$ riscv64-unknown-elf-gcc -S riscv_fib.c

riscv_fib.c

unsigned int fib(unsigned int n)
{
    if (n < 2) return n;
    else return fib(n - 1) + fib(n - 2);
}

riscv_fib.s

	.file	"riscv_fib.c"
	.option nopic
	.text
	.align	2
	.globl	fib
	.type	fib, @function
fib:
	add	sp,sp,-48
	sd	ra,40(sp)
	sd	s0,32(sp)
	sd	s1,24(sp)
	add	s0,sp,48
	sw	a0,-36(s0)
	lw	a4,-36(s0)
	li	a5,1
	bgtu	a4,a5,.L2
	lw	a5,-36(s0)
	j	.L3
.L2:
	lw	a5,-36(s0)
	addw	a5,a5,-1
	mv	a0,a5
	call	fib
	mv	s1,a0
	lw	a5,-36(s0)
	addw	a5,a5,-2
	mv	a0,a5
	call	fib
	mv	a5,a0
	addw	a5,s1,a5
.L3:
	mv	a0,a5
	ld	ra,40(sp)
	ld	s0,32(sp)
	ld	s1,24(sp)
	add	sp,sp,48
	jr	ra
	.size	fib, .-fib
	.ident	"GCC: (GNU) 6.1.0"