[cfe-dev] [RFC][OpenCL] Allow users to add supported OpenCL extensions by pragma

[Bader, Alexey via cfe-dev]

Hi Brian,

> Alexey, this proposal does change the behavior of the compiler.  It informs clang that there is an OpenCL extension that can be enabled and disabled whose name was not known at the time clang was built.  After being registered, clang must process enable and disable pragmas for the new extension exactly as it does the built-in KHR extensions.

What I meant is that I don't quite understand how it's useful developers. Clang is supposed to check for errors and produce LLVM IR for the back-end.

The only value I see that this registering allows developers use "#pragma ... enable" and suppress clang's warning about unsupported extension. But since clang doesn't provide any additional error checking for the registered extension nor produce different LLVM IR, I don't see any reason to use "#pragma ... enable" in the first place. As an alternative solution we can introduce a compiler knob that will disable this diagnostics.

My understanding is that the only useful OpenCL extension supported by clang are 'cl_khr_fp16', 'cl_khr_fp64' and 'cl_khr_*_atomics', since they enable compilation of half/double/atomic types. The rest are useless for the front-end compiler. If developer enable 'cl_khr_gl_sharing' in the OpenCL program - it will have zero effect on compilation results.

I don't this it's required to have compiler directive for every OpenCL extension. It makes sense to have compiler directive only for extensions that modify OpenCL kernel language semantics.

Thanks,
Alexey

-----Original Message-----
From: Sumner, Brian [mailto:Brian.Sumner at amd.com] 
Sent: Monday, June 27, 2016 5:29 PM
To: C Bergström <cbergstrom at pathscale.com>; Bader, Alexey <alexey.bader at intel.com>
Cc: Anastasia Stulova <Anastasia.Stulova at arm.com>; Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>; cfe-dev (cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>; Pan, Xiuli <xiuli.pan at intel.com>; nd <nd at arm.com>; Jeroen Ketema <j.ketema at imperial.ac.uk>
Subject: RE: [cfe-dev] [RFC][OpenCL] Allow users to add supported OpenCL extensions by pragma

Christopher,  we can certainly modify clang, and have.  But then we have to carry those modifications around and try to get them into the clang code base.  Is it OK for clang to carry around every OpenCL vendor's vendor specific additions?  AMD has quite a few extensions, and so do Intel, Qualcomm, and others.   Wouldn't it be better for clang to provide a mechanism to allow extensions to be defined in a way that avoids clang having to carry around each and every one of those vendor extensions?

Alexey, this proposal does change the behavior of the compiler.  It informs clang that there is an OpenCL extension that can be enabled and disabled whose name was not known at the time clang was built.  After being registered, clang must process enable and disable pragmas for the new extension exactly as it does the built-in KHR extensions.

Brian

-----Original Message-----
From: C Bergström [mailto:cbergstrom at pathscale.com]
Sent: Monday, June 27, 2016 2:56 AM
To: Bader, Alexey
Cc: Anastasia Stulova; Sumner, Brian; Liu, Yaxun (Sam); cfe-dev (cfe-dev at lists.llvm.org); Pan, Xiuli; nd; Jeroen Ketema
Subject: Re: [cfe-dev] [RFC][OpenCL] Allow users to add supported OpenCL extensions by pragma

I don't care 2-ways about OpenCL, but adding non-standard pragma is a slippery slope. Please don't add extensions because some vendor is incapable of doing slight modifications to the source. If instead of a pragma you offered a build configuration, macros or something else to make this easier that I could totally understand.

On Mon, Jun 27, 2016 at 3:43 PM, Bader, Alexey via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> Hi,
>
>
>
> Sorry for being late to the discussion, but I didn’t get the use case 
> of the proposed feature.
>
> Why vendor/library developer would need compiler to support for some 
> pragma that doesn’t change to behavior of the compiler?
>
>
>
> Thanks,
>
> Alexey
>
>
>
> From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
> Sent: Tuesday, June 14, 2016 6:56 PM
> To: Sumner, Brian <Brian.Sumner at amd.com>; Liu, Yaxun (Sam) 
> <Yaxun.Liu at amd.com>; cfe-dev (cfe-dev at lists.llvm.org) 
> <cfe-dev at lists.llvm.org>; Bader, Alexey <alexey.bader at intel.com>; Pan, 
> Xiuli <xiuli.pan at intel.com>
> Cc: Jeroen Ketema <j.ketema at imperial.ac.uk>; nd <nd at arm.com>
> Subject: RE: [RFC][OpenCL] Allow users to add supported OpenCL 
> extensions by pragma
>
>
>
> Thanks, Brian!
>
>
>
> Yes, perhaps we could try to make the extension list fully dynamic. 
> Although I would imagine it’s still useful to have standard extensions 
> from the Spec directly in Clang to be available for the cases without 
> standard header include.
>
>
>
> Cheers,
>
> Anastasia
>
>
>
> From: Sumner, Brian [mailto:Brian.Sumner at amd.com]
> Sent: 14 June 2016 16:18
> To: Anastasia Stulova; Liu, Yaxun (Sam); cfe-dev 
> (cfe-dev at lists.llvm.org); Bader, Alexey (alexey.bader at intel.com); Pan, 
> Xiuli
> Cc: Jeroen Ketema; nd
> Subject: RE: [RFC][OpenCL] Allow users to add supported OpenCL 
> extensions by pragma
>
>
>
> Hi Anastasia,
>
>
>
> To answer your last question, this would be specific to clang.  It 
> will allow those of us using clang for OpenCL languages to implement 
> many of our vendor specific extensions without any other changes to 
> clang.  I expect a few of the current KHR extensions that do not 
> involve new or special types could be moved into opencl-c.h using this mechanism if desired as well.
>
>
>
> Thanks,
>
> Brian
>
>
>
> From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
> Sent: Monday, June 13, 2016 11:25 AM
> To: Liu, Yaxun (Sam); cfe-dev (cfe-dev at lists.llvm.org); Bader, Alexey 
> (alexey.bader at intel.com); Pan, Xiuli
> Cc: Sumner, Brian; Jeroen Ketema; nd
> Subject: RE: [RFC][OpenCL] Allow users to add supported OpenCL 
> extensions by pragma
>
>
>
> Interesting idea! Just to be clear, you are suggesting to add parsing 
> of the following into Clang:
>
>
>
> #pragma OPENCL EXTENSION the_new_extension_name : register
>
>
>
> Which would add custom the_new_extension_name to the list of known and 
> supported OpenCL extensions dynamically?
>
>
>
> Normally, target triple that combines RT and specific hardware type 
> would be used for those purposes.
>
> But I believe it shouldn’t be too complicated to add this pragma 
> considering that OpenCL already has similar ones.
>
>
>
> This approach could offer some degree of flexibility to the library 
> implementations,
>
> which would otherwise have to add separate triple representing their 
> library ABIs.
>
>
>
> Would this be allowed in any OpenCL code as some sort of Clang 
> extension or is there a plan to add this into Spec?
>
>
>
> Thanks,
>
> Anastasia
>
>
>
> From: Liu, Yaxun (Sam) [mailto:Yaxun.Liu at amd.com]
> Sent: 10 June 2016 19:40
> To: cfe-dev (cfe-dev at lists.llvm.org); Anastasia Stulova; Bader, Alexey 
> (alexey.bader at intel.com); Pan, Xiuli
> Cc: Sumner, Brian; Jeroen Ketema
> Subject: [RFC][OpenCL] Allow users to add supported OpenCL extensions 
> by pragma
>
>
>
> Currently Clang defines supported OpenCL extensions for each target 
> based on triple and CPU. As a default configuration this seems sufficient.
>
>
>
> However if vendors and library developers want to add support of their 
> own extensions, they have to modify Clang, which is very inconvenient.
>
>
>
> We need a flexible and expressive way to add supported extensions for 
> vendors and library developers.
>
>
>
> Brian has a proposal which introduces a pragma to add supported extensions:
>
>
>
> #pragma OPENCL EXTENSION the_new_extension_name : register
>
>
>
> This pragma tells clang the name of a new OpenCL extension and request 
> that it process it just like any other OpenCL extension, e.g.
> recognize
>
>
>
> #pragma OPENCL EXTENSION the_new_extension_name : enable/disable
>
>
>
> in subsequent code.
>
>
>
> Since this pragma can be used in the header file of vendors or library 
> developers’ OpenCL implementations, it can provide flexible and 
> expressive way to represent supported extensions when combined with 
> other preprocessor constructs.
>
>
>
> Any feedbacks? Thanks.
>
>
>
> Sam
>
>
>
>
>
>
>
>
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