[cfe-dev] [RFC] implementation of _Float16

Sjoerd Meijer via cfe-dev cfe-dev at lists.llvm.org
Wed May 10 03:18:56 PDT 2017


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?

[1] ARM ARM: https://static.docs.arm.com/ddi0487/b/DDI0487B_a_armv8_arm.pdf
[2] ARM C Language Extensions 2.1: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053d/IHI0053D_acle_2_1.pdf
[3] ISO/IEC TS 18661-3  interchange and extended types:  http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1945.pdf

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