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

Liu, Yaxun (Sam) via cfe-dev cfe-dev at lists.llvm.org
Wed Oct 17 09:31:28 PDT 2018


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


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 (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.


Yaxun (Sam) Liu

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