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

[Andrew Haley]

On 11/15/19 9:15 AM, Pengfei Li (Arm Technology China) wrote:

>>> This patch aligns with the implementation in [1] which makes the
>>> x86_64
>>> r12 register allocatable. Please let me know if I have missed anything
>>> for AArch64.
>>
>> We don't generally use r27 for compressed class pointers.
> 
> Do you mean that r27 is only used for encoding/decoding oops but not for
> any klass pointers?

Almost always, yes.

> I looked at the AArch64 code and find it also used in
> MacroAssembler::encode_klass_not_null() if the compressed mode is
> not zero-based.

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.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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