[aarch64-port-dev ] RFR(M): 8233743: AArch64: Make r27 conditionally allocatable

[Nick Gasson]

Hi Andrew,

> 
> I see
> 
>    if (use_XOR_for_compressed_class_base) {
>      if (CompressedKlassPointers::shift() != 0) {
>        eor(dst, src, (uint64_t)CompressedKlassPointers::base());
>        lsr(dst, dst, LogKlassAlignmentInBytes);
>      } else {
>        eor(dst, src, (uint64_t)CompressedKlassPointers::base());
>      }
>      return;
>    }
> 
>    if (((uint64_t)CompressedKlassPointers::base() & 0xffffffff) == 0
>        && CompressedKlassPointers::shift() == 0) {
>      movw(dst, src);
>      return;
>    }
> 
>    ... followed by code which does use r27.
> 
> Do you ever see r27 being used? If so, I'd be interested to know how
> this gets triggered and what command-line arguments you use. It's
> rather inefficient.
> 

Oddly enough the test case runtime/memory/ReadFromNoaccessArea.java now 
hits this. I see:

CompressedKlassPointers::base() => 0xffff0b4b5000
CompressedKlassPointers::shift() => 3

The itable stub calls MacroAssembler::load_klass() twice which then 
calls the above decode_klass_not_null() with dst==src if 
UseCompressedClassPointers is true. So we do the saving/restoring 
rheapbase dance twice which blows up the size of the itable stub beyond 
the estimated 152B max size.

The key is that this test passes -XX:HeapBaseMinAddress=33G. That in 
conjunction with the recent changes to where the CDS archive is loaded 
hits this code path (I don't see this with -Xshare:off).


Thanks,
Nick