RFR: 8201318: Introduce GCThreadLocalData to abstract GC-specific data belonging to a thread

[Aleksey Shipilev]

On 04/10/2018 06:50 PM, Aleksey Shipilev wrote:
> On 04/10/2018 05:59 PM, Robbin Ehn wrote:
>> On 2018-04-10 17:47, Aleksey Shipilev wrote:
>>> On 04/10/2018 05:25 PM, Robbin Ehn wrote:
>>>> Had quick look, what I saw looked good. (not a full review)
>>>> Is there a reason for moving the gc data to 'zero offset' in Thread?
>>>
>>> Oh! I missed that, and I fully agree with this move. At least one reason I see, smaller offsets
>>> against TLS open up opportunities for denser code-generation when e.g. GC barriers poll thread-local
>>> data. Right now SATB barrier generates something like "cmpb $0x0, 0x3d8(%r15)", while it could
>>> generate just "cmpb $0x0, 0x0(%r15)" now :)
>>
>> Yes, but it pushes down e.g.:
>> 333   volatile void* _polling_page;                 // Thread local polling page
>> Which may not matter, as long as it's on the first page I suppose.
> 
> I think the major concern should be the instruction size. On x86 what matters is what category
> immediate offset falls into. Some hand-crafted assembly:
> 
>    0:	48 89 42 7f          	mov    %rax,0x7f(%rdx)
>    4:	48 89 82 80 00 00 00 	mov    %rax,0x80(%rdx)
>    b:	80 7a 7f 00          	cmpb   $0x0,0x7f(%rdx)
>    f:	80 ba 80 00 00 00 00 	cmpb   $0x0,0x80(%rdx)
>   16:	80 7a 7f 41          	cmpb   $0x41,0x7f(%rdx)
>   1a:	80 ba 80 00 00 00 41 	cmpb   $0x41,0x80(%rdx)
>   21:	f6 42 7f 00          	testb  $0x0,0x7f(%rdx)
>   25:	f6 82 80 00 00 00 00 	testb  $0x0,0x80(%rdx)
>   2c:	f6 42 7f 41          	testb  $0x41,0x7f(%rdx)
>   30:	f6 82 80 00 00 00 41 	testb  $0x41,0x80(%rdx)
> 
> 
> In our case, we want to pack the most used fields under first 128 bytes. Maybe we should put polling
> page at offset 0, and trim GCTLD to 96 bytes?

Should have used %r15, of course, which does not change the picture qualitatively:

   0:	49 89 47 7f          	mov    %rax,0x7f(%r15)
   4:	49 89 87 80 00 00 00 	mov    %rax,0x80(%r15)
   b:	41 80 7f 7f 00       	cmpb   $0x0,0x7f(%r15)
  10:	41 80 bf 80 00 00 00 	cmpb   $0x0,0x80(%r15)
  17:	00
  18:	41 80 7f 7f 41       	cmpb   $0x41,0x7f(%r15)
  1d:	41 80 bf 80 00 00 00 	cmpb   $0x41,0x80(%r15)
  24:	41
  25:	41 f6 47 7f 00       	testb  $0x0,0x7f(%r15)
  2a:	41 f6 87 80 00 00 00 	testb  $0x0,0x80(%r15)
  31:	00
  32:	41 f6 47 7f 41       	testb  $0x41,0x7f(%r15)
  37:	41 f6 87 80 00 00 00 	testb  $0x41,0x80(%r15)
  3e:	41

-Aleksey