[LLVMdev] Loads moving across barriers

[Andrew Trick]

On Dec 20, 2013, at 6:46 PM, Matt Arsenault <arsenm2 at gmail.com> wrote:

> 
> On Dec 4, 2013, at 8:25 PM, Andrew Trick <atrick at apple.com> wrote:
> 
>> 
>> On Dec 4, 2013, at 5:19 PM, Matt Arsenault <Matthew.Arsenault at amd.com> wrote:
>> 
>>> On 12/04/2013 04:29 PM, Andrew Trick wrote:
>>>> On Dec 4, 2013, at 3:33 PM, Matt Arsenault <Matthew.Arsenault at amd.com> wrote:
>>>> 
>>>>> On 11/11/2013 03:13 PM, Andrew Trick wrote:
>>>>>> On Nov 9, 2013, at 1:39 PM, Matt Arsenault <arsenm2 at gmail.com> wrote:
>>>>>> 
>>>>>>> On Nov 9, 2013, at 3:14 AM, Chandler Carruth <chandlerc at google.com> wrote:
>>>>>>> 
>>>>>>>> Perhaps you're instead trying to say that with certain address spaces "noalias" (and by inference, "restrict" at the language level) has a different semantic model than other address spaces? While it's less worrisome than the first interpretation, I still don't really like it.
>>>>>>>> 
>>>>>>> This sounds right. With the constant address space, anything you do is OK since it’s constant. Private address space is supposed to be totally inaccessible from other workitems, so parallel modifications aren’t a concern. The others require explicit synchronization which noalias would need to be aware of.
>>>>>> FWIW, it seems generally useful to me to have a nomemfence function attribute and intrinsic property. We should avoid memory optimization (and possibly other optimization) across these regardless of alias analysis.
>>>>>> 
>>>>> I'm think I'll try implementing this. Ideally it would be parameterized over the address space, so it makes more sense for it to be a memfence attribute rather than a nomemfence. You would then have an arbitrary number of memfence(N) attributes for each required address space.
>>>> So for correctness, would we need to tag all functions with memfence(0..M) until we can prove otherwise? That seem heinous.
>>> I was thinking the absence of it would mean no memfence in any address space, which is the current behavior. This adds the option of fencing.
>>>> Better to have an optional attribute that can be added to expose optimization. Is it important in practice to optimize the case of memfence(I) + nomemfence(J)?
>>> I think it would be important for the GPU case. You never need a memfence for private address space / addrspace 0, but you frequently want them for local or global. The local or global writes can't be reordered, but it could be very beneficial to move the private accesses across fences which might help reduce register usage.
>>> 
>>>>  If so, is there a problem with nomemfence(N)?
>>> nomemfence is the current assumption made on an arbitrary call, and it's the common case. Specifying the absence of a fence seems backwards of how this is used and more cumbersome to deal with. To match the current behavior, it would require littering nomemfence for any possible address space everywhere. In OpenCL you specify your fences, so it would be more straightforward to map that. If I have a memfence intrinsic, I just need to mark it with the fence attribute, and then propogate it to its callers. There would generally only be a few of them in any program compared to fenceless calls. To implement this with nomemfence, I would have to mark every function with at least 4 nomemfences, and remove them when encountering the memfence intrinsic.
>> 
>> 
>> Sure, but the program still needs to be correct if you skip attribute propagation.
>> -Andy
> 
> Is this a requirement for an attribute? This would be a problem for the already existing noduplicate. If a function has a call to a noduplicate function, the calling function could still be duplicated if the attribute isn’t propagated which isn’t allowed.

Others can weigh in here. This is just my understanding. Attribute propagation has to be optional because we can’t assume inter-procedural optimization runs for correct codegen. What if the memfence resides in a different module?

In the case of noduplicate, the only reason to propagate AFAICT would be to suppress inlining. It seems reasonable enough to expect attribute propagation to happen before inlining. So I don't think noduplicate is an issue in practice.

I think "memfence" could be an issue if we use the attribute to summarize LLVM atomic load/store and fence instructions (in addition to OpenCL barriers).

If the semantics you are proposing won't apply to general memory ordering constraints, then at least the name should be changed to specifically refer to OpenCL barriers.

-Andy

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