4.0 KiB
NEORV32 Zfinx Floating-Point Extension
The NEORV32 floating-point unit (FPU) implements the Zfinx RISC-V extension. The extension can be
enabled via the CPU_EXTENSION_RISCV_Zfinx top configuration generic.
The RISC-V Zfinx single-precision floating-point extensions uses the integer register file x instead
of a dedicated floating-point f register file (which is defined by the RISC-V F single-precision
floating-point extension). Hence, the standard data transfer instructions from the F extension are
not available in Zfinx:
- floating-point load/store operations (
FLW,FSW) and their compressed versions - integer register file
x<-> floating point register filefmove operations (FMV.W.X,FMV.X.W)
ℹ️ See the according section of the NEORV32 data sheet for more information.
Intrinsic Library
The NEORV32 Zfinx floating-point extension can still be used using the provided intrinsic library. This
library uses "custom" inline assembly instructions wrapped within normal C-language functions. Each original
instruction of the extension can be utilized using an according intrinsic function.
For example, the floating-point addition instruction FADD.S can be invoked using the according intrinsic function:
float riscv_intrinsic_fadds(float rs1, float rs2)
The pure-software emulation instruction, which uses the standard built-in functions to execute all
floating-point operations, is available via wrapper function. The emulation function for the FADD.S instruction is:
float riscv_emulate_fadds(float rs1, float rs2)
The emulation functions as well as the available intrinsics for the Zfinx extension are located in
neorv32_zfinx_extension_intrinsics.h. The provided test program main.c verifies all currently implemented
Zfinx instructions by checking the functionality against the pure software-based emulation model (GCC soft-float library).
Exemplary Test Output
<<< Zfinx extension test >>>
SILENT_MODE enabled (only showing actual errors)
Test cases per instruction: 1000000
NOTE: The NEORV32 FPU does not support subnormal numbers yet. Subnormal numbers are flushed to zero.
#0: FCVT.S.WU (unsigned integer to float)...
Errors: 0/1000000 [ok]
#1: FCVT.S.W (signed integer to float)...
Errors: 0/1000000 [ok]
#2: FCVT.WU.S (float to unsigned integer)...
Errors: 0/1000000 [ok]
#3: FCVT.W.S (float to signed integer)...
Errors: 0/1000000 [ok]
#4: FADD.S (addition)...
Errors: 0/1000000 [ok]
#5: FSUB.S (subtraction)...
Errors: 0/1000000 [ok]
#6: FMUL.S (multiplication)...
Errors: 0/1000000 [ok]
#7: FMIN.S (select minimum)...
Errors: 0/1000000 [ok]
#8: FMAX.S (select maximum)...
Errors: 0/1000000 [ok]
#9: FEQ.S (compare if equal)...
Errors: 0/1000000 [ok]
#10: FLT.S (compare if less-than)...
Errors: 0/1000000 [ok]
#11: FLE.S (compare if less-than-or-equal)...
Errors: 0/1000000 [ok]
#12: FSGNJ.S (sign-injection)...
Errors: 0/1000000 [ok]
#13: FSGNJN.S (sign-injection NOT)...
Errors: 0/1000000 [ok]
#14: FSGNJX.S (sign-injection XOR)...
Errors: 0/1000000 [ok]
#15: FCLASS.S (classify)...
Errors: 0/1000000 [ok]
# unsupported FDIV.S (division) [illegal instruction]...
<RTE> Illegal instruction @ PC=0x000006A8, INST=0x18A484D3 </RTE>
[ok]
# unsupported FSQRT.S (square root) [illegal instruction]...
<RTE> Illegal instruction @ PC=0x000006E0, INST=0x580484D3 </RTE>
[ok]
# unsupported FMADD.S (fused multiply-add) [illegal instruction]...
<RTE> Illegal instruction @ PC=0x0000071E, INST=0x1EA484C3 </RTE>
[ok]
# unsupported FMSUB.S (fused multiply-sub) [illegal instruction]...
<RTE> Illegal instruction @ PC=0x0000075C, INST=0x1EA484C7 </RTE>
[ok]
# unsupported FNMSUB.S (fused negated multiply-sub) [illegal instruction]...
<RTE> Illegal instruction @ PC=0x0000079A, INST=0x1EA484CF </RTE>
[ok]
# unsupported FNMADD.S (fused negated multiply-add) [illegal instruction]...
<RTE> Illegal instruction @ PC=0x000007D8, INST=0x1EA484CF </RTE>
[ok]
[Zfinx extension verification successful!]