[cfe-dev] [RFC] implementation of _Float16

Wed May 10 06:19:20 PDT 2017

Hi Hal, Martin,

Thanks for the feedback.
Yes, the issue indeed is that '__fp16' is already used to implement a storage-only type.  And earlier I wrote that I don't expect LLVM IR changes, but now I am not so sure anymore if both types map onto the same half LLVM IR type. With  two half precision types, __fp16 and _Float16, where one is a storage only type and the other a native type, somehow the distinction between these two must be made I think.

Cheers,
Sjoerd.

From: Martin J. O'Riordan [mailto:martin.oriordan at movidius.com]
Sent: 10 May 2017 14:13
To: 'Hal Finkel'; Sjoerd Meijer
Cc: 'clang developer list'
Subject: RE: [cfe-dev] [RFC] implementation of _Float16

Our Out-of-Tree target implements fully native FP16 operations based on '__fp16' (scalar and SIMD vector), so is the issue for ARM that '__fp16' is already used to implement a storage-only type and that another type is needed to differentiate between a native and a storage-only type?  Once the 'f16' type appears in the IR (and the vector variants) the code-generation is straightforward enough.

Certainly we have had to make many changes to CLang and to LLVM to fully implement this including suppression of implicit conversion to 'double', but nothing scary or obscure.  Many of these changes are simply to enable something that is already normal for OpenCL, but to do so for C and C++.

More controversially we also added a "synonym" for this using 'short float' rather than '_Float16' (or OpenCL's 'half'), and created a parallel set of the ISO C library functions using 's' to suffix the usual names (e.g. 'tan', 'tanf', 'tanl' plus 'tans').  The 's' suffix was unambiguous (though we actually use the double-underscore prefix, e.g. '__tans' to avoid conflict with the user's names) and the type 'short float' was available too without breaking anything.  Enabling the 'h' suffix for FP constants (again from OpenCL) makes the whole fit smoothly with the normal FP types.

However, for variadic functions (such as 'printf') we do promote to 'double' because there are no formatting specifiers available for 'half' any more than there is support for 'float' - it is also consistent with 'va_arg' usage for 'char' and 'short' as 'int'.  My feeling is that using implementation defined types 'float', 'double' and 'long double' can be extended to include 'short float' without dictating that they have any particular bit-sizes (e.g. FP16 for 'half').

This solution has worked very well over the past few years and is symmetric with the other floating-point data types.

There are some issues with C++ and overloading because '__fp16' to other FP types (and INT types) is not ranked in exactly the same way as for example 'float' is to other FP types; but this is really only because it is not a 1st class citizen of the type-system and the rules would need to be specified to make this valid.  I have not tried to fix this as it works reasonably well as it is, and it would really be an issue for the C++ committee to decide if they ever choose to adopt another FP data type.  I did add it to the traits in the C++ library though so that it is considered legal for floating-point types.

I'd love to see this adopted as a formal type in a future version of ISO C and ISO C++.

            MartinO

From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Hal Finkel via cfe-dev
Sent: 10 May 2017 11:39
To: Sjoerd Meijer <Sjoerd.Meijer at arm.com<mailto:Sjoerd.Meijer at arm.com>>; cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [RFC] implementation of _Float16

On 05/10/2017 05:18 AM, Sjoerd Meijer via cfe-dev wrote:
Hi,

ARMv8.2-A introduces as an optional extension half-precision data-processing instructions for Advanced SIMD and floating-point in both AArch64 and AArch32 states [1], and we are looking into implementing C/C++-language support for these new ARMv8.2-A half-precision instructions.

We would like to introduce a new Clang type. The reason is that we e.g. cannot use type __fp16 (defined in the ARM C Language Extensions [2]) because it is a storage type only. This means when using standard C operators values of __fp16 type promote to float when used in arithmetic operations, which we would like to avoid for the ARMv8.2-A half-precision instructions. Please note that the LLVM IR already has a half precision type, onto which for example __fp16 is mapped, so there are no changes or additions required for the LLVM IR.

As a new Clang type we would like to propose _Float16 as defined in a C11 extension, see [3]. Arithmetic is well defined, it is not only a storage type as __fp16. Our question is whether a partial implementation, just implementing this type and not claiming (full) C11 conformance is acceptable?

I would very much like to see fp16 as a first-class floating-point type in Clang and LLVM (i.e. handling that is not just a storage type). Doing this in Clang in a way that is specified by C11 seems like the right approach. I don't see why implementing this would be predicated on implementing other parts of C11.

 -Hal
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