sp -4

[riscv-non-isa]

sp -4

[lenary]

Commonly you need to set sp in the assembly code that runs before your C code runs. This is called the CRT, and ensures the right environment is presented to C programs.

This is code is conventionally provided by crt0.o and crtbegin.o (and some exit-related functionality is provided by crtend.o), in newlib's libc (used by not providing -nostartfiles) however these are likely wrong for your platform (unless you're using a toolchain specific to your MCU) and instead you will have to hand-write some assembly and a linker script to provide the same functionality.

[quantrpeter]

yes, you are right. I didn't init the stack but the cpu just keep running to next instruction. May be this is the characteristics of RISC-V