23 “Zfinx”, “Zdinx”, “Zhinx”, “Zhinxmin”: Standard Extensions for Floating-Point in Integer Registers, Version 1.0.0-rc
This chapter is in the Frozen state. Change is extremely unlikely. A high threshold will be used, and a change will only occur because of some truly critical issue being identified during the public review cycle. Any other desired or needed changes can be the subject of a follow-on new extension. For more info see: http://riscv.org/spec-state.
This chapter defines the “Zfinx” extension (pronounced “z-f-in-x”)
that provides instructions similar to those in the standard
floating-point F extension for single-precision floating-point
instructions but which operate on the x registers instead of the
f registers. This chapter also defines the “Zdinx”,
“Zhinx”, and “Zhinxmin” extensions that provide similar
instructions for other floating-point precisions.
The Zfinx extension adds all of the instructions that the F extension adds, except for the transfer instructions FLW, FSW, FMV.W.X, FMV.X.W, C.FLW[SP], and C.FSW[SP].
Zfinx software uses integer loads and stores to transfer floating-point values from and to memory. Transfers between registers use either integer arithmetic or floating-point sign-injection instructions.
The Zfinx variants of these F-extension instructions have the same semantics,
except that whenever such an instruction would have accessed an f
register, it instead accesses the x register with the same number.
23.1 Processing of Narrower Values
Floating-point operands of width w < XLEN bits occupy bits
w-1:0 of an x register.
Floating-point operations on w-bit operands ignore operand bits
XLEN-1:w .
Floating-point operations that produce w < XLEN-bit results fill bits XLEN-1:w with copies of bit w-1 (the sign bit).
The NaN-boxing scheme employed in the f registers was designed to
efficiently support recoded floating-point formats.
Recoding is less practical for Zfinx, though, since the same registers
hold both floating-point and integer operands.
Hence, the need for NaN boxing is diminished.
Sign-extending 32-bit floating-point numbers when held in RV64 x
registers matches the existing RV64 calling conventions, which require all
32-bit types to be sign-extended when passed or returned in x registers.
To keep the architecture more regular, we extend this pattern to 16-bit
floating-point numbers in both RV32 and RV64.
23.2 Zdinx
The Zdinx extension provides analogous double-precision floating-point instructions. The Zdinx extension requires the Zfinx extension.
The Zdinx extension adds all of the instructions that the D extension adds, except for the transfer instructions FLD, FSD, FMV.D.X, FMV.X.D, C.FLD[SP], and C.FSD[SP].
The Zdinx variants of these D-extension instructions have the same semantics,
except that whenever such an instruction would have accessed an f
register, it instead accesses the x register with the same number.
23.3 Processing of Wider Values
Double-precision operands in RV32Zdinx
are held in aligned x-register pairs, i.e.,
register numbers must be even.
Use of misaligned (odd-numbered) registers for double-width floating-point
operands is reserved.
Regardless of endianness, the lower-numbered register holds the low-order
bits, and the higher-numbered register holds the high-order bits: e.g., bits
31:0 of a double-precision operand in RV32Zdinx might be held in register
x14, with bits 63:32 of that operand held in x15.
When a double-width floating-point result is written to x0, the entire
write takes no effect: e.g., for RV32Zdinx, writing a double-precision result
to x0 does not cause x1 to be written.
When x0 is used as a double-width floating-point operand, the entire
operand is zero—i.e., x1 is not accessed.
Load-pair and store-pair instructions are not provided, so transferring double-precision operands in RV32Zdinx from or to memory requires two loads or stores. Register moves need only a single FSGNJ.D instruction, however.
23.4 Zhinx
The Zhinx extension provides analogous half-precision floating-point instructions. The Zhinx extension requires the Zfinx extension.
The Zhinx extension adds all of the instructions that the Zfh extension adds, except for the transfer instructions FLH, FSH, FMV.H.X, and FMV.X.H.
The Zhinx variants of these Zfh-extension instructions have the same semantics,
except that whenever such an instruction would have accessed an f
register, it instead accesses the x register with the same number.
23.5 Zhinxmin
The Zhinxmin extension provides minimal support for 16-bit half-precision
floating-point instructions that operate on the x registers.
The Zhinxmin extension requires the Zfinx extension.
The Zhinxmin extension includes the following instructions from the Zhinx extension: FCVT.S.H and FCVT.H.S. If the Zdinx extension is present, the FCVT.D.H and FCVT.H.D instructions are also included.
In the future, an RV64Zqinx quad-precision extension could be defined analogously to RV32Zdinx. An RV32Zqinx extension could also be defined but would require quad-register groups.
23.6 Privileged Architecture Implications
In the standard privileged architecture defined in Volume II, the
mstatus field FS is hardwired to 0 if the Zfinx extension is
implemented, and FS no longer affects the trapping behavior of
floating-point instructions or fcsr accesses.
The misa bits F, D, and Q are hardwired to 0 when the Zfinx
extension is implemented.
A future discoverability mechanism might be used to probe the existence of the Zfinx, Zhinx, and Zdinx extensions.
The F extension uses separate
fregisters for floating-point computation, to reduce register pressure and simplify the provision of register-file ports for wide superscalars. However, the additional 128 B of architectural state increases the minimal implementation cost. By eliminating thefregisters, the Zfinx extension substantially reduces the cost of simple RISC-V implementations with floating-point instruction-set support. Zfinx also reduces context-switch cost.In general, software that assumes the presence of the F extension is incompatible with software that assumes the presence of the Zfinx extension, and vice versa.