> On Aug 21, 2016, at 11:14 AM, Philip Reames <listmail at
philipreames.com> wrote:
>
> On 08/17/2016 03:05 PM, Mehdi Amini wrote:
>>
>>> On Aug 17, 2016, at 2:08 PM, Zhuravlyov, Konstantin
<Konstantin.Zhuravlyov at amd.com <mailto:Konstantin.Zhuravlyov at
amd.com>> wrote:
>>>
>>> >Why not going with a metadata attachment directly and kill the
"singlethread" keyword? Something like:
>>> >Something like:
>>> > cmpxchg i32* %addr, i32 42, i32 0 monotonic monotonic, 3,
!memory.scope{!42}
>>> > cmpxchg i32* %addr, i32 42, i32 0 monotonic monotonic, 3,
!memory.scope{!43}
>>>
>>> >...
>>>
>>> >!42 = !{"singlethread"}
>>> >!43 = !{"L2"}
>>>
>>> >It is not clear to me if there is any correctness issues to
dropping metadata?
>>>
>>> Yes, we cannot use the metadata approach since this metadata can be
dropped during the processing of one module but not dropped in the processing of
a second module, potentially resulting in inconsistent scopes for synchronizing
operations leading to data races and subsequently leading to correctness issues.
>>>
>>
>> Right, I saw Sameer's explanation for that earlier, and we
shouldn’t move forward (without Philip’s opinion on the topic as he expressed
concerns).
> Given my current time commitments, having me on the critical path for any
proposal is not a good idea. I'm willing to step aside here as long as the
proposal is well reviewed by someone who's familiar with the memory model.
Hal, Sanjoy, JF, Chandler, and Danny would all be reasonable alternates.
OK, good to know. I put you on the path because you wrote:
"I am opposed to extending the instructions without a strong motivating
case. I don't care anywhere near as much about metadata based schemes, but
extending the instruction semantics imposes a much larger burden on the rest of
the community. That burden has to be well justified and supported."
It is not clear to me right now if the "use case" makes it "well
justified" or not (an alternative being using intrinsic for OpenCL as
Justin Lebar mentioned). I don’t feel I can answer this, so adding CC Chandler
and Sanjoy to begin with.
—
Mehdi
> Mehdi, if things get to the point where you think they're good to go
and no one else has chimed in, ping me. I'm not going to be following until
then, but I'll make the time for a final pass through if no one else has
first.
>>
>> But you stripped out the second part of my email where I wrote "It
seems you’re going back to integer, which I don’t really like for reasons
mentioned earlier in this thread, and that I don’t feel you addressed here”. Why
can’t `synchscope` take a string literal?
>>
>>
>> —
>> Mehdi
>>
>>
>>
>>
>>
>>>
>>>
>>> From: Zhuravlyov, Konstantin
>>> Sent: Wednesday, August 17, 2016 4:29:30 PM
>>> To: Sameer Sahasrabuddhe; Philip Reames
>>> Cc: Mehdi Amini; Liu, Yaxun (Sam); Ke Bai; Mekhanoshin, Stanislav;
Sumner, Brian; llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>; Tye, Tony
>>> Subject: RE: [llvm-dev] Memory scope proposal
>>>
>>> Hi,
>>>
>>> I have updated the review here:
>>> https://reviews.llvm.org/D21723
<https://reviews.llvm.org/D21723>
>>>
>>> As Sameer pointed out, the motivation is:
>>> In OpenCL 2.x, two atomic operations on the same atomic object need
to have the same scope to prevent a data race. This derives from the definition
of "inclusive scope" in OpenCL 2.x. Encoding OpenCL 2.x scope as
metadata in LLVM IR would be a problem because there cannot be a "safe
default value" to be used when the metadata is dropped. If the
"largest" scope is used as the default, then the optimizer must
guarantee that the metadata is dropped from every atomic operation in the whole
program, or not dropped at all.
>>>
>>> Thanks,
>>> Konstantin
>>>
>>> From: Sameer Sahasrabuddhe [mailto:sameer at sbuddhe.net
<mailto:sameer at sbuddhe.net>]
>>> Sent: Sunday, July 10, 2016 4:06 AM
>>> To: Philip Reames <listmail at philipreames.com
<mailto:listmail at philipreames.com>>
>>> Cc: Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini
at apple.com>>; Liu, Yaxun (Sam) <Yaxun.Liu at amd.com
<mailto:Yaxun.Liu at amd.com>>; Ke Bai <kebai613 at gmail.com
<mailto:kebai613 at gmail.com>>; Mekhanoshin, Stanislav
<Stanislav.Mekhanoshin at amd.com <mailto:Stanislav.Mekhanoshin at
amd.com>>; Sumner, Brian <Brian.Sumner at amd.com
<mailto:Brian.Sumner at amd.com>>; llvm-dev at lists.llvm.org
<mailto:llvm-dev at lists.llvm.org>; Zhuravlyov, Konstantin
<Konstantin.Zhuravlyov at amd.com <mailto:Konstantin.Zhuravlyov at
amd.com>>; Tye, Tony <Tony.Tye at amd.com <mailto:Tony.Tye at
amd.com>>
>>> Subject: Re: [llvm-dev] Memory scope proposal
>>>
>>>
>>> On Mon, Jul 4, 2016 at 5:09 AM, Philip Reames via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>> I will comment - as one of the few people actually working on
llvm's atomic implementation with any regularity - that I am opposed to
extending the instructions without a strong motivating case. I don't care
anywhere near as much about metadata based schemes, but extending the
instruction semantics imposes a much larger burden on the rest of the community.
That burden has to be well justified and supported.
>>>
>>> In OpenCL 2.x, two atomic operations on the same atomic object need
to have the same scope to prevent a data race. This derives from the definition
of "inclusive scope" in OpenCL 2.x. Encoding OpenCL 2.x scope as
metadata in LLVM IR would be a problem because there cannot be a "safe
default value" to be used when the metadata is dropped. If the
"largest" scope is used as the default, then the optimizer must
guarantee that the metadata is dropped from every atomic operation in the whole
program, or not dropped at all.
>>>
>>> Hence the original attempt to extend LLVM atomic instructions with
a broader scope field.
>>>
>>> Sameer.
>>
>
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