AARCH64: 8139041: Redundant DMB instructions (CORRECTED )

[Vladimir Kozlov]

On 10/10/15 2:47 AM, Christian Thalinger wrote:
>
>> On Oct 9, 2015, at 5:37 AM, Andrew Haley <aph at redhat.com> wrote:
>>
>> Hi,
>>
>> On 10/09/2015 03:59 PM, Roland Westrelin wrote:
>>
>>>> There is a much simpler way: remove adjacent barriers in
>>>> MacroAssembler.  Thanks to the way that the AArch64 ISA is designed,
>>>> barriers can be merged simply by ORing them together.  Of course, this
>>>> technique works for C1 and C2, and it adds essentially nothing to the
>>>> compilation time.
>>>>
>>>> http://cr.openjdk.java.net/~aph/8139041/
>>>>
>>>> One thing which may be controversial is that I've added a field to
>>>> CodeBuffer to keep track of barriers and labels.  I had to do this
>>>> because when we're compiling there is (AFAICS) essentially nowhere
>>>> else to keep the state.
>
> I don’t think it matters to have an additional field in CodeBuffer.  It’s a temporary data structure and we use much more memory for graphs.
>
> I would even go that far and questioning putting it under #ifdef AARCH64.

+1 that.

Vladimir

>
>>>
>>> Isn’t your new field a bit like:
>>>
>>>   address       insts_mark() const       { return _insts.mark();       }
>>>   void      set_insts_mark()             {        _insts.set_mark();   }
>>>   void    clear_insts_mark()             {        _insts.clear_mark(); }
>>>
>>> which is used in very few locations AFAIK. Do you think you could reuse that one?
>>
>> Yes, that's what it's based on.  I guess that is possible in theory,
>> but AbstractAssembler::InstructionMark() looks like this:
>>
>>     InstructionMark(AbstractAssembler* assm) : _assm(assm) {
>>       assert(assm->inst_mark() == NULL, "overlapping instructions");
>>       _assm->set_inst_mark();
>>     }
>>
>> so any instruction which leaves the mark set will trigger an assertion
>> failure the next time InstructionMark is used.  I suppose that in
>> extremis I could make every instruction which is not a memory barrier
>> clear the mark, but ewww.  :(
>>
>> I suppose I could define an AArch64-specific version of
>> InstructionMark which does not have this assert, but I'm not sure I
>> like that either.
>>
>> [An aside: we use InstructionMark unnecessarily in AArch64, and it's
>> on my list of things to remove, but that's for another day.]
>>
>> Thanks for looking at this,
>>
>> Andrew.
>