[LLVMdev] [RFC][PATCH][OPENCL] synchronization scopes redux

[Chandler Carruth]

On Tue, Jan 6, 2015 at 8:06 PM, Sahasrabuddhe, Sameer <
sameer.sahasrabuddhe at amd.com> wrote:

>
> On 1/7/2015 8:59 AM, Chandler Carruth wrote:
>
>
> Essentially, I think target-independent optimizations are still
> attractive, but we might want to just force them to go through an actual
> target-implemented API to interpret the scopes rather than making the
> interpretation work from first principles. I just worry that the targets
> are going to be too different and we may fail to accurately predict future
> targets' needs.
>
>
> If we have a target-implemented API, then just opaque numbers should also
> be sufficient, right? For the API, all we care about is queries that
> interesting optimizations will want answered from the target. This could be
> at the instruction level: "is it okay to remove this atomic store with
> scope n1 that is immediately followed by atomic store with scope n2?". Or
> it could be at the scope level: "does scope n2 include scope n1"?
>

I think it is significantly more friendly (and easier to debug mistakes) if
the textual IR uses human readable names. We already have a hard time due
to the totally opaque nature of address spaces -- there are magical address
spaces for segment stuff in x86.

The strings are only opaque to the target-independent optimizer. While
integers and strings are equally friendly to the code in the target,
strings are significantly more friendly to humans reading the IR.


The other advantage is that it makes it much harder to accidentally write
code that relies on the particular values for the integers. =]


>
>
>   I think the "strings" can be made relatively clean.
>
>  What I'm imagining is something very much like the target-specific
> attributes which are just strings and left to the target to interpret, but
> are cleanly factored so that the strings are wrapped up in a nice opaque
> attribute that is used as the sigil everywhere in the IR. We could do this
> with metadata, and technically this fits the model of metadata if we make
> the interpretation of the absence of metadata be "system". However, I'm
> quite hesitant to rely on metadata here as it hasn't always ended up
> working so well for us. ;]
>
>
> Metadata was the first thing to be considered internally at AMD. But it
> was quickly shot down because the Research guys were unwilling to accept
> the possibility of scope being lost and replaced by a default "system"
> scope. Current models are useful only when all atomic accesses for a given
> location use the same scope throughout the application, i.e., all threads
> running on all agents. So it is not okay for the compiler to "promote" the
> scope in just one kernel unless it has access to the entire application;
> the result is undefined. This is true for OpenCL source as well as HSAIL
> target. This may change in the near furture:
>
> HRF-Relaxed: Adapting HRF to the complexities of industrial heterogeneous
> memory models
> http://benedictgaster.org/?page_id=278
>
> But even then, it will be difficult to say if the same models can be
> applied to heterogeneous systems that don't resemble OpenCL or HSAIL.
>

Yea, I'm not really surprised by this.


>
>   I'd be interested in your thoughts and others' thoughts on how me might
> encode an opaque string-based scope effectively. If we can find a
> reasonably clean way of doing it, it seems like the best approach at this
> point:
>
>  - It ensures we have no bitcode stability problems.
> - It makes it easy to define a small number of IR-specified values like
> system/crossthread/allthreads/whatever and singlethread, and doing so isn't
> ever awkward due to any kind of baked-in ordering.
> - In practice in the real world, every target is probably going to just
> take this and map it to an enum that clearly spells out the rank for their
> target, so I suspect it won't actually increase the complexity of things
> much.
>
>
> I seem to be missing something here about the need for strings. If they
> are opaque anyway, and they are represented by sigils, then the sigils
> themselves are all that matter, right? Then the encoding is just a number...
>

See above for why I'd prefer not to use a raw number in the IR.


>
>
>
>
>> But while the topic is wide open, here's another possibly whacky
>> approach: we let the scopes be integers, and add a "scope layout" string
>> similar to data-layout. The string encodes the ordering of the integers. If
>> it is empty, then simple numerical comparisons are sufficient. Else the
>> string spells out the exact ordering to be used. Any known current target
>> will be happy with the first option. If some target inserts an intermediate
>> scope in the future, then that version switches from empty to a fully
>> specified string. The best part is that we don't even need to do this right
>> now, and only come up with a "scope layout" spec when we really hit the
>> problem for some future target.
>
>
> This isn't a bad approach, but it seems even more complex. I think I'd
> rather go with the fairly boring one where the IR just encodes enough data
> for the target to answer queries about the relationship between scopes.
>
>
> I am not really championing scope layout strings over a target-implemented
> API, but it seems less work to me rather than more. The relationship
> between scopes is just an SWO, and it can be represented as a graph. A
> practical target will have a very small number of scopes, say not more than
> 16. It should be possible to encode this into a graphviz-style string. Then
> instead of having every target implement an API, they just have to specify
> the relationship as a string.
>

I see where you're going here, and it sounds feasible, but it honestly
seems much *more* work and certainly more complex for the IR. We can always
add such a representation to communicate the relationships if it becomes
important, but I'd rather communicate via a boring target API to start with
I think.
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