RFR (S) 8078438: Interpreter should support conditional card marks (UseCondCardMark)

[Andrew Haley]

On 04/28/2015 03:02 PM, Andrew Dinn wrote:
> On 28/04/15 14:28, Mikael Gerdin wrote:
>>> Hmm.  We were told otherwise here before.  Does the code which scans
>>> the card table use a load barrier after each read of the card?  If not
>>> the StoreStore won't have any effect anyway.
>>
>> Consider the following sequence of events:
>> Mutator:        CMS-preclean:
>> # o.x == NULL
>> o.x = a           
>> card[o >> 9] = 0x0    finds card[o >> 9] == 0x0
>>             reads o.x, finds NULL
>>             writes card[o >> 9] = 0x1
>> #            o.x == a becomes visible
>>
>> CMS-remark occurs, o.x is not scanned because card[o >> 9] == 0x1 which
>> is not dirty.
>>
>> There's no load barrier in the card scanning code that I'm aware of.
> 
> So, you are saying that CMS depends on the fact that x86/Sparc provide TSO?
> 
> If so then I believe for AArch64 that this means:
> 
> On the writing side we need to implement StoreCM using either
> 
>   dmb  ish
>   strb zr, ...
> 
> or, preferably
> 
>   strbl zr, ...
> 
> On the reading side we need the GC thread to execute an acquiring load
> when it reads the card, either
> 
>   ldrb ...
>   dmb ishld
> 
> or, preferably
> 
>   ldarb ...
> 
> How exactly does the GC code do the load -- using generated code or
> using a C library routine?
> 
> n.b. I am just completing an update to the AArch64 volatile write rules
> which deal with volatile object puts. The latest patch moves form
> generating a sequence like this
> 
>   dmb	ish
>   lsr	x10, x13, #9
>   str	w14, [x13,#40]
>   strb	wzr, [xmethod,x10,lsl #0]
>   dmb	ish
> 
> to producing something like this
> 
>   add	x10, x13, #0x28
>   stlr	w14, [x10]
>   lsr	x10, x13, #9
>   strb	wzr, [x11,x10,lsl #0]
> 
> The strb here is the current translation of the StoreCM.
> 
> From what you are saying it appears that we need to implement these
> sequences as either:
> 
>   dmb	ish
>   lsr	x10, x13, #9
>   str	w14, [x13,#40]
>   dmb ish
>   strb	wzr, [xmethod,x10,lsl #0]
>   dmb	ish

Eek!  No!

> or, preferably
> 
>   add	x10, x13, #0x28
>   stlr	w14, [x10]
>   lsr	x10, x13, #9
>   stlrb	wzr, [x11,x10,lsl #0]

That's more like it.

We'd want to use conditional card marking, I think.  It is possible
also to do the release conditionally on
CMSCollectorCardTableModRefBSExt::_requires_release.

Andrew.