Support of half precision floating-point numbers in RISC-V ISA: Zfh and Zfhmin

Floating-point support in RISC-V ISA

RISC-V does not mandate the support of any floating-point operations as part of the base ISA (RV32I and RV64I) but single precision and double-precision extensions (e.g. RV64F and RV64D) were among the first to be specified. The F extension added 32 floating-point registers, floating-point arithmetic operations, moves and conversions instructions. The D extension (which requires the F extension) extended this set of operation to also support double precision. There also exists a Q extension for quad-precision. 

In reality, there are multiple F and D extensions: RV32F (to extend the 32-bit base ISA) and RV64F (for the 64-bit base ISA), similarly there are RV32D and RV64D. RV32D has the particularity of adding 64-bit wide floating-point registers when the associated general purpose registers are only 32-bit wide. This flexibility of RISC-V ISA is reviewed in the post RISC-V Register Files.

Initially, smaller floaitng-point formats, such as half precision, were not supported. 

Half precision used to be lagging behind in term of ISA and hardware support. It was only specified has a storage format in the 2008 revision of the IEEE-754 standard (the IEEE standard specifying floating-point formats and operations). The momentum of deep learning and convolutional neural networks has kick-started a renewed interest for small number formats and in particular half precision (among many others). 

Note: In the IEEE-754 2019 revision, half precision is still not defined has a basic format but as an interchange format. Although the standard is more permissive in terms of which formats can admit arithmetic operations.

RISC-V International (the association behind the RISC-V specification(s)) recently ratified two extensions specifying sets of instructions for half-precision support: Zfh and Zfhmin. The later being defined as a subset of the former, we will review Zfh first.

Zfh: full half-precision support

Zfh extends floating-point registers (FRF) to support half precision. It requires the F extension, thus no new register file, nor any register width extension is required since the floating-point registers are already wide enough to contain single precision values.

As other single-precision values in RV32D/RV64D, half precision values are stored in larger FLEN registers using RISC-V NaN boxing scheme (more on that in a future post). 

This new extension defines:

• Instruction to move data (with or without conversions):
• flh: load from memory into an F register
• fsh: store to memory from an F register
• fmv.h.x, fmv.x.h: bit pattern move between X and F register files (unmodified)
• fcvt.h.(w/l)[u], fcvt.(w/l)[u].h: conversions  between X and F register files (from/to integer)
• Arithmetic operations: fadd.h, fsub.h, fmul.h, fsqrt.h, fdiv.h, fmin.h, fmax.h, f(n)madd/f(n)msub.h
• Floating-point comparisons: fcmp.h
• Conversions between half precision and other floating-point formats
• Miscellaneous: fclass.h, fsgn(/n/x).h

All arithmetic operations operate on uniform I/Os: all operands are half precision values, and the output is an half precision result. They share their opcode with the corresponding F/D instructions: the fmt fields (bits 25 and 26) value 2b'10 (2) encodes half precision (value 2b'00 for single precision, 2b'01 for double, and 2b'11 for quad but who need such a precision !).

The following diagram represents the data move instructions:

The availability of move to/from and conversion with 64-bit operands is conditioned on the availability of RV64 (general purpose move and integer conversions) and D extension (floating-point conversions).

Although certainly anecdotal, the availability of the Q extension (floating-point quad precision, a.k.a 128-bit format): instructions for conversions from/to quad precision are defined: fcvt.h.q and fcvt.q.h.

Zfhmin: reduced half-precision support

The extension Zfhmin can be seen as a subset of Zfh. It only mandates support for half-precision load and store, bit pattern moves from/to integer register file (no conversions with integer formats) and conversions with other floating-point formats. It represents a total of 6 instructions (extended to 8 with conversion from/to double precision format if the D extension is supported).

    Zfhmin constitutes a reduced set of instruction which can be used by platforms where computing with half-precision values directly is not required but which still require the capability to manipulate them, in particular in memory, before converting them to a larger format for an eventual computation.

Vector support for half precision: Zvfh and Zvfhmin

The RISC-V Vector extension (RVV) version 1.0 specified vector support for single and double precision (SEW=32 and 64 bits). A draft of the specification (link to source) introduces Zvfh which is an extension of all the floating-point instruction to half-precision , including conversions with other formats, and Zvfhmin which is a really reduced subset of operations. As is the case for other floating-point format, half precision is supported by a specific value of the vsew field of the vtype configuration register: vsew=1. This encoding corresponding to a Selected Element Width (SEW) of 16 bits and is identical for both integer and floating point formats.

Zvfhmin only mandates the support of conversion between half and single precision: it extends the support of vfwcvt.f.f.v (widening half-to-float) and vfncvt.f.f.w (narrowing float-to-half) to SEW=16-bit.

Zvfh extends to half-precision all the floating-point vector operations, floating-point reductions, floating-point moves., a brief description of those operations can be found on Part 2 of our series RISC-V Vector Extension in a Nutshell.

Both Zvfhmin and Zvfh mandates the support of single precision element in vectors. On top of this support Zvfh mandates at least Zfhmin on the scalar floating-point side.

Half precision in RVA22 profile

The RISC-V consortium defines profiles. These profiles aim at defining a common set of mandatory extensions and a reduced set of optional extensions which can be used by hardware and software providers to build a compatible ecosystem without having to deal with more specialized ISA extensions. Profile descriptions can be found on RISC-V github.

RVA22 is the most recent profile, it is dedicated for 64-bit application processors

Zfhmin is part of the mandatory extensions of the RVA22 profile, while Zfh is an optional extension (which supersede Zfhmin when selected). This means that all application processors targeting compatibility with the RISC-V ecosystem must have a minimal support for half precision, and than extended support is part of the extended profile. Neither of the vector extensions Zvfh nor Zvfhmin are required in the RVA22 profile.

Conclusion

RISC-V Half-precision support in scalar operations has already been ratified (as extensions Zfh and Zfhmin) and his part of the latest application profile (RVA22). There exist draft specifications for the support of half-precision in vector operations: Zvfh and Zvfhmin. Other formats, such as BFloat16 or more esoteric number formats, should follow in the coming years.

RISC-V is an open community so do not hesitate to sign-in to stay up-to-date and participate to the effort: http://riscv.org.

Reference:

• RISC-V specification: "Zfh" and "Zfhmin" Standard Extensions for Half-Precision Floating-Point 
• RISC-V RVA22 profiles https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc#6-rva22-profiles
• RISC-V vector draft specification for Zvfh section
• RISC-V vector draft specification for Zvfhmin section