[cfe-dev] OpenCL Generic Address Spaces - CodeGen (was Re: [llvm-dev] AliasAnalysis does not look though a memcpy)

[Finkel, Hal J. via cfe-dev]

On 12/6/18 1:36 PM, Richard Smith wrote:
Hi Andrew,

Can you please provide a reference to the relevant part of the OpenCL specification describing this feature? This sounds like an extremely surprising and problematic language design choice, and I'd like to make sure we're not misinterpreting the specification.

(Some specific things that are unclear here: Where can GAS pointers be used? Can I put them in a struct? Can I make an array of them? Are all array elements required to point to the same address space? Are they mutable? Can I assign pointers from multiple different address spaces to the same GAS pointer? Must functions taking GAS pointers be defined in the same translation unit as the call? Can different GAS parameters resolve to different address spaces? Can you take the address of a function taking GAS pointers?)

Andrew, please correct me if I'm wrong... It looks like the answer to all of Richard's questions is: yes . This doesn't look like template instantiation. I retract my recommendation in that regard.

  https://www.khronos.org/registry/OpenCL/specs/2.2/html/OpenCL_C.html#the-generic-address-space

which explicitly says that you get to do this:

kernel void bar(global int *g, local int *l)
{
    int *var;

    if (...)
        var = g;
    else
        var = l;
    *var = 42;
    ...
}

where the address space associated with a particular variable can be control-dependent. Also, it can change over time:

global int *gp;
local int *lp;
private int *pp;

int *p;
p = gp; // legal
p = lp; // legal
p = pp; // legal

If you can't represent these directly (e.g., your global address space is also your generic address space) then you might need a fat-pointer representation which you optimize, where possible, by propagating AS info where possible.

 -Hal

On Thu, 6 Dec 2018 at 11:07, Finkel, Hal J. via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
Hi, Andrew,

I'd like to fork this part of the thread and move it to cfe-dev. My best
advice is to handle this in Clang, not LLVM, and I've cc'd Richard and
John for their advice. More inline...

On 12/6/18 10:46 AM, Andrew Savonichev wrote:
> ...
>>>>>> Can you say more about the use case?
>>>>> OpenCL C has a notion of Generic Address Space (GAS), allowing you to
>>>>> cast a pointer from any (named) address space to a GAS pointer. Then you
>>>>> can use this GAS pointer instead of a named AS pointer. Compiler is
>>>>> responsible to infer the original address space of a GAS pointer when it
>>>>> is actually used (for load/store), otherwise this is a compilation
>>>>> error.
>>>> That seems scary :-) -- Can this inference not be done syntactically in
>>>> Clang?
>>>>
>>> From a frontend perspective, a GAS pointer is just a pointer:
>>>
>>>   void foo(int *p, int *q) { *p = 42; *q = 43; };
>>>
>>> Until Clang reaches a call site, it has no idea about real address
>>> spaces of `p' and `q'. When we do reach a call site, `foo()' can already
>>> be CodeGen'ed, so we can't really change anything.
>> Is this supposed to work like template instantiation? Are you guaranteed
>> to only get one (unique) set of address spaces for the function
>> arguments?
>>
> Yes, just like in C++ template, if `foo' is called with different sets
> of address spaces, a compiler have to create different function
> instantiations for each set.

I think that you should handle this in Clang using TreeTransform, in a
sense, just like C++ template instantiation. See
lib/Sema/TreeTransform.h, and there are a number of examples in lib/Sema
of transforms using this infrastructure. Using TreeTransform you would
create variants of each function with the right address spaces, based on
usage, and then emit them all during CodeGen. because you'd do this
prior to code generation, you don't need to worry about the emission
ordering.

 -Hal

>
>> We can change the order that functions are emitted in Clang if necessary.
>>
> I haven't thought this is actually configurable. I'd really appreciate
> if you can give me a pointer on how to do this.
>
>
>
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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