RFR: 8346832: runtime/CompressedOops/CompressedCPUSpecificClassSpaceReservation.java fails on RISC-V

[Fei Yang]

Hi, please review this small change fixing a test failure.

This test first expects `tryReserveForUnscaled` and `tryReserveForZeroBased` from the process output. In addition, when test without CDS, it also expects `reserve_between (range [0x0000000800000000-0x0000100000000000)` which is not lower than zero-based limit. But the process output gives `reserve_between (range [0x0000000100000000-0x0000100000000000)`, which is not expected.

Previously, we have the code handling this case in `CompressedKlassPointers::reserve_address_space_for_compressed_classes`.
But I find that the code is gone after JDK-8305895: Implement JEP 450: Compact Object Headers. This is the related change:


diff --git a/src/hotspot/cpu/riscv/compressedKlass_riscv.cpp b/src/hotspot/cpu/riscv/compressedKlass_riscv.cpp
index cffadb4189b..7c8d6b8f5bb 100644
--- a/src/hotspot/cpu/riscv/compressedKlass_riscv.cpp
+++ b/src/hotspot/cpu/riscv/compressedKlass_riscv.cpp
@@ -56,9 +56,9 @@ char* CompressedKlassPointers::reserve_address_space_for_compressed_classes(size
     result = reserve_address_space_for_zerobased_encoding(size, aslr);
   }

-  // Failing that, optimize for case (3) - a base with only bits set between [33-44)
+  // Failing that, optimize for case (3) - a base with only bits set between [32-44)
   if (result == nullptr) {
-    const uintptr_t from = nth_bit(32 + (optimize_for_zero_base ? LogKlassAlignmentInBytes : 0));
+    const uintptr_t from = nth_bit(32);
     constexpr uintptr_t to = nth_bit(44);
     constexpr size_t alignment = nth_bit(32);
     result = reserve_address_space_X(from, to, size, alignment, aslr);


It seems to me more reasonable to add the optimize_for_zero_base check back and replace the LogKlassAlignmentInBytes with
`CompressedKlassPointers::max_shift()`. max_shift() is used in `CompressedKlassPointers::reserve_address_space_for_zerobased_encoding` to calculate zerobased_max as well [1].

Testing: same test passes with fastdebug build on linux-riscv64 platform. Tagging @tstuefe 

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/oops/compressedKlass.cpp#L201

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

Commit messages:
 - 8346832: runtime/CompressedOops/CompressedCPUSpecificClassSpaceReservation.java fails on RISC-V

Changes: https://git.openjdk.org/jdk/pull/22879/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22879&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346832
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22879.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22879/head:pull/22879

PR: https://git.openjdk.org/jdk/pull/22879