[LLVMdev] [cfe-dev] OpenCL support

[Villmow, Micah]

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Peter Collingbourne
> Sent: Monday, December 06, 2010 2:56 PM
> To: David Neto
> Cc: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] [cfe-dev] OpenCL support
> 
> Hi David,
> 
> On Mon, Dec 06, 2010 at 11:14:42AM -0500, David Neto wrote:
> > >> It
> > >> seems it would be a good idea to transform the code so that uses
> of x
> > >> become loads and stores from memory, and the address for that
> memory
> > >> is returned by a builtin function that itself is dependent on work
> > >> group ids.
> > >>
> > >> I'm just learning Clang now, so I'm not prepared to say how that
> would
> > >> be done.  Is it okay to transform the AST before semantic
> analysis?
> > >> Where should I start looking?  (I would guess lib/Sema...)
> > >
> > > This transformation may be useful for a CPU based OpenCL
> > > implementation, but would not be appropriate in Sema for a few
> > > reasons.  The first is that the AST should at all times be an
> accurate
> > > representation of the input source code.
> > >
> > > The second is that such a transformation would be specific to the
> > > OpenCL implementation -- not only would it be inappropriate for
> > > GPUs but there are a number of feasible CPU based implementation
> > > techniques which we shouldn't have to teach Sema or in fact any
> part
> > > of Clang about.
> > >
> > > The best place to do this transformation would be at the LLVM level
> > > with an implementation specific transformation pass.
> >
> > Ok.  Now I'm even more convinced that your patch [1] is incorrect
> because:
> > (a) it's specific to GPU-style implementations of OpenCL, not the
> > generic semantics of OpenCL.
> > (b) it pushes target-specific assumptions into Sema.  But you've just
> > argued that the AST should reflect the original source code as much
> as
> > possible.
> 
> Yes, that's why I don't like the patch so much :)  It was really
> designed to work with the current infrastructure, which isn't
> very well suited to more exotic languages like OpenCL.
> 
> > On (a):   I understand that ARM is preparing to contribute a more
> > complete OpenCL front-end to Clang.   It would be great to nail down
> a
> > common front end with generic OpenCL semantics, and let later stages
> > (Clang's CodeGen? LLVM IR pass?) handle more target-specific
> > assumptions.  E.g. it would be nice to standardize on how Clang
> > handles OpenCL's local, global, etc. etc. etc.  E.g. just agreeing on
> > address space numbering would be a step forward.  (e.g. global is 1,
> > local is 2...)
> 
> +llvmdev, as this is also a LLVM-relevant issue.
> 
> I agree.  We should set a standard for address spaces in LLVM - a low
> range for 'standard' address spaces (with a defined semantics for each
> value in that range) and a high range for target-specific spaces.
> It looks like address spaces are already being used this way to a
> certain extent in the targets (X86 uses 256 -> GS, 257 -> FS).  And
> I think 256 'standard' address spaces should be enough, but I'm happy
> to be proven wrong :)
> 
[Villmow, Micah] This would be very beneficial to define these. The only main issue is the
default address space. In OpenCL it is private, in LLVM, it is closer to global than private.


> > What do I think your patch should look like?  It's true that the
> > diag::err_as_qualified_auto_decl is inappropriate for OpenCL when
> it's
> > the __local addres space.
> >
> > But we need to implement the semantics somehow.  Conceptually I think
> > of it as a CL source-to-source transformation that lowers
> > function-scope-local-address-space variables into a more primitive
> > form.
> >
> > I think I disagree that the Clang is an inappropriate spot for
> > implementing this type of transform: Clang "knows" the source
> language
> > semantics, and has a lot of machinery required for the transform.
> > Also, Clang also knows a lot about the target machine (e.g. type
> > sizes, builtins, more?).
> >
> > So I believe the "auto var in different address space" case should be
> > allowed in the AST in the OpenCL case, and the local-lowering
> > transform should be applied in CodeGen.  Perhaps the lowering is
> > target-specific, e.g. GPU-style, or more generic style as I proposed.
> >
> > Thoughts?
> 
> I've been rethinking this and perhaps coming around to this way
> of thinking.  Allocating variables in the __local address space
> is really something that can't be represented at the LLVM level,
> at least in a standard form.
[Villmow, Micah] We ran across this problem in our OpenCL implementation. However, you can create a global variable with an '__local' address space and it works fine. There is an issue with collision between auto-arrays in different kernels, but that can be solved with a little name mangling. There are other ways to do this, for example, by converting local auto-arrays into kernel local pointer arguments with a known size.


> 
> But to a certain extent both auto and static storage-classes are wrong
> here.  Auto implies that each invocation of the function gets its own
> variable, while static implies that all invocations share a variable.
> 
> Perhaps the right thing to do here is to introduce a new storage-class
> for __local variables (let's call it the 'wg-local' storage-class).
> A variable cannot be made wg-local with a storage-class specifier but
> function-scope-local-address-space variables would be made so in a
> similar way to my original patch.  The target would then be required
> to define at CodeGen the semantics of declaring wg-local variables and
> loading and storing from local address space in the way you propose.
[Villmow, Micah] I'd move away from adding a new storage class as using the address space alone is sufficient to handle OpenCL's __local address space.


> 
> A side effect of this is that we will require a mapping of
> target-unsupported address spaces to supported address spaces in
> CodeGen.  For example, a CPU based implementation should map the
> local address space to 0.
> 
> Thanks,
> --
> Peter
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