[RFC] riscv32 compiler flags

[Heinrich Schuchardt]

Hello Rick, hello Leo,

in origin master we have moved to compiling riscv32 using the GCC the 
32bit toolchain provided by kernel.org. This works fine with GCC-11.

Tom now tried to move to GCC-12. Here libgcc.a is compiled with:

readelf -h 
/opt/gcc-12.2.0-nolibc/riscv32-linux/lib/gcc/riscv32-linux/12.2.0/libgcc.a
Flags: 0x5, RVC, double-float ABI

while up to now our code is compiled with

readelf -h arch/riscv/cpu/start.o
Flags: 0x1, RVC, soft-float ABI

This leads to a linking error:

/opt/gcc-12.2.0-nolibc/riscv32-linux/bin/riscv32-linux-ld.bfd: 
/opt/gcc-12.2.0-nolibc/riscv32-linux/bin/../lib/gcc/riscv32-linux/12.2.0/libgcc.a(_lshrdi3.o): 
can't link double-float modules with soft-float module

This can be reproduced with

sudo docker pull trini/u-boot-gitlab-ci-runner:jammy-20221003-08Oct2022
sudo docker run -ti 
trini/u-boot-gitlab-ci-runner:jammy-20221003-08Oct2022 /bin/bash

CROSS_COMPILE=/opt/gcc-12.2.0-nolibc/riscv32-linux/bin/riscv32-linux-
cd ~
git clone https://source.denx.de/u-boot/u-boot.git
cd u-boot/
git checkout origin/WIP/update-to-llvm-14-gcc-12.2

Building U-Boot with

     -march=rv32imafdc -mabi=ilp32d

instead of

     -march=rv32iac -mabi=ilp32

results in binaries that can be linked to libgcc.a.

Neither U-Boot nor the linked libgcc code actually uses floating-point 
numbers. So the generated code probably is just the same.

We already have CONFIG_RISCV_ISA_C and CONFIG_RISCV_ISA_A to tune the 
build flags for U-Boot.

I will try to create a patch that adds configuration settings for the F 
and D extension.

Best regards

Heinrich