]

Running


java -XX:+SegmentedCodeCache -XX:ReservedCodeCacheSize=10M -XX:NonNMethodCodeHeapSize=6M \
     -XX:ProfiledCodeHeapSize=5M -XX:NonProfiledCodeHeapSize=5M -version


on a machine with more than 285 cores, this would fail with the message that the specified `NonNMethodCodeHeapSize` is too small to fit all compiler buffers (instead of failing because the sum of the heaps is larger than the `ReservedCodeCacheSize`). Hence, the calculated compiler count is too high. This is due to CompilationPolicy::initialize() checking how many compiler buffers fit into the `ReservedCodeCacheSize`. However, in the case above, this is significantly larger than `NonNMethodCodeHeapSize` (especially on a debug build) and causes a check changed in #17244 to fail. That check was changed to check that all compiler buffers fit into the `NonNMethodCodeHeap` instead of the `NonNMethodCodeHeap` having at least a size of `CodeCacheMinimumUseSpace`.

# Changes

This PR fixes the calculation of the `CICompilerCount` ergonomic. Firstly, @shipilev kindly provided a fix for the compiler buffer size used in the calculation is also correct if we only have C2. Secondly, `NonNMethodHeapSize` is used as the maximum buffer size available for compilers buffers in the calculation of the maximum number of compiler threads instead of `ReservedCodeCacheSize`. Therefore, the check failing in the explanation above can never fail because we set the number of compiler threads only so high that they will always fit into the `NonNMethodCodeHeap`.

This change changes how many compiler threads are created by the `CICompilerCount` ergonomic. For the default value `NonNMethodCodeHeapSize=5M`this limit is 24 compiler threads on a system 285 cores or more for product builds and 20 threads for debug builds on a system with 145 cores or more.

# Testing

- [ ] [Github Actions](https://github.com/mhaessig/jdk/actions/runs/15733154809)
- [ ] tier1 through tier3 plus Oracle internal testing on our supported platforms

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Depends on: https://git.openjdk.org/jdk/pull/25791

Commit messages:
 - Calculate buffer size correctly for c2_only
 - Caclulate how many compiler buffers fit into NonNMethodCodeHeap
 - Clarify endif

Changes: https://git.openjdk.org/jdk/pull/25872/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354727
  Stats: 10 lines in 1 file changed: 7 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25872.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25872/head:pull/25872

PR: https://git.openjdk.org/jdk/pull/25872