RVfplib - Optimized RISC-V FP emulation for 32-bit processors

RVfplib is an optimized RISC-V library for FP arithmetic on 32-bit integer processors, for single and double-precision FP. RVfplib is IEEE 754 compliant, with the following exceptions:

It does not support exception flags.
It does not support signaling NaNs, all the produced NaNs are quiet in the form of 0x7FC00000 and 0x7FF8000000000000.
The only provided rounding mode is RNE.

RVfplib is available in 4 versions:

RVfplib, optimized for performance. It is compatible with RV32IM[C] processors.
RVfplib_s, optimized for low code-size. It is compatible with RV32IM[C] processors.
RVfplib_nd, optimized for performance, with no support for subnormal values (they are considered correctly signed zeroes). It is compatible with RV32EM[C] processors.
RVfplib_nd_s, optimized for low code-size, with no support for subnormal values (they are considered correctly signed zeroes). It is compatible with RV32EM[C] processors.

The optimizations for performance are input-dependent, and in certain contextes can lead to slower code. If in doubt, use the the code optimized for size.

The functions are aliased with those automatically linked from libgcc, e.g. the single-precision addition is called __addsf3.

Dependencies

To build the library, a RISC-V RV32IM[C] toolchain is needed. Make sure that riscv32-unknown-elf-gcc is available in the used shell. To do so, add to the PATH variable the path to the bin directory of your riscv32-unknown-elf toolchain. If this directory is called ${RISCV_TOOLS_BIN_DIR}, then execute the following command in the shell that will be used to build the library.

export PATH="${RISCV_TOOLS_BIN_DIR}:$PATH"

For custom needs, modify the Makefile.

The library itself has no external dependencies.

Build the library

In the following, the directory in which this file is stored will be identified as $(BASE_DIR)

To build the library, open a terminal in this directory and execute:

make

This command will build the four libraries in $(BASE_DIR)/build/lib/.

Use the library

The following steps work with a RISC-V GCC toolchain.

To use the library, add:

-L $(BASE_DIR)/build/lib -nolibc -lc

to the linker flags, and:

-lrvfp

to the linker flags, but only after the source files and the other external needed libraries.

For example, to compile a main.c program with RVfplib, use:

riscv32-unknown-elf-gcc -march=rv32imc main.c -L $(BASE_DIR)/build/lib -nolibc -lc -lm -lrvfp

Use another RVfplib version

To link one of the other three RVfplib versions, just replace

-lrvfp

with one of the following:

-lrvfp_s -lrvfp_nd -lrvfp_nd_s

Source

The source code of the functions is kept in src/asm/, whereas src/c/ contains test-related C code.

Test

It's possible to select which function to test commenting/un-commenting the related line in the file script/topScript_test.sh.

Then, from $(BASE_DIR), launch:

./script/topScript_test.sh size - To test the size-optimized library

./script/topScript_test.sh performance - To test the performance-optimized library

The test depends upon riscv32-unknown-elf- tools, TestFloat programs, SPIKE, PK, and Python3. The TestFloat programs and Python3 should be accessible from the shell. For the others, make sure to define and export the following variables before calling the script:

GCC_TOOLS_BIN_PATH : the bin path to your GCC tools
SPIKE_PATH : the path to your SPIKE program
PK_PATH : the path to your Proxy Kernel program

Acknowledgements

Part of this library is inspired by the Arm FP support provided within libgcc:

pulp-platform/RVfplib