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

Andrew Savonichev via cfe-dev cfe-dev at lists.llvm.org
Fri Dec 7 09:37:01 PST 2018


Finkel, Hal J. writes:
> 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.
>
Sorry, GAS compile-time resolution (inference) is not a mandatory as I
believed, and OpenCL 2.0 compiler is allowed to fallback to dynamic
resolution (generate instructions which can work with any AS).

Though it is still beneficial to infer as much as possible at compile
time.

As Hal already mentioned, GAS is described here:
  https://www.khronos.org/registry/OpenCL/specs/2.2/html/OpenCL_C.html#the-generic-address-space

> (Some specific things that are unclear here:
> Where can GAS pointers be used?
>
Everywhere where a named pointer can be used. Only casts from GAS to a
named AS are disallowed.

> Can I put them in a struct? Can I make an array of them?
>
Yes.

> Are all array elements required to point to the same address space?
>
Specification does not explicitly forbid this, so the answer should be:
no, elements of an array can have different address space.

> Are they mutable? Can I assign pointers from multiple different
> address spaces to the same GAS pointer?
>
Yes.

> Must functions taking GAS pointers be defined in the same translation
> unit as the call?
>
No, they can be defined in different translation units.

> Can different GAS parameters resolve to different address spaces?
>
Yes.

> Can you take the address of a function taking GAS pointers?)
>
Luckily, function pointers are not allowed in OpenCL at all.

> 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.
>
Sorry, I was not clear about this. Specification is not clear about this
either, but from my understanding, this should work like a templates
when a GAS pointer is a function argument: if a function is called twice
with parameters of different AS, we have to duplicate the function.

>   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.
>
So you suggest to optimize address spaces in LLVM IR, and if the
optimization fails somewhere, then generate a generic code in a device
backend. Is this accurate?

This is probably the best approach, and from my understanding, this is
how it works for many OpenCL 2.0 implementations.

What I'm trying to figure out, is how to write an analysis that can
statically infer all address spaces in an "average code".

So it looks like I cannot modify Clang to make IR more friendly for this
analysis, and I need to run some optimizations to infer an address space
in more complicated cases.

> 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.

-- 
Andrew

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