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

[RealFYang]

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

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Issue

• JDK-8346832: runtime/CompressedOops/CompressedCPUSpecificClassSpaceReservation.java fails on RISC-V (Bug - P4)

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[tstuefe]

Hi @RealFYang!

Would not removing the zero-based search be simpler then? We would forfeit the search for an area within 4g-32g then, but if I recall correctly, for bases between 4G and 16TB we use addiw + slli. In other words, we don't take advantage of zero base nonzero shift.

If I recall that incorrectly, then you are right, we may just skip searching between 4Gb and 32Gb since we just did that in the zero-based step.