# Lab2: RISC-V RV32I[MA] emulator with ELF support
contributed by < [geniuseric](https://github.com/geniuseric) >

## Requirement
[Assignment2: RISC-V Toolchain](https://hackmd.io/@sysprog/2021-arch-homework2)

## Palindrome Number
- I pick up the assembly program from [黃瑋盛](https://hackmd.io/@N9qHU_eLRvKyfDfJk8cDXA/Bk3xctfBF).
- This is the question from [LeetCode 9](https://leetcode.com/problems/palindrome-number/).
- An integer is a **palindrome** when it reads the same backward as forward. For example, `121` is palindrome while `123` is not.
- The reason why I choose this question is because **palindrome** is a new vocabulary to me. The meaning of this word also sounds intersting to me.

## Implementation
- Rewrite original C code into modified C code which can execute with [rv32emu](https://github.com/sysprog21/rv32emu).
- The original C code reverses the integer by all digits, I improve the code which it only reverses the integer by half of all digits. For example, `1221` is a palindrome number. The original C code has to check `1`、`2`、`2`、`1` four digits, but the modified C code only needs to check `1`、`2` two digits to know if it is a palindrome.
- In order to execute the C code in the emulator, the first problem I encounter is how to print an integer on the terminal. Therefore, I add `printf("%d", val)` at the line **883** in `emurv32i.c` and it works fine.
- The second problem I encounter is that there are undefined reference to `__modsi3` and `__divsi3`. Therefore, I have to write `mod` and `div` functions by myself.
- The **Original C Code** and the **Modified C Code** are shown below.

### Original C Code

#include<stdio.h>
    
int is_Palindrome(int x){
    if(x >= 0){
        int tmp = x;
        int rv_Int = 0;
        while(tmp){
            rv_Int *= 10;
            rv_Int += tmp % 10;
            tmp /= 10;
        }
        return x == rv_Int;
    }
    return 0;
}

int main(){
    int data[5] = {0, 123456789, 12321, -878, -789};
    for(int i=0; i<(sizeof(data)/sizeof(int)); i++){
        if(is_Palindrome(data[i]))
            printf("%d is palindrome\n", data[i]);
        else
            printf("%d is not palindrome\n", data[i]);
    }
    return 0;
}

### Modified C Code

# define SIZE 4

int div(int a, int b){ // d = a / b
    int d = 0;
    while (a >= b){
        a -= b;
        d += 1;
    }
    return d;
}

int mod(int a, int b){ // m = a / b
    int m = a;
    while (m >= b)
        m -= b;
    return m;
}

int isPalindrome(int x){
    if (x == 0)
        return 1;
    else if (x < 0 || mod(x, 10) == 0)
        return 0;
    else{
        int temp = x;
        int back_x = 0;
        while (temp > back_x){
            back_x = back_x * 10 + mod(temp, 10);
            temp = div(temp, 10);	
        }		
        return (back_x == temp) || (div(back_x, 10) == temp);
    }
}

void _start(){
    int data[SIZE] = {-88, 10, 12321, 258852};
    volatile char *tx_char = (volatile char *) 0x40002000;
    volatile int *tx_int = (volatile int *) 0x40000008;
    const char *isTrue = " is a palindrome!\n";
    const char *isFalse = " is not a palindrome.\n";
    int counter;
    for (int i = 0; i < SIZE; i++){
        int res = isPalindrome(data[i]);
        *tx_int = data[i];
        counter = 0;
        if (res == 1){    		
            while(*isTrue){
                *tx_char = *isTrue;
                isTrue++;
                counter++;
            }
            isTrue -= counter;
        }
        else{
            while(*isFalse){
                *tx_char = *isFalse;
                isFalse++;
                counter++;
            } 
            isFalse -= counter;  
        } 	
    }    
}

## Disassemble the ELF
- Modify the arguments specified by RV32I_CFLAGS to experiment.
- `-O1` and `-O3` are for speed optimization and `-Os` is for size optimization.
- Since the modified C code is very different from the original C code, the comparison between original handwritten and compiler optimized assembly code is unfair.
- However, the differences between different compiler optimized assembly codes are important.
- There is a bug when I use `-Os` for optimization.

### Compiler optimized with `-O0`
#### *objdump*

isPalindrome:     file format elf32-littleriscv


Disassembly of section .text:

00010074 <div>:
   10074:	fd010113          	addi	sp,sp,-48
   10078:	02812623          	sw	s0,44(sp)
   1007c:	03010413          	addi	s0,sp,48
   10080:	fca42e23          	sw	a0,-36(s0)
   10084:	fcb42c23          	sw	a1,-40(s0)
   10088:	fe042623          	sw	zero,-20(s0)
   1008c:	0200006f          	j	100ac <div+0x38>
   10090:	fdc42703          	lw	a4,-36(s0)
   10094:	fd842783          	lw	a5,-40(s0)
   10098:	40f707b3          	sub	a5,a4,a5
   1009c:	fcf42e23          	sw	a5,-36(s0)
   100a0:	fec42783          	lw	a5,-20(s0)
   100a4:	00178793          	addi	a5,a5,1
   100a8:	fef42623          	sw	a5,-20(s0)
   100ac:	fdc42703          	lw	a4,-36(s0)
   100b0:	fd842783          	lw	a5,-40(s0)
   100b4:	fcf75ee3          	bge	a4,a5,10090 <div+0x1c>
   100b8:	fec42783          	lw	a5,-20(s0)
   100bc:	00078513          	mv	a0,a5
   100c0:	02c12403          	lw	s0,44(sp)
   100c4:	03010113          	addi	sp,sp,48
   100c8:	00008067          	ret

000100cc <mod>:
   100cc:	fd010113          	addi	sp,sp,-48
   100d0:	02812623          	sw	s0,44(sp)
   100d4:	03010413          	addi	s0,sp,48
   100d8:	fca42e23          	sw	a0,-36(s0)
   100dc:	fcb42c23          	sw	a1,-40(s0)
   100e0:	fdc42783          	lw	a5,-36(s0)
   100e4:	fef42623          	sw	a5,-20(s0)
   100e8:	0140006f          	j	100fc <mod+0x30>
   100ec:	fec42703          	lw	a4,-20(s0)
   100f0:	fd842783          	lw	a5,-40(s0)
   100f4:	40f707b3          	sub	a5,a4,a5
   100f8:	fef42623          	sw	a5,-20(s0)
   100fc:	fec42703          	lw	a4,-20(s0)
   10100:	fd842783          	lw	a5,-40(s0)
   10104:	fef754e3          	bge	a4,a5,100ec <mod+0x20>
   10108:	fec42783          	lw	a5,-20(s0)
   1010c:	00078513          	mv	a0,a5
   10110:	02c12403          	lw	s0,44(sp)
   10114:	03010113          	addi	sp,sp,48
   10118:	00008067          	ret

0001011c <isPalindrome>:
   1011c:	fd010113          	addi	sp,sp,-48
   10120:	02112623          	sw	ra,44(sp)
   10124:	02812423          	sw	s0,40(sp)
   10128:	02912223          	sw	s1,36(sp)
   1012c:	03010413          	addi	s0,sp,48
   10130:	fca42e23          	sw	a0,-36(s0)
   10134:	fdc42783          	lw	a5,-36(s0)
   10138:	00079663          	bnez	a5,10144 <isPalindrome+0x28>
   1013c:	00100793          	li	a5,1
   10140:	0b40006f          	j	101f4 <isPalindrome+0xd8>
   10144:	fdc42783          	lw	a5,-36(s0)
   10148:	0007cc63          	bltz	a5,10160 <isPalindrome+0x44>
   1014c:	00a00593          	li	a1,10
   10150:	fdc42503          	lw	a0,-36(s0)
   10154:	f79ff0ef          	jal	ra,100cc <mod>
   10158:	00050793          	mv	a5,a0
   1015c:	00079663          	bnez	a5,10168 <isPalindrome+0x4c>
   10160:	00000793          	li	a5,0
   10164:	0900006f          	j	101f4 <isPalindrome+0xd8>
   10168:	fdc42783          	lw	a5,-36(s0)
   1016c:	fef42623          	sw	a5,-20(s0)
   10170:	fe042423          	sw	zero,-24(s0)
   10174:	0440006f          	j	101b8 <isPalindrome+0x9c>
   10178:	fe842703          	lw	a4,-24(s0)
   1017c:	00070793          	mv	a5,a4
   10180:	00279793          	slli	a5,a5,0x2
   10184:	00e787b3          	add	a5,a5,a4
   10188:	00179793          	slli	a5,a5,0x1
   1018c:	00078493          	mv	s1,a5
   10190:	00a00593          	li	a1,10
   10194:	fec42503          	lw	a0,-20(s0)
   10198:	f35ff0ef          	jal	ra,100cc <mod>
   1019c:	00050793          	mv	a5,a0
   101a0:	00f487b3          	add	a5,s1,a5
   101a4:	fef42423          	sw	a5,-24(s0)
   101a8:	00a00593          	li	a1,10
   101ac:	fec42503          	lw	a0,-20(s0)
   101b0:	ec5ff0ef          	jal	ra,10074 <div>
   101b4:	fea42623          	sw	a0,-20(s0)
   101b8:	fec42703          	lw	a4,-20(s0)
   101bc:	fe842783          	lw	a5,-24(s0)
   101c0:	fae7cce3          	blt	a5,a4,10178 <isPalindrome+0x5c>
   101c4:	fe842703          	lw	a4,-24(s0)
   101c8:	fec42783          	lw	a5,-20(s0)
   101cc:	00f70e63          	beq	a4,a5,101e8 <isPalindrome+0xcc>
   101d0:	00a00593          	li	a1,10
   101d4:	fe842503          	lw	a0,-24(s0)
   101d8:	e9dff0ef          	jal	ra,10074 <div>
   101dc:	00050713          	mv	a4,a0
   101e0:	fec42783          	lw	a5,-20(s0)
   101e4:	00e79663          	bne	a5,a4,101f0 <isPalindrome+0xd4>
   101e8:	00100793          	li	a5,1
   101ec:	0080006f          	j	101f4 <isPalindrome+0xd8>
   101f0:	00000793          	li	a5,0
   101f4:	00078513          	mv	a0,a5
   101f8:	02c12083          	lw	ra,44(sp)
   101fc:	02812403          	lw	s0,40(sp)
   10200:	02412483          	lw	s1,36(sp)
   10204:	03010113          	addi	sp,sp,48
   10208:	00008067          	ret

0001020c <_start>:
   1020c:	fc010113          	addi	sp,sp,-64
   10210:	02112e23          	sw	ra,60(sp)
   10214:	02812c23          	sw	s0,56(sp)
   10218:	04010413          	addi	s0,sp,64
   1021c:	000107b7          	lui	a5,0x10
   10220:	3b878793          	addi	a5,a5,952 # 103b8 <_start+0x1ac>
   10224:	0007a603          	lw	a2,0(a5)
   10228:	0047a683          	lw	a3,4(a5)
   1022c:	0087a703          	lw	a4,8(a5)
   10230:	00c7a783          	lw	a5,12(a5)
   10234:	fcc42223          	sw	a2,-60(s0)
   10238:	fcd42423          	sw	a3,-56(s0)
   1023c:	fce42623          	sw	a4,-52(s0)
   10240:	fcf42823          	sw	a5,-48(s0)
   10244:	400027b7          	lui	a5,0x40002
   10248:	fcf42e23          	sw	a5,-36(s0)
   1024c:	400007b7          	lui	a5,0x40000
   10250:	00878793          	addi	a5,a5,8 # 40000008 <__global_pointer$+0x3ffee438>
   10254:	fcf42c23          	sw	a5,-40(s0)
   10258:	000107b7          	lui	a5,0x10
   1025c:	38c78793          	addi	a5,a5,908 # 1038c <_start+0x180>
   10260:	fef42623          	sw	a5,-20(s0)
   10264:	000107b7          	lui	a5,0x10
   10268:	3a078793          	addi	a5,a5,928 # 103a0 <_start+0x194>
   1026c:	fef42423          	sw	a5,-24(s0)
   10270:	fe042023          	sw	zero,-32(s0)
   10274:	0f40006f          	j	10368 <_start+0x15c>
   10278:	fe042783          	lw	a5,-32(s0)
   1027c:	00279793          	slli	a5,a5,0x2
   10280:	ff040713          	addi	a4,s0,-16
   10284:	00f707b3          	add	a5,a4,a5
   10288:	fd47a783          	lw	a5,-44(a5)
   1028c:	00078513          	mv	a0,a5
   10290:	e8dff0ef          	jal	ra,1011c <isPalindrome>
   10294:	fca42a23          	sw	a0,-44(s0)
   10298:	fe042783          	lw	a5,-32(s0)
   1029c:	00279793          	slli	a5,a5,0x2
   102a0:	ff040713          	addi	a4,s0,-16
   102a4:	00f707b3          	add	a5,a4,a5
   102a8:	fd47a703          	lw	a4,-44(a5)
   102ac:	fd842783          	lw	a5,-40(s0)
   102b0:	00e7a023          	sw	a4,0(a5)
   102b4:	fe042223          	sw	zero,-28(s0)
   102b8:	fd442703          	lw	a4,-44(s0)
   102bc:	00100793          	li	a5,1
   102c0:	06f71e63          	bne	a4,a5,1033c <_start+0x130>
   102c4:	02c0006f          	j	102f0 <_start+0xe4>
   102c8:	fec42783          	lw	a5,-20(s0)
   102cc:	0007c703          	lbu	a4,0(a5)
   102d0:	fdc42783          	lw	a5,-36(s0)
   102d4:	00e78023          	sb	a4,0(a5)
   102d8:	fec42783          	lw	a5,-20(s0)
   102dc:	00178793          	addi	a5,a5,1
   102e0:	fef42623          	sw	a5,-20(s0)
   102e4:	fe442783          	lw	a5,-28(s0)
   102e8:	00178793          	addi	a5,a5,1
   102ec:	fef42223          	sw	a5,-28(s0)
   102f0:	fec42783          	lw	a5,-20(s0)
   102f4:	0007c783          	lbu	a5,0(a5)
   102f8:	fc0798e3          	bnez	a5,102c8 <_start+0xbc>
   102fc:	fe442783          	lw	a5,-28(s0)
   10300:	40f007b3          	neg	a5,a5
   10304:	fec42703          	lw	a4,-20(s0)
   10308:	00f707b3          	add	a5,a4,a5
   1030c:	fef42623          	sw	a5,-20(s0)
   10310:	04c0006f          	j	1035c <_start+0x150>
   10314:	fe842783          	lw	a5,-24(s0)
   10318:	0007c703          	lbu	a4,0(a5)
   1031c:	fdc42783          	lw	a5,-36(s0)
   10320:	00e78023          	sb	a4,0(a5)
   10324:	fe842783          	lw	a5,-24(s0)
   10328:	00178793          	addi	a5,a5,1
   1032c:	fef42423          	sw	a5,-24(s0)
   10330:	fe442783          	lw	a5,-28(s0)
   10334:	00178793          	addi	a5,a5,1
   10338:	fef42223          	sw	a5,-28(s0)
   1033c:	fe842783          	lw	a5,-24(s0)
   10340:	0007c783          	lbu	a5,0(a5)
   10344:	fc0798e3          	bnez	a5,10314 <_start+0x108>
   10348:	fe442783          	lw	a5,-28(s0)
   1034c:	40f007b3          	neg	a5,a5
   10350:	fe842703          	lw	a4,-24(s0)
   10354:	00f707b3          	add	a5,a4,a5
   10358:	fef42423          	sw	a5,-24(s0)
   1035c:	fe042783          	lw	a5,-32(s0)
   10360:	00178793          	addi	a5,a5,1
   10364:	fef42023          	sw	a5,-32(s0)
   10368:	fe042703          	lw	a4,-32(s0)
   1036c:	00300793          	li	a5,3
   10370:	f0e7d4e3          	bge	a5,a4,10278 <_start+0x6c>
   10374:	00000013          	nop
   10378:	00000013          	nop
   1037c:	03c12083          	lw	ra,60(sp)
   10380:	03812403          	lw	s0,56(sp)
   10384:	04010113          	addi	sp,sp,64
   10388:	00008067          	ret

#### *readelf*

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:               0x1020c
  Start of program headers:          52 (bytes into file)
  Start of section headers:          1504 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         2
  Size of section headers:           40 (bytes)
  Number of section headers:         9
  Section header string table index: 8

#### *size*

   text	   data	    bss	    dec	    hex	filename
    852	      8	      0	    860	    35c	isPalindrome

### Compiler optimized with `-O1`
#### *objdump*

isPalindrome:     file format elf32-littleriscv


Disassembly of section .text:

00010074 <div>:
   10074:	00050793          	mv	a5,a0
   10078:	00b54c63          	blt	a0,a1,10090 <div+0x1c>
   1007c:	00000513          	li	a0,0
   10080:	40b787b3          	sub	a5,a5,a1
   10084:	00150513          	addi	a0,a0,1
   10088:	feb7dce3          	bge	a5,a1,10080 <div+0xc>
   1008c:	00008067          	ret
   10090:	00000513          	li	a0,0
   10094:	00008067          	ret

00010098 <mod>:
   10098:	00b54663          	blt	a0,a1,100a4 <mod+0xc>
   1009c:	40b50533          	sub	a0,a0,a1
   100a0:	feb55ee3          	bge	a0,a1,1009c <mod+0x4>
   100a4:	00008067          	ret

000100a8 <isPalindrome>:
   100a8:	00050693          	mv	a3,a0
   100ac:	0a050663          	beqz	a0,10158 <isPalindrome+0xb0>
   100b0:	00000513          	li	a0,0
   100b4:	0206c063          	bltz	a3,100d4 <isPalindrome+0x2c>
   100b8:	00900793          	li	a5,9
   100bc:	00d7de63          	bge	a5,a3,100d8 <isPalindrome+0x30>
   100c0:	00068513          	mv	a0,a3
   100c4:	ff650513          	addi	a0,a0,-10
   100c8:	fea7cee3          	blt	a5,a0,100c4 <isPalindrome+0x1c>
   100cc:	00000793          	li	a5,0
   100d0:	00051663          	bnez	a0,100dc <isPalindrome+0x34>
   100d4:	00008067          	ret
   100d8:	00000793          	li	a5,0
   100dc:	00900613          	li	a2,9
   100e0:	0140006f          	j	100f4 <isPalindrome+0x4c>
   100e4:	ff670713          	addi	a4,a4,-10
   100e8:	00168693          	addi	a3,a3,1
   100ec:	fee64ce3          	blt	a2,a4,100e4 <isPalindrome+0x3c>
   100f0:	02d7d863          	bge	a5,a3,10120 <isPalindrome+0x78>
   100f4:	00279713          	slli	a4,a5,0x2
   100f8:	00f707b3          	add	a5,a4,a5
   100fc:	00179713          	slli	a4,a5,0x1
   10100:	06d65063          	bge	a2,a3,10160 <isPalindrome+0xb8>
   10104:	00068793          	mv	a5,a3
   10108:	ff678793          	addi	a5,a5,-10
   1010c:	fef64ee3          	blt	a2,a5,10108 <isPalindrome+0x60>
   10110:	00f707b3          	add	a5,a4,a5
   10114:	00068713          	mv	a4,a3
   10118:	00000693          	li	a3,0
   1011c:	fc9ff06f          	j	100e4 <isPalindrome+0x3c>
   10120:	00100513          	li	a0,1
   10124:	fad788e3          	beq	a5,a3,100d4 <isPalindrome+0x2c>
   10128:	00900713          	li	a4,9
   1012c:	02f75263          	bge	a4,a5,10150 <isPalindrome+0xa8>
   10130:	00000713          	li	a4,0
   10134:	00900613          	li	a2,9
   10138:	ff678793          	addi	a5,a5,-10
   1013c:	00170713          	addi	a4,a4,1
   10140:	fef64ce3          	blt	a2,a5,10138 <isPalindrome+0x90>
   10144:	40e686b3          	sub	a3,a3,a4
   10148:	0016b513          	seqz	a0,a3
   1014c:	00008067          	ret
   10150:	00000713          	li	a4,0
   10154:	ff1ff06f          	j	10144 <isPalindrome+0x9c>
   10158:	00100513          	li	a0,1
   1015c:	00008067          	ret
   10160:	00d707b3          	add	a5,a4,a3
   10164:	00000693          	li	a3,0
   10168:	f89ff06f          	j	100f0 <isPalindrome+0x48>

0001016c <_start>:
   1016c:	fc010113          	addi	sp,sp,-64
   10170:	02112e23          	sw	ra,60(sp)
   10174:	02812c23          	sw	s0,56(sp)
   10178:	02912a23          	sw	s1,52(sp)
   1017c:	03212823          	sw	s2,48(sp)
   10180:	03312623          	sw	s3,44(sp)
   10184:	03412423          	sw	s4,40(sp)
   10188:	03512223          	sw	s5,36(sp)
   1018c:	03612023          	sw	s6,32(sp)
   10190:	01712e23          	sw	s7,28(sp)
   10194:	fa800793          	li	a5,-88
   10198:	00f12023          	sw	a5,0(sp)
   1019c:	00a00793          	li	a5,10
   101a0:	00f12223          	sw	a5,4(sp)
   101a4:	000037b7          	lui	a5,0x3
   101a8:	02178793          	addi	a5,a5,33 # 3021 <div-0xd053>
   101ac:	00f12423          	sw	a5,8(sp)
   101b0:	0003f7b7          	lui	a5,0x3f
   101b4:	32478793          	addi	a5,a5,804 # 3f324 <__global_pointer$+0x2d874>
   101b8:	00f12623          	sw	a5,12(sp)
   101bc:	00010993          	mv	s3,sp
   101c0:	01010b93          	addi	s7,sp,16
   101c4:	00010437          	lui	s0,0x10
   101c8:	28440413          	addi	s0,s0,644 # 10284 <_start+0x118>
   101cc:	000104b7          	lui	s1,0x10
   101d0:	29c48493          	addi	s1,s1,668 # 1029c <_start+0x130>
   101d4:	40000b37          	lui	s6,0x40000
   101d8:	00100a93          	li	s5,1
   101dc:	40002937          	lui	s2,0x40002
   101e0:	0400006f          	j	10220 <_start+0xb4>
   101e4:	0004c783          	lbu	a5,0(s1)
   101e8:	02078263          	beqz	a5,1020c <_start+0xa0>
   101ec:	00000713          	li	a4,0
   101f0:	00f90023          	sb	a5,0(s2) # 40002000 <__global_pointer$+0x3fff0550>
   101f4:	00148493          	addi	s1,s1,1
   101f8:	00170713          	addi	a4,a4,1
   101fc:	0004c783          	lbu	a5,0(s1)
   10200:	fe0798e3          	bnez	a5,101f0 <_start+0x84>
   10204:	40e484b3          	sub	s1,s1,a4
   10208:	0100006f          	j	10218 <_start+0xac>
   1020c:	00000713          	li	a4,0
   10210:	ff5ff06f          	j	10204 <_start+0x98>
   10214:	40e40433          	sub	s0,s0,a4
   10218:	00498993          	addi	s3,s3,4
   1021c:	033b8e63          	beq	s7,s3,10258 <_start+0xec>
   10220:	0009aa03          	lw	s4,0(s3)
   10224:	000a0513          	mv	a0,s4
   10228:	e81ff0ef          	jal	ra,100a8 <isPalindrome>
   1022c:	014b2423          	sw	s4,8(s6) # 40000008 <__global_pointer$+0x3ffee558>
   10230:	fb550ae3          	beq	a0,s5,101e4 <_start+0x78>
   10234:	00044783          	lbu	a5,0(s0)
   10238:	00000713          	li	a4,0
   1023c:	fc078ce3          	beqz	a5,10214 <_start+0xa8>
   10240:	00f90023          	sb	a5,0(s2)
   10244:	00140413          	addi	s0,s0,1
   10248:	00170713          	addi	a4,a4,1
   1024c:	00044783          	lbu	a5,0(s0)
   10250:	fe0798e3          	bnez	a5,10240 <_start+0xd4>
   10254:	fc1ff06f          	j	10214 <_start+0xa8>
   10258:	03c12083          	lw	ra,60(sp)
   1025c:	03812403          	lw	s0,56(sp)
   10260:	03412483          	lw	s1,52(sp)
   10264:	03012903          	lw	s2,48(sp)
   10268:	02c12983          	lw	s3,44(sp)
   1026c:	02812a03          	lw	s4,40(sp)
   10270:	02412a83          	lw	s5,36(sp)
   10274:	02012b03          	lw	s6,32(sp)
   10278:	01c12b83          	lw	s7,28(sp)
   1027c:	04010113          	addi	sp,sp,64
   10280:	00008067          	ret

#### *readelf*

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:               0x1016c
  Start of program headers:          52 (bytes into file)
  Start of section headers:          1216 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         2
  Size of section headers:           40 (bytes)
  Number of section headers:         9
  Section header string table index: 8

#### *size*

   text	   data	    bss	    dec	    hex	filename
    571	      1	      0	    572	    23c	isPalindrome

### Compiler optimized with `-O3`
#### *objdump*

isPalindrome:     file format elf32-littleriscv


Disassembly of section .text:

00010074 <div>:
   10074:	00050793          	mv	a5,a0
   10078:	00000513          	li	a0,0
   1007c:	00b7ca63          	blt	a5,a1,10090 <div+0x1c>
   10080:	40b787b3          	sub	a5,a5,a1
   10084:	00150513          	addi	a0,a0,1
   10088:	feb7dce3          	bge	a5,a1,10080 <div+0xc>
   1008c:	00008067          	ret
   10090:	00008067          	ret

00010094 <mod>:
   10094:	00b54663          	blt	a0,a1,100a0 <mod+0xc>
   10098:	40b50533          	sub	a0,a0,a1
   1009c:	feb55ee3          	bge	a0,a1,10098 <mod+0x4>
   100a0:	00008067          	ret

000100a4 <isPalindrome>:
   100a4:	00050693          	mv	a3,a0
   100a8:	08050c63          	beqz	a0,10140 <isPalindrome+0x9c>
   100ac:	00000513          	li	a0,0
   100b0:	0006ce63          	bltz	a3,100cc <isPalindrome+0x28>
   100b4:	00900793          	li	a5,9
   100b8:	00d7dc63          	bge	a5,a3,100d0 <isPalindrome+0x2c>
   100bc:	00068513          	mv	a0,a3
   100c0:	ff650513          	addi	a0,a0,-10
   100c4:	fea7cee3          	blt	a5,a0,100c0 <isPalindrome+0x1c>
   100c8:	00051463          	bnez	a0,100d0 <isPalindrome+0x2c>
   100cc:	00008067          	ret
   100d0:	00000793          	li	a5,0
   100d4:	00900613          	li	a2,9
   100d8:	00279593          	slli	a1,a5,0x2
   100dc:	00f587b3          	add	a5,a1,a5
   100e0:	00179593          	slli	a1,a5,0x1
   100e4:	06d65663          	bge	a2,a3,10150 <isPalindrome+0xac>
   100e8:	00068793          	mv	a5,a3
   100ec:	ff678793          	addi	a5,a5,-10
   100f0:	fef64ee3          	blt	a2,a5,100ec <isPalindrome+0x48>
   100f4:	00068713          	mv	a4,a3
   100f8:	00f587b3          	add	a5,a1,a5
   100fc:	00000693          	li	a3,0
   10100:	ff670713          	addi	a4,a4,-10
   10104:	00168693          	addi	a3,a3,1
   10108:	fee64ce3          	blt	a2,a4,10100 <isPalindrome+0x5c>
   1010c:	fcd7c6e3          	blt	a5,a3,100d8 <isPalindrome+0x34>
   10110:	00100513          	li	a0,1
   10114:	faf68ce3          	beq	a3,a5,100cc <isPalindrome+0x28>
   10118:	00900713          	li	a4,9
   1011c:	02f75663          	bge	a4,a5,10148 <isPalindrome+0xa4>
   10120:	00000713          	li	a4,0
   10124:	00900613          	li	a2,9
   10128:	ff678793          	addi	a5,a5,-10
   1012c:	00170713          	addi	a4,a4,1
   10130:	fef64ce3          	blt	a2,a5,10128 <isPalindrome+0x84>
   10134:	40d70733          	sub	a4,a4,a3
   10138:	00173513          	seqz	a0,a4
   1013c:	00008067          	ret
   10140:	00100513          	li	a0,1
   10144:	00008067          	ret
   10148:	00000713          	li	a4,0
   1014c:	fe9ff06f          	j	10134 <isPalindrome+0x90>
   10150:	00b687b3          	add	a5,a3,a1
   10154:	00000693          	li	a3,0
   10158:	fc1ff06f          	j	10118 <isPalindrome+0x74>

0001015c <_start>:
   1015c:	ff010113          	addi	sp,sp,-16
   10160:	fa800793          	li	a5,-88
   10164:	00f12023          	sw	a5,0(sp)
   10168:	00a00793          	li	a5,10
   1016c:	00f12223          	sw	a5,4(sp)
   10170:	000037b7          	lui	a5,0x3
   10174:	02178793          	addi	a5,a5,33 # 3021 <div-0xd053>
   10178:	00f12423          	sw	a5,8(sp)
   1017c:	0003f7b7          	lui	a5,0x3f
   10180:	32478793          	addi	a5,a5,804 # 3f324 <__global_pointer$+0x2d844>
   10184:	000105b7          	lui	a1,0x10
   10188:	00010837          	lui	a6,0x10
   1018c:	00f12623          	sw	a5,12(sp)
   10190:	00010e13          	mv	t3,sp
   10194:	01010e93          	addi	t4,sp,16
   10198:	2b458593          	addi	a1,a1,692 # 102b4 <_start+0x158>
   1019c:	2cc80813          	addi	a6,a6,716 # 102cc <_start+0x170>
   101a0:	400028b7          	lui	a7,0x40002
   101a4:	40000f37          	lui	t5,0x40000
   101a8:	00900613          	li	a2,9
   101ac:	000e2303          	lw	t1,0(t3)
   101b0:	0e030063          	beqz	t1,10290 <_start+0x134>
   101b4:	00034c63          	bltz	t1,101cc <_start+0x70>
   101b8:	0e665463          	bge	a2,t1,102a0 <_start+0x144>
   101bc:	00030793          	mv	a5,t1
   101c0:	ff678793          	addi	a5,a5,-10
   101c4:	fef64ee3          	blt	a2,a5,101c0 <_start+0x64>
   101c8:	02079e63          	bnez	a5,10204 <_start+0xa8>
   101cc:	006f2423          	sw	t1,8(t5) # 40000008 <__global_pointer$+0x3ffee528>
   101d0:	0005c783          	lbu	a5,0(a1)
   101d4:	00000713          	li	a4,0
   101d8:	00078e63          	beqz	a5,101f4 <_start+0x98>
   101dc:	00f88023          	sb	a5,0(a7) # 40002000 <__global_pointer$+0x3fff0520>
   101e0:	0015c783          	lbu	a5,1(a1)
   101e4:	00170713          	addi	a4,a4,1
   101e8:	00158593          	addi	a1,a1,1
   101ec:	fe0798e3          	bnez	a5,101dc <_start+0x80>
   101f0:	40e585b3          	sub	a1,a1,a4
   101f4:	004e0e13          	addi	t3,t3,4
   101f8:	fbce9ae3          	bne	t4,t3,101ac <_start+0x50>
   101fc:	01010113          	addi	sp,sp,16
   10200:	00008067          	ret
   10204:	00030693          	mv	a3,t1
   10208:	00000793          	li	a5,0
   1020c:	00279513          	slli	a0,a5,0x2
   10210:	00f507b3          	add	a5,a0,a5
   10214:	00179513          	slli	a0,a5,0x1
   10218:	00050793          	mv	a5,a0
   1021c:	08d65663          	bge	a2,a3,102a8 <_start+0x14c>
   10220:	00068793          	mv	a5,a3
   10224:	ff678793          	addi	a5,a5,-10
   10228:	fef64ee3          	blt	a2,a5,10224 <_start+0xc8>
   1022c:	00068713          	mv	a4,a3
   10230:	00f507b3          	add	a5,a0,a5
   10234:	00000693          	li	a3,0
   10238:	ff670713          	addi	a4,a4,-10
   1023c:	00168693          	addi	a3,a3,1
   10240:	fee64ce3          	blt	a2,a4,10238 <_start+0xdc>
   10244:	fcd7c4e3          	blt	a5,a3,1020c <_start+0xb0>
   10248:	04d78863          	beq	a5,a3,10298 <_start+0x13c>
   1024c:	00000713          	li	a4,0
   10250:	00f65863          	bge	a2,a5,10260 <_start+0x104>
   10254:	ff678793          	addi	a5,a5,-10
   10258:	00170713          	addi	a4,a4,1
   1025c:	fef64ce3          	blt	a2,a5,10254 <_start+0xf8>
   10260:	006f2423          	sw	t1,8(t5)
   10264:	f6d716e3          	bne	a4,a3,101d0 <_start+0x74>
   10268:	00084783          	lbu	a5,0(a6)
   1026c:	00000713          	li	a4,0
   10270:	f80782e3          	beqz	a5,101f4 <_start+0x98>
   10274:	00f88023          	sb	a5,0(a7)
   10278:	00184783          	lbu	a5,1(a6)
   1027c:	00170713          	addi	a4,a4,1
   10280:	00180813          	addi	a6,a6,1
   10284:	fe0798e3          	bnez	a5,10274 <_start+0x118>
   10288:	40e80833          	sub	a6,a6,a4
   1028c:	f69ff06f          	j	101f4 <_start+0x98>
   10290:	000f2423          	sw	zero,8(t5)
   10294:	fd5ff06f          	j	10268 <_start+0x10c>
   10298:	006f2423          	sw	t1,8(t5)
   1029c:	fcdff06f          	j	10268 <_start+0x10c>
   102a0:	00030693          	mv	a3,t1
   102a4:	00000793          	li	a5,0
   102a8:	00d787b3          	add	a5,a5,a3
   102ac:	00000693          	li	a3,0
   102b0:	f9dff06f          	j	1024c <_start+0xf0>

#### *readelf*

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:               0x1015c
  Start of program headers:          52 (bytes into file)
  Start of section headers:          1264 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         2
  Size of section headers:           40 (bytes)
  Number of section headers:         9
  Section header string table index: 8

#### *size*

   text	   data	    bss	    dec	    hex	filename
    619	      1	      0	    620	    26c	isPalindrome

## Check the result
- The results and statistics of execution flow are shown below.

### Compiler optimized with `-O0`

./emu-rv32i isPalindrome
-88 is not a palindrome.
10 is not a palindrome.
12321 is a palindrome!
258852 is a palindrome!

>>> Execution time: 122751361 ns
>>> Instruction count: 703301 (IPS=5729476)
>>> Jumps: 87494 (12.44%) - 57 forwards, 87437 backwards
>>> Branching T=87426 (99.96%) F=31 (0.04%)

### Compiler optimized with `-O1`

./emu-rv32i isPalindrome
-88 is not a palindrome.
10 is not a palindrome.
12321 is a palindrome!
258852 is a palindrome!

>>> Execution time: 58340944 ns
>>> Instruction count: 205361 (IPS=3520015)
>>> Jumps: 87417 (42.57%) - 15 forwards, 87402 backwards
>>> Branching T=87395 (99.94%) F=52 (0.06%)

### Compiler optimized with `-O3`

./emu-rv32i isPalindrome
-88 is not a palindrome.
10 is not a palindrome.
12321 is a palindrome!
258852 is a palindrome!

>>> Execution time: 36123699 ns
>>> Instruction count: 205306 (IPS=5683415)
>>> Jumps: 87401 (42.57%) - 4 forwards, 87397 backwards
>>> Branching T=87397 (99.95%) F=47 (0.05%)

## Compare different codes and results
- `-O1` has the least code lines, `-O0` has the most.
- `-O0` uses registers `s0-s1`, `a0-a5`, `sp`, `ra`. `-O1` uses registers `s0-s7`, `a0-a5`, `sp`, `ra`. `-O3` uses registers `t1, t3-t5`, `a0-a7`, `sp`.
- `-O3` uses the least stack size, `-O0` uses the most.
- `-O3` has the least instruction counts, `-O0` has the most.
- `-O3` and `-O1` have similar instruction counts, the execution time of them are also similar. `-O0` has the longest execution time.

| | -O0 | -O1 | -O3 |
| -------- | -------- | -------- | -------- |
| Code line | 202 | 136 | 148 |
| Register Number | 10 | 16 | 13 |
| Stack Size | 48 | 64 | 16 |
| Instruction count | 703301 | 205361 | 205306 |
| Execution time | 122751361 ns | 58340944 ns| 36123699 ns |