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

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 06/06/16 18:58, Pranith Kumar wrote:
> On Mon, Jun 6, 2016 at 11:49 AM, Sergey Fedorov <serge.fdrv@gmail.com> wrote:
>> On 06/06/16 18:47, Pranith Kumar wrote:
>>> On Mon, Jun 6, 2016 at 11:44 AM, Sergey Fedorov <serge.fdrv@gmail.com> wrote:
>>>> On 03/06/16 21:27, Pranith Kumar wrote:
>>>>> On Thu, Jun 2, 2016 at 5:18 PM, Richard Henderson <rth@twiddle.net> 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.
>> Do you mean *barriers* or *memory access* operations implying memory
>> ordering?
> I meant the latter. I know no arch which has acquire/release barriers.
> Sorry for the confusion.

So I am. By the way, what's the difference between sequential consistent
*barrier* and a combination of all the other barriers?

Kind regards,
Sergey