src/hotspot/share/oops/compressedKlass.cpp

*** 73,10 ***
--- 73,14 ---
        RANGE2FMTARGS(_klass_range_start, _klass_range_end),
        p2i(_base), _shift, _lowest_valid_narrow_klass_id, _highest_valid_narrow_klass_id);
  #define ASSERT_HERE(cond) assert(cond, " (%s)", tmp);
  #define ASSERT_HERE_2(cond, msg) assert(cond, msg " (%s)", tmp);
  
+   // There is no technical reason preventing us from using other klass pointer bit lengths,
+   // but it should be a deliberate choice
+   ASSERT_HERE(_narrow_klass_pointer_bits == 32 || _narrow_klass_pointer_bits == 19);
+ 
    // All values must be inited
    ASSERT_HERE(_max_shift != -1);
    ASSERT_HERE(_klass_range_start != (address)-1);
    ASSERT_HERE(_klass_range_end != (address)-1);
    ASSERT_HERE(_lowest_valid_narrow_klass_id != (narrowKlass)-1);

*** 214,11 ***
  
    // Calculate Base and Shift:
  
    if (UseCompactObjectHeaders) {
  
!     // In compact object header mode, with 22-bit narrowKlass, we don't attempt for
      // zero-based mode. Instead, we set the base to the start of the klass range and
      // then try for the smallest shift possible that still covers the whole range.
      // The reason is that we want to avoid, if possible, shifts larger than
      // a cacheline size.
      _base = addr;
--- 218,16 ---
  
    // Calculate Base and Shift:
  
    if (UseCompactObjectHeaders) {
  
!     // This handles the case that we - experimentally - reduce the number of
+     // class pointer bits further, such that (shift + num bits) < 32.
+     assert(len <= (size_t)nth_bit(narrow_klass_pointer_bits() + max_shift()),
+            "klass range size exceeds encoding, len: " SIZE_FORMAT ", narrow_klass_pointer_bits: %d, max_shift: %d", len, narrow_klass_pointer_bits(), max_shift());
+ 
+     // In compact object header mode, with 19-bit narrowKlass, we don't attempt for
      // zero-based mode. Instead, we set the base to the start of the klass range and
      // then try for the smallest shift possible that still covers the whole range.
      // The reason is that we want to avoid, if possible, shifts larger than
      // a cacheline size.
      _base = addr;