Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier

[Pranith Kumar]

On Mon, Jun 6, 2016 at 11:44 AM, Sergey Fedorov <address@hidden> wrote:
> On 03/06/16 21:27, Pranith Kumar wrote:
>> On Thu, Jun 2, 2016 at 5:18 PM, Richard Henderson <address@hidden> wrote:
>>>
>>> What if we have tcg_canonicalize_memop (or some such) split off the barriers
>>> into separate opcodes.  E.g.
>>>
>>> MO_BAR_LD_B = 32        // prevent earlier loads from crossing current op
>>> MO_BAR_ST_B = 64        // prevent earlier stores from crossing current op
>>> MO_BAR_LD_A = 128       // prevent later loads from crossing current op
>>> MO_BAR_ST_A = 256       // prevent later stores from crossing current op
>>> MO_BAR_LDST_B = MO_BAR_LD_B | MO_BAR_ST_B
>>> MO_BAR_LDST_A = MO_BAR_LD_A | MO_BAR_ST_A
>>> MO_BAR_MASK = MO_BAR_LDST_B | MO_BAR_LDST_A
>>>
>>> // Match Sparc MEMBAR as the most flexible host.
>>> TCG_BAR_LD_LD = 1       // #LoadLoad barrier
>>> TCG_BAR_ST_LD = 2       // #StoreLoad barrier
>>> TCG_BAR_LD_ST = 4       // #LoadStore barrier
>>> TCG_BAR_ST_ST = 8       // #StoreStore barrier
>>> TCG_BAR_SYNC  = 64      // SEQ_CST barrier
>> I really like this format. I would also like to add to the frontend:
>>
>
> Actually, the acquire barrier is a combined load-load + load-store
> barrier; and the release barrier is a combo of load-store + store-store
> barriers.
>

All the above are two-way barriers. Acquire/Release are one-way
barriers. So we cannot combine the above to get acquire/release
semantics without being conservative.

Thanks,
-- 
Pranith