# Lab2: RISC-V RV32I[MA] emulator with ELF support: Power Function

###### tags: `RISC-V`

Choosing power function from [曾紹銘](https://hackmd.io/o1yeSGzDQO2LmbKXEoBC_g?view).

## Rewrite in C language

void _start(){
    volatile char* tx = (volatile char*) 0x40002000;
    int base = 3;
    int exp = 5;
    int result = 1;
    
    while(exp > 0){
        if(exp & 0x1)
            result *= base;
        exp >>= 1;
        base *= base;
    }
    
    int out = result;
    *tx = out;
}

## Comparing different optimization and manual in GNU Toolchain

### execute the elf file

The result in integer is not convert to char, so the output format is wrong.
I was shocked by the counts of instruction.
- produced by gcc with -O3

$ riscv-none-embed-gcc -march=rv32i -mabi=ilp32 -O3 -nostdlib -o pow power.c
$ ./emu-rv32i pow
�
>>> Execution time: 19543 ns
>>> Instruction count: 5 (IPS=255846)
>>> Jumps: 1 (20.00%) - 0 forwards, 1 backwards
>>> Branching T=0 (-nan%) F=0 (-nan%)

- produced by gcc with -Os
I encountered some errors, but they have not been resolved.

$ riscv-none-embed-gcc -march=rv32i -mabi=ilp32 -Os -nostdlib -o pow power.c
/home/ccs100203/riscv-none-embed-gcc/8.2.0-3.1/bin/../lib/gcc/riscv-none-embed/8.2.0/../../../../riscv-none-embed/bin/ld: /tmp/ccjM40Oy.o: in function `.L3':
power.c:(.text+0x38): undefined reference to `__mulsi3'
/home/ccs100203/riscv-none-embed-gcc/8.2.0-3.1/bin/../lib/gcc/riscv-none-embed/8.2.0/../../../../riscv-none-embed/bin/ld: /tmp/ccjM40Oy.o: in function `.L2':
power.c:(.text+0x4c): undefined reference to `__mulsi3'
collect2: error: ld returned 1 exit status

### objdump

$ riscv-none-embed-objdump -d pow

The optimization of gcc was amazing, the code size was greatly compressed.

- produced by gcc with -O3

pow:     file format elf32-littleriscv


Disassembly of section .text:

00010054 <_start>:
   10054:       400027b7                lui     a5,0x40002
   10058:       ff300713                li      a4,-13
   1005c:       00e78023                sb      a4,0(a5) # 40002000 <__global_pointer$+0x3fff079c>
   10060:       00008067                ret

- produced by manual

.data
x: .word 3
y: .word 5				# unsigned
str1: .string " ^ "
str2: .string " = "

.text
main:
	lw s0, x			# Set s0 equal to the parameter x
	lw s1, y			# Set s1 equal to the parameter y
	mv a0, s0			# Set arguments
	mv a1, s1
	jal power			# return the result a0
	mv s2, a0			# Set s2 equal to the result
	j print
power:
	addi sp, sp, -12
	sw ra, 0(sp)
	sw s0, 4(sp)
	sw s1, 8(sp)
	mv s0, a0			# s0 -> parameter x
	mv s1, a1			# s1 -> parameter y
	li s2, 1			# s2 -> res, init = 1
loop:
	bge x0, s1, Ret		# if (0 >= y) break
	andi t0, s1, 0x1	# t0 -> y & 0x1
	li t1, 0x1			# t1 -> 0x1
	li ra, 0x54			# Set ra equal to srli s1, s1, 1
	beq t0, t1, odd		# y is odd
	srli s1, s1, 1		# y_u >> 1, y /= 2
	mul s0, s0, s0		# x = x * x
	j loop				# goto loop
odd:
	mul s2, s2, s0		# res = res * x
	jr ra				# goto srli s1, s1, 1
Ret:
	mv a0, s2			# Set return reg a0 
	lw ra, 0(sp)		# Restore ra
	lw s0, 4(sp)		# Restore s0
	lw s1, 8(sp)		# Restore s1
	addi sp, sp, 12		# Free space on the stack for the 3 words
	jr ra				# Return to the caller
print:
	mv t0, a0			# Set tmp equal to a0
	mv a0, s0			# prepare to print x
	li a7, 1			# print int
	ecall
	la a0, str1			# prepare to print string ^ 
	li a7, 4			# print string
 	ecall
	mv a0, s1			# prepare to print y
	li a7, 1			# print int
	ecall
	la a0, str2			# prepare to print string =
	li a7, 4 			# print string
	ecall
	mv a0, s2			# prepare to print result
	li a7, 1			# print int
	ecall

### readeif

$ riscv-none-embed-readelf -h pow

- produced by gcc with -O3

ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           RISC-V
  Version:                           0x1
  Entry point address:               0x10054
  Start of program headers:          52 (bytes into file)
  Start of section headers:          480 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         1
  Size of section headers:           40 (bytes)
  Number of section headers:         6
  Section header string table index: 5

### size

$ riscv-none-embed-size pow

- produced by gcc with -O3

   text    data     bss     dec     hex filename
     16       0       0      16      10 pow