[cfe-dev] [OpenCL] Representing OpenCL half type by _Float16 type

Thu Oct 25 07:41:39 PDT 2018

Hi Brian,

Generally, all builtin types are configurable in Clang based on the target (incl int or float). We have a way to enforce the language specific settings using TargetInfo::adjust. We currently already force the half width to 2 bytes there.

However, subclass targets can still override this logic. Not something we recommend to do though.

Anastasia

________________________________
From: Sumner, Brian <Brian.Sumner at amd.com>
Sent: 23 October 2018 00:25
To: Anastasia Stulova; Liu, Yaxun (Sam); Bader, Alexey; cfe-dev (cfe-dev at lists.llvm.org)
Cc: nd
Subject: RE: [OpenCL] Representing OpenCL half type by _Float16 type

Hi,

The doc Sam referenced seems to be implying that sizeof(_Float16) is allowed to be 4 if the target doesn’t support “native” half precision operations.  Is that correct?  That would be a showstopper for OpenCL where sizeof(half) must be 2.

Brian

From: Anastasia Stulova <Anastasia.Stulova at arm.com>
Sent: Monday, October 22, 2018 3:42 PM
To: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>; Bader, Alexey <alexey.bader at intel.com>; Sumner, Brian <Brian.Sumner at amd.com>; cfe-dev (cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>
Cc: nd <nd at arm.com>
Subject: Re: [OpenCL] Representing OpenCL half type by _Float16 type

Hi Sam,

I would disagree regarding the requirements on the half implementation. Section 6.1.1 explicitly says:

"The half data type must conform to the IEEE 754-2008 half precision storage format."

Do you suggest to change the spec to refer to ISO standard instead of IEEE for half type? I am a bit concerned about backwards compatibility though in case current implementations are relying on the higher precision arithmetic. Do you know the implications on the precision and ulp bounds?

Cheers,

Anastasia

________________________________

From: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>
Sent: 22 October 2018 15:35
To: Bader, Alexey; Anastasia Stulova; Sumner, Brian; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>)
Subject: RE: [OpenCL] Representing OpenCL half type by _Float16 type

Right.

Sam

From: Bader, Alexey <alexey.bader at intel.com<mailto:alexey.bader at intel.com>>
Sent: Monday, October 22, 2018 4:53 AM
To: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>; 'anastasia.stulova at arm.com' <anastasia.stulova at arm.com<mailto:anastasia.stulova at arm.com>>; Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>) <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
Subject: RE: [OpenCL] Representing OpenCL half type by _Float16 type

The proposal sounds good to me.

I assume that by changing OpenCL half data type representation from __fp16 to _Float16, we align it with cl_khr_fp16 extension. There should be no functional changes for programs not using this extension. Right?

From: Liu, Yaxun (Sam) [mailto:Yaxun.Liu at amd.com]
Sent: Friday, October 19, 2018 6:30 PM
To: Bader, Alexey <alexey.bader at intel.com<mailto:alexey.bader at intel.com>>; 'anastasia.stulova at arm.com' <anastasia.stulova at arm.com<mailto:anastasia.stulova at arm.com>>; Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>) <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
Subject: RE: [OpenCL] Representing OpenCL half type by _Float16 type

Hi Alexey,

We have semantic checks to enforce the restrictions on half type when cl_khr_fp16 is not enabled. We could keep these checks if we switch to _Float16.

Sam

From: Bader, Alexey <alexey.bader at intel.com<mailto:alexey.bader at intel.com>>
Sent: Friday, October 19, 2018 9:16 AM
To: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>; 'anastasia.stulova at arm.com' <anastasia.stulova at arm.com<mailto:anastasia.stulova at arm.com>>; Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>) <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
Subject: RE: [OpenCL] Representing OpenCL half type by _Float16 type

Hi Sam,

I assume OpenCL half type mapped to __fp16 as OpenCL specification defines it as “A 16-bit floating-point conforming to the IEEE 754-2008 half precision storage format”. This aligned with __fp16 description from clang documentation.

OpenCL cl_khr_fp16 extension enables arithmetic operations on ‘half’ data types and I think it’s reasonable to map ‘half’ data type to _Float16 if cl_khr_fp16 is enabled.

Is it possible to map ‘half’ data type to _Float16 and keep the restrictions of OpenCL core specification?

Thanks,

Alexey

From: Liu, Yaxun (Sam) [mailto:Yaxun.Liu at amd.com]
Sent: Wednesday, October 17, 2018 7:31 PM
To: 'anastasia.stulova at arm.com' <anastasia.stulova at arm.com<mailto:anastasia.stulova at arm.com>>; Bader, Alexey <alexey.bader at intel.com<mailto:alexey.bader at intel.com>>; Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>) <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
Subject: [OpenCL] Representing OpenCL half type by _Float16 type

Hi,

Clang supports two 16 bit floating point builtin types: __fp16 and _Float16. Their differences are here

https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point

Currently clang supports OpenCL half type as __fp16, one significant disadvantage of which is that arithmetic operations are emitted as fp32 instead of fp16 instructions in LLVM IR. Another disadvantage is interoperability with C++ programs, which uses _Float16 mostly since it is an ISO standard whereas __fp16 is not.

OpenCL spec v 2.0 s 6.1.1.1 (https://www.khronos.org/registry/OpenCL/specs/opencl-2.0-openclc.pdf ) defines half type. There is no requirement that it has to be __fp16 or _Float16 type in clang. Actually, it seems both __fp16 and _Float16 can be used to represent OpenCL half type.

Considering the advantages of _Float16 type, I am wondering whether we should consider using _Float16 to represent OpenCL half type.

Any comments are welcome.

Thanks.

Yaxun (Sam) Liu

AMD

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