RFR: JDK-8202016: Use obj+offset in interpreter array access

[Andrew Dinn]

On 14/05/18 11:15, Roman Kennke wrote:
> Am 07.05.2018 um 21:47 schrieb Roman Kennke:
>> In the TemplateTable::aastore() and
>> InterpreterMacroAssembler::load_resolved_reference_at_index(), the
>> element address is flattened, and then passed to the BarrierSetAssembler
>> for actual access. GCs like Shenandoah need the original obj though.
>>
>> The proposed change replaces the flattening with base+index+disp
>> addressing, and removes the shift and add computations. In the case of
>> aastore, we need to re-fetch the rcx/index from the stack because it
>> gets trashed on the way.
>>
>> Testing: passed: hotspot/jtreg:tier1
>>
>> Can I please get a review?
>> Thanks, Roman
Well, that x86 code change looks ok/ish/ -- although the need to reload
from the stack is a tad disappointing.

However, I am left wondering why this is not a problem on other
architectures? Indeed, looking at AArch64 I believe it is actually a
serious headache. Method aastore looks like this:

  Address element_address(r4, arrayOopDesc::base_offset_in_bytes(T_OBJECT));

  index_check(r3, r2);     // kills r1
  __ lea(r4, Address(r3, r2, Address::uxtw(UseCompressedOops? 2 : 3)));
  . . .

  // Get the value we will store
  __ ldr(r0, at_tos());
  // Now store using the appropriate barrier
  do_oop_store(_masm, element_address, r0, IN_HEAP | IN_HEAP_ARRAY);

The Address for r4 only includes a displacement and the index register
offset has to be explicitly added to the base register using an lea. Of
course, that's because AArch64 doesn't like an Address with both a disp
and a index register. You can have one or the other but not both.

So, if the abstraction you are relying on is to rely on do_oop_store to
unpack an object base address, offset and index register from its
Address argument then I am afraid you are on to a loser. What do you
propose to do about that? and should it not be done as part of this fix?

regards,


Andrew Dinn
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