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  1. JDK
  2. JDK-8193266

AArch64: TestOptionsWithRanges.java SIGSEGV


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      The TestOptionsWithRanges.java fails as when the test launches the following commandline, there is a SIGSEGV:

      java -server -XX:+UseG1GC -XX:+UnlockDiagnosticVMOptions -XX:SharedArchiveFile=TestOptionsWithRanges.jsa -Xshare:dump -XX:SharedBaseAddress=2147483648

      See the attached TestOptionsWithRanges.jtr for the full results.

      The crash occurs in the code generated by:
       void TemplateTable::_return(TosState state)
      for " _return_register_finalizer" - this is why the initializer of Object is failing.
      The failing code is generated by

      We execute in decode_klass_not_null the clause triggered by MacroAssembler::use_XOR_for_compressed_class_base .
      If that is set, we generated a shift (optional) and then a eor to generate the address.

      The MacroAssembler initializer sets the flag with the following logic:

            = (operand_valid_for_logical_immediate(false /*is32*/,
               && ((uint64_t)Universe::narrow_klass_base()
                   > (1u << log2_intptr(CompressedClassSpaceSize))));

      So if the narrow_klass_base is larger than the highest bit of the CompressedClassSpaceSize then logically we eor the base address onto the address.
      Like so:
         0x000003ff98959da8: ldr x1, [x24]
         0x000003ff98959dac: ldr w3, [x1, #8]
         0x000003ff98959db0: lsl x3, x3, #3
         0x000003ff98959db4: eor x3, x3, #0x80000000

      We get the situation where a bad address is generated when the class is accessed.
      Given x3 starting as 0x18002a86, we should get the following:
      (0x18002a86L <<3)+0x80000000L = 0x140015430

      Instead the calculation is :
      (0x18002a86L <<3l) ^ 0x80000000L = 0x40015430

      Which is incorrect, and causes a crash.

      The problem is because the CompressedClassSpaceSize doesn't encompass the full range of memory allocated for classes.

      The method MetaspaceShared::initialize_dumptime_shared_and_meta_space does the initialization, and allocates 4GB on LP64 systems. The "CompressedClassSpaceSize" is set here.

      The dumping process reports the following:

      narrow_klass_base = 0x00000000 8000 0000, narrow_klass_shift = 3
      Allocated temporary class space: 1073741824 bytes at 0x0000000 1 4000 0000 <- 0x80000000 + 3GB
      Allocated shared space: 3221225472 bytes at 0x00000000 8000 0000 < +3GB

      The address of the class is within the temporary class space. Plainly if the classes are being allocated within a memory space of 4GB, we're not going to be able to generate addresses by XORing a base value that is at or below the most significant bits of the unshifted address.

      I have modified the constructor of MacroAssembler to set use_XOR_for_compressed_class_base using 4GB as the size of the compressed class area, ignoring the parameter. Superficially this tests ok. Using eor will be valid with base addresses over 4GB.

      I don't think the CompressedClassSpaceSize parameter is being respected - I'll do more testing.




            • Assignee:
              smonteith Stuart Monteith
              smonteith Stuart Monteith
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