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# Homework 3 : SoftCPU
###### tags: `Computer Architecture 2022`
## Prerequisites
First, we need to finish [Lab2: RISC-V RV32I[MACF] emulator with ELF support](https://hackmd.io/@sysprog/SJAR5XMmi).
1. Prepare GNU Toolchain for RISC-V. See The xPack GNU RISC-V Embedded GCC
```
$ cd /tmp
$ wget https://github.com/xpack-dev-tools/riscv-none-elf-gcc-xpack/releases/download/v12.2.0-1/xpack-riscv-none-elf-gcc-12.2.0-1-linux-x64.tar.gz
$ tar zxvf xpack-riscv-none-elf-gcc-12.2.0-1-linux-x64.tar.gz
$ cp -af xpack-riscv-none-elf-gcc-12.2.0-1 $HOME/riscv-none-elf-gcc
```
2. Configure $PATH
```
$ cd $HOME/riscv-none-elf-gcc
$ echo "export PATH=`pwd`/bin:$PATH" > setenv
```
3. Once step (1) and (2) are complete, you can simply update $PATH environment variable via:
```
$ cd $HOME
$ source riscv-none-elf-gcc/setenv
```
4. Then, do the steps of [Lab3: srv32 - RISCV RV32IM Soft CPU](https://hackmd.io/@sysprog/S1Udn1Xtt)
```
export CROSS_COMPILE=riscv-none-elf-
sudo apt install autoconf automake autotools-dev curl gawk git \
build-essential bison flex texinfo gperf libtool patchutils bc git \
libmpc-dev libmpfr-dev libgmp-dev gawk zlib1g-dev libexpat1-dev
git clone --recursive https://github.com/riscv/riscv-gnu-toolchain
cd riscv-gnu-toolchain
mkdir -p build && cd build
../configure --prefix=/opt/riscv --with-isa-spec=20191213 \
--with-multilib-generator="rv32i-ilp32--;rv32im-ilp32--;rv32imac-ilp32--;rv32im_zicsr-ilp32--;rv32imac_zicsr-ilp32--;rv64imac-lp64--;rv64imac_zicsr-lp64--"
make -j$(nproc)
```
When I do `make -j$(nproc)` , I find a problem. It said it can't create directory.
```
mkdir: 無法建立 「/opt/riscv」 目錄: 拒絕不符權限的操作
mkdir: 無法建立 「/opt/riscv」 目錄: 拒絕不符權限的操作
make[2]: *** [Makefile:2818：installdirs] 錯誤 1
make[2]: 離開目錄「/home/hung/riscv-gnu-toolchain/build/build-gdb-newlib」
make[1]: *** [Makefile:2558：install] 錯誤 2
make[1]: 離開目錄「/home/hung/riscv-gnu-toolchain/build/build-gdb-newlib」
make: *** [Makefile:537：stamps/build-gdb-newlib] 錯誤 2
```
I manually created the directory and set permissions.
```
cd $HOME
cd /opt
sudo mkdir riscv
sudo chmod 777 /opt/riscv
```
And check if the directory have read and write permissions. After doing this, it can run smoothly.
```
hung@hung-MS-7B79:/opt$ ls -l
總用量 4
drwxrwxrwx 2 root root 4096 11月 21 23:49 riscv
```
5. Install the dependent packages.
```
sudo apt install build-essential lcov ccache libsystemc-dev
```
6. Install `verilator` and set environment variable
```diff
cd $HOME
git clone https://github.com/verilator/verilator
cd verilator
git checkout stable
export VERILATOR_ROOT=`pwd`
+ autoconf // Create ./configure script, or it can find the directory of configure
./configure
make
export VERILATOR_ROOT=$HOME/verilator
export PATH=$VERILATOR_ROOT/bin:$PATH
```
7. Get the source
```
git clone https://github.com/sysprog21/srv32
```
## Run RTL sim and ISS sim
Then we can use the following command to run RTL sim and ISS sim.
```
cd srv32
make hw3
```
The following result is from my [Homework 2 - RISC-V Toolchain](https://hackmd.io/KqhB3hrVRHGjFbx91qQIcg)
### HW2 Simulation results
RTL Simulation result
```
Input:[1,2,3,4,5,]
Output:[5,4,3,2,1,]
Input:[1,2,]
Output:[2,1,]
Input:[1,2,3,4,5,6,7,8,9,10,]
Output:[10,9,8,7,6,5,4,3,2,1,]
Excuting 34642 instructions, 45808 cycles, 1.322 CPI
Program terminate
- ../rtl/../testbench/testbench.v:434: Verilog $finish
Simulation statistics
=====================
Simulation time : 0.213 s
Simulation cycles: 45819
Simulation speed : 0.215113 MHz
```
ISS (Instruction Set Simulator) result
```
Input:[1,2,3,4,5,]
Output:[5,4,3,2,1,]
Input:[1,2,]
Output:[2,1,]
Input:[1,2,3,4,5,6,7,8,9,10,]
Output:[10,9,8,7,6,5,4,3,2,1,]
Excuting 34642 instructions, 45808 cycles, 1.322 CPI
Program terminate
Simulation statistics
=====================
Simulation time : 0.017 s
Simulation cycles: 45808
Simulation speed : 2.688 MHz
```
### HW3 C code
And I choose another problem from [leetcode 15 - 3Sum](https://leetcode.com/problems/3sum/). The following is my C code
```
#include "stdlib.h"
#include "stdio.h"
void bubblesort(int* nums, int numsSize){
for (int i = 0; i < numsSize - 1; i++){
for (int j = 0; j < numsSize - i - 1; j++){
if (nums[j] > nums[j + 1]){
int temp = nums[j];
nums[j] = nums[j + 1];
nums[j + 1] = temp;
}
}
}
}
int BinarySearch (int *a, int head, int tail, int x){
int left = head, right = tail-1;
while (left <= right) {
int middle = (left + right)/2;
if (x < a[middle]) right = middle - 1;
else if (x > a[middle]) left = middle + 1;
else return middle;
}
return -1;
}
int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes){
int time = 0;
bubblesort(nums,numsSize);
int** ans = malloc(sizeof(int*));
for(int i = 0;i<numsSize-2;i++){
for(int j = numsSize-1;j>=i+2;j--){
int total = 0 - nums[i] - nums[j];
int index = BinarySearch(nums,i+1,j,total);
if(index >= 0){
ans = realloc(ans, sizeof(int*)*(time+1));
ans[time] = (int*) malloc(sizeof(int) * 3);
ans[time][0] = nums[i];
ans[time][1] = nums[index];
ans[time][2] = nums[j];
time++;
}
while(nums[j] == nums[j-1]){
j--;
if(j <= i+1)
break;
}
}
while(nums[i] == nums[i+1]){
i++;
if(i >= numsSize-2)
break;
}
}
*returnSize = time;
*returnColumnSizes = malloc(sizeof(int*) * (*returnSize));
for (int i = 0; i < (*returnSize); i++) {
(*returnColumnSizes)[i] = 3;
}
return ans;
}
void printnums(int* nums, int numsSize){
printf("intput : [");
for(int i = 0;i<numsSize;i++){
printf(" %d ",nums[i]);
}
printf("]\n");
}
void printans(int** ans, int* returnSize, int** returnColumnSizes){
for(int i = 0;i<(*returnSize);i++){
printf("output : [");
for(int j = 0;j<(*returnColumnSizes)[i];j++){
printf(" %d ",ans[i][j]);
}
printf("]\n");
}
}
int main(){
int nums[] = {-1,0,1,2,-1,4};
int numsSize = 6;
printnums(nums,numsSize);
int* returnSize = malloc(sizeof(int));
int** returnColumnSizes = malloc(sizeof(int*));
int** ans = threeSum(nums,numsSize,returnSize,returnColumnSizes);
printans(ans,returnSize,returnColumnSizes);
}
```
### HW3 assembly code
This is my RISC-V assembly code.
:::spoiler
```
.data
test1:.word -7,-5,3,5,-2,0,1,-1
len1: .word 8
returnSize: .word 0x40000000
returnColumnSize: .word 0x40000100
start_input_line:.string "Input:["
start_output_line:.string "Output:["
endline:.string "]\n"
.text
main:
la a0,test1
lw a1,len1
jal printinput
la a0,test1
lw a1,len1
lw a2,returnSize
lw a3,returnColumnSize
jal threeSum
mv a1,a2
mv a2,a3
jal printans
j exit
printinput:
addi sp,sp,-8
sw a0,4(sp)
sw ra,0(sp)
li t0,0
la a0, start_input_line
li a7, 4
ecall
printinputloop:
slli t1,t0,2
lw s1,4(sp)
add s1,s1,t1
lw a0,0(s1)
jal printf
addi t0,t0,1
blt t0,a1,printinputloop
la a0, endline
li a7, 4
ecall
lw ra,0(sp)
addi sp,sp,8
jr ra
printans:
addi sp,sp,-8
sw a0,4(sp)
sw ra,0(sp)
lw t5,0(a1)
li t3,0
printoutputloop:
li t0,0
la a0, start_output_line
li a7, 4
ecall
lw t4,0(a2)
printoutput:
slli t1,t0,2
lw s1,4(sp)
add s1,s1,t1
lw a0,0(s1)
jal printf
addi t0,t0,1
blt t0,t4,printoutput
la a0, endline
li a7, 4
ecall
addi a2,a2,4
lw a0,4(sp)
addi a0,a0,12
sw a0,4(sp)
addi t3,t3,1
blt t3,t5,printoutputloop
lw ra,0(sp)
addi sp,sp,8
jr ra
sort:
addi sp,sp,-4
sw a0,0(sp)
li t1,0
addi a1,a1,-1
sortloop1:
bge t1,a1,done # t1 = i outerloop
sub t3,a1,t1 # t3 = numsSize-i-1
li,t2,0
lw a0,0(sp)
sortloop2:
bge t2,t3,sortloop1done # t2 = j innerloop
lw t4,0(a0) # load nums[j]
lw t5,4(a0) # load nums[j+1]
bge t4,t5,swap
j sortloop2done
swap:
sw t5,0(a0)
sw t4,4(a0)
sortloop2done:
addi a0,a0,4
addi t2,t2,1
j sortloop2
sortloop1done:
addi t1,t1,1
j sortloop1
done:
lw a0,0(sp)
addi sp,sp,4
jr ra
BinarySearch:
addi sp,sp,-8
sw ra,4(sp)
sw a0,0(sp)
mv t0,a1 # left = hand
addi t1,a2,-1 # tail - 1
Searchloop:
lw a0,0(sp)
blt t1,t0,BSdone # if right < left go to BSdone
add t3,t0,t1 # middle = (left+right)/2
srli t3,t3,1
slli t5,t3,2
add a0,a0,t5
lw t4,0(a0) # a[middle]
blt a3,t4, atleft # if a[middle] < x , go to atleft
beq a3,t4,get # if a[middle] == x , go to get
addi t0,t3,1
j Searchloop
atleft:
addi t1,t3,-1
j Searchloop
get:
lw ra,4(sp)
addi sp,sp,8
mv a0,t3
jr ra
BSdone:
lw ra,4(sp)
addi sp,sp,8
li t2,-1
mv a0,t2
jr ra
threeSum:
addi sp,sp,-24
sw a3,20(sp)
sw a2,16(sp)
sw a1,8(sp)
sw ra,4(sp)
sw a0,0(sp)
li s0,0x20000000 #int** ans
sw s0,12(sp)
li s1,0 # s1 = time
jal sort
lw a1,8(sp)
li s10,-1 # t4 = i
threeSumOuterloop:
addi s10,s10,1
addi s9,a1,-2 # s9 = numsSize-2
bge s10,s9,threeSumEnd # if i >= numsSize-2, go to threeSumEnd
mv s11,a1 # s11 = numsSize
threeSumInnerloop:
addi s11,s11,-1
addi t6,s10,2
bge s11,t6,Search # if j >= i+2 go to Search, else go to Outerloop
j findOutersame
Search:
lw a0,0(sp)
slli s2,s10,2
add a0,a0,s2
lw s2,0(a0) # s2 = nums[i]
lw a0,0(sp)
slli s3,s11,2
add a0,a0,s3
lw s3,0(a0) # s3 = nums[j]
add s4,s2,s3
sub s4,x0,s4 # s4 = total
lw a0,0(sp)
addi a1,s10,1
mv a2,s11
mv a3,s4
jal BinarySearch
mv s8,a0 # s8 = index
lw a0,0(sp)
lw a1,8(sp)
lw ra,4(sp)
bge s8,x0,gettriplets
findInnersame:
slli s5,s11,2
lw a0,0(sp)
add a0,a0,s5
lw s5,0(a0) # s5 = nums[j]
lw s6,-4(a0)
beq s5,s6,findInner
j threeSumInnerloop
findInner:
addi s11,s11,-1
li s6,1
ble s11,s6,threeSumInnerloop
j findInnersame
findOutersame:
slli s6,s10,2
lw a0,0(sp)
add a0,a0,s6
lw s5,0(a0) # s6 = nums[j]
lw s6,4(a0)
beq s5,s6,findOuter
j threeSumOuterloop
findOuter:
addi s10,s10,1
bgt s10,s9,threeSumOuterloop
j findOutersame
gettriplets:
mv a0,s8
li s7,12
mul s7,s1,s7
lw s0,12(sp)
add s0,s0,s7
slli s8,s8,2
lw a0,0(sp)
add s8,a0,s8
lw s8,0(s8) # num[index]
sw s2,0(s0)
sw s8,4(s0)
sw s3,8(s0)
lw a0,0(sp)
addi s1,s1,1
j findInnersame
threeSumEnd:
lw a3,20(sp)
lw a2,16(sp)
sw s1,0(a2)
li t3,3
columnsize:
sw t3,0(a3)
addi s1,s1,-1
addi a3,a3,4
li t4,1
bge s1,t4,columnsize
lw s0,12(sp)
mv a0,s0
lw ra,4(sp)
lw a3,20(sp)
lw a2,16(sp)
addi sp,sp,24
jr ra
printf:
li a7,1 # print int
ecall
li a0, 9 # tab
li a7, 11 # print char
ecall
jr ra
exit:
li a7, 10 # end the program
ecall
```
:::
### HW3 simulation result
RTL Simulation result
```
intput : [ -1 3 2 0 -2 -4 3 4 1 0 -3 ]
output : [ -4 0 4 ]
output : [ -4 1 3 ]
output : [ -3 -1 4 ]
output : [ -3 0 3 ]
output : [ -3 1 2 ]
output : [ -2 -1 3 ]
output : [ -2 0 2 ]
output : [ -1 0 1 ]
Excuting 46245 instructions, 61775 cycles, 1.335 CPI
Program terminate
- ../rtl/../testbench/testbench.v:434: Verilog $finish
Simulation statistics
=====================
Simulation time : 0.256 s
Simulation cycles: 61786
Simulation speed : 0.241352 MHz
```
ISS (Instruction Set Simulator) result
```
./rvsim --memsize 128 -l trace.log ../sw/hw3_2/hw3_2.elf
intput : [ -1 3 2 0 -2 -4 3 4 1 0 -3 ]
output : [ -4 0 4 ]
output : [ -4 1 3 ]
output : [ -3 -1 4 ]
output : [ -3 0 3 ]
output : [ -3 1 2 ]
output : [ -2 -1 3 ]
output : [ -2 0 2 ]
output : [ -1 0 1 ]
Excuting 46245 instructions, 61775 cycles, 1.336 CPI
Program terminate
Simulation statistics
=====================
Simulation time : 0.013 s
Simulation cycles: 61775
Simulation speed : 4.861 MHz
```
## Waveform analysis
We can use the following command to create a wave.fst file.
```
sudo apt-get gtkwave # install gtkwave
cd srv32/sim
make hw3.run
cd sim && ./sim +dump
```
Then we can use gtkwave to open the file.
### Analysis
||| IF/ID | EX | WB |
| -------- | -------- | -------- |-------- | -------- |
| next_pc| fetch_pc (imem_addr)| if_pc |ex_pc|wb_pc|
### Branch Penalty
#### branch
```
20: 0002a023 sw zero,0(t0)
24: 00428293 addi t0,t0,4
28: fe62ece3 bltu t0,t1,20 <_bss_clear>
2c: 00040117 auipc sp,0x40
30: fd410113 addi sp,sp,-44 # 40000 <_stack>
34: 268000ef jal ra,29c <main>
38: 6f41006f j 1072c <exit>
```
First, let discuss how the branch work. In `line 28`, if `t0` > `t1`, it will jump to `line 20`. The `branch_taken` will be set to 1. The following result is from gtkwave.
If `branch_taken` equal 1, that mean two instructions should be killed. In next clock, it will fetch the correct instruction.
| stage | c1 | c2 | c3|c4|c5|
| -------- | -------- | -------- |-------- |-------- |-------- |
| IF/ID | bltu | auipc | addi (30)|sw|addi (24)
| EX | addi | bltu (taken) | NOP|NOP|sw|
| WB | sw | addi | bltu|NOP|NOP|
#### branch if not taken
In `line 28`, if `t0` < `t1`, it will not jump to `line20`. It will go to the next instruction. In `srv32`, branch always fetch next instruction, so it will not kill any instruction
| stage | c1 | c2 | c3|c4|
| -------- | -------- | -------- |-------- |-------- |
| IF/ID | bltu | auipc | addi (30)| jal|
| EX | addi | bltu | auipc|addi (30)|
| WB | sw | addi | bltu|auipc|
#### jump
In `line 34` , it will jump to `main` and link `ra`. When execute `jal`, it also set `branch_taken` to 1. That mean two instructions should be killed.
| stage | c1 | c2 | c3|c4|
| -------- | -------- | -------- |-------- |-------- |
| IF/ID | jal | j | NOP|main|
| EX | addi | jal (taken) | NOP|NOP|
| WB | auipc | addi | jal|NOP|
### Forwarding
We can observe that `auipc` instruction need to pass the value to `addi` and `addi` need to pass the value to `add`.
```
3f4: 00021997 auipc s3,0x21
3f8: a9c98993 addi s3,s3,-1380 # 20e90 <__malloc_av_>
3fc: 00f987b3 add a5,s3,a5
```
The following is the result of `gtkwave`.
In this case, We can observe that if instruction need bypass. The previous instruction will transform the value of `ex_result` to `reg_rdata1`. We also can observe that if instruction need bypass, `wb_dst_sel` and `ex_src1_sel` will be the same. The implementation of register forwarding is as follow so we can know how the bypass work.
```
assign reg_rdata1[31: 0] = (ex_src1_sel == 5'h0) ? 32'h0 :
(!wb_flush && wb_alu2reg &&
(wb_dst_sel == ex_src1_sel)) ? // register forwarding
(wb_mem2reg ? wb_rdata : wb_result) :
regs[ex_src1_sel];
assign reg_rdata2[31: 0] = (ex_src2_sel == 5'h0) ? 32'h0 :
(!wb_flush && wb_alu2reg &&
(wb_dst_sel == ex_src2_sel)) ? // register forwarding
(wb_mem2reg ? wb_rdata : wb_result) :
regs[ex_src2_sel];
```
Than we can see there are full bypassing.
## Software optimizations
After the analysis of gtkwave, we know that branch will waste two cycle. I try to use `loop unrolling` to reduce cycles. The following is the modified C code. But I find that if the input array size is too small, it will take more cycle to complete it. Because I need some condition to make `loop unrolling` correct.
```diff
int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes){
int time = 0;
bubblesort(nums,numsSize);
int** ans = malloc(sizeof(int*));
for(int i = 0;i<numsSize-2;i++){
for(int j = numsSize-1;j>=i+2;j--){
int total = 0 - nums[i] - nums[j];
int index = BinarySearch(nums,i+1,j,total);
if(index >= 0){
ans = realloc(ans, sizeof(int*)*(time+1));
ans[time] = (int*) malloc(sizeof(int) * 3);
ans[time][0] = nums[i];
ans[time][1] = nums[index];
ans[time][2] = nums[j];
time++;
}
while(nums[j] == nums[j-1]){
j--;
if(j <= 1)
break;
}
}
- while(nums[i] == nums[i+1]){
- i++;
- if(i >= numsSize-2)
- break;
- }
+ i = i+1;
+ while(nums[i-1] == nums[i]){
+ i++;
+ if(i >= numsSize-1)
+ break;
+ }
+ if(i >= numsSize-1)
+ break;
+ for(int j = numsSize-1;j>=i+2;j--){
+ int total = 0 - nums[i] - nums[j];
+ int index = BinarySearch(nums,i+1,j,total);
+ if(index >= 0){
+ ans = realloc(ans, sizeof(int*)*(time+1));
+ ans[time] = (int*) malloc(sizeof(int) * 3);
+ ans[time][0] = nums[i];
+ ans[time][1] = nums[index];
+ ans[time][2] = nums[j];
+ time++;
+ }
+ while(nums[j] == nums[j-1]){
+ j--;
+ if(j <= i+1)
+ break;
+ }
+ }
+ while(nums[i] == nums[i+1]){
+ i++;
+ if(i >= numsSize-2)
+ break;
+ }
}
*returnSize = time;
*returnColumnSizes = malloc(sizeof(int*) * (*returnSize));
for (int i = 0; i < (*returnSize); i++) {
(*returnColumnSizes)[i] = 3;
}
return ans;
}
```
RTL Simulation result
```
intput : [ -1 3 2 0 -2 -4 3 4 1 0 -3 ]
output : [ -4 0 4 ]
output : [ -4 1 3 ]
output : [ -3 -1 4 ]
output : [ -3 0 3 ]
output : [ -3 1 2 ]
output : [ -2 -1 3 ]
output : [ -2 0 2 ]
output : [ -1 0 1 ]
Excuting 46224 instructions, 61746 cycles, 1.335 CPI
Program terminate
- ../rtl/../testbench/testbench.v:434: Verilog $finish
Simulation statistics
=====================
Simulation time : 0.256 s
Simulation cycles: 61757
Simulation speed : 0.241238 MHz
```
ISS (Instruction Set Simulator) result
```
./rvsim --memsize 128 -l trace.log ../sw/hw3_2/hw3_2.elf
intput : [ -1 3 2 0 -2 -4 3 4 1 0 -3 ]
output : [ -4 0 4 ]
output : [ -4 1 3 ]
output : [ -3 -1 4 ]
output : [ -3 0 3 ]
output : [ -3 1 2 ]
output : [ -2 -1 3 ]
output : [ -2 0 2 ]
output : [ -1 0 1 ]
Excuting 46224 instructions, 61746 cycles, 1.336 CPI
Program terminate
Simulation statistics
=====================
Simulation time : 0.014 s
Simulation cycles: 61746
Simulation speed : 4.545 MHz
```
I try to add some code to make the loop as sooner it ends as possible (Add break). It will not jump to another instruction and judge if the loop is complete.
```diff
int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes){
int time = 0;
bubblesort(nums,numsSize);
int** ans = malloc(sizeof(int*));
for(int i = 0;i<numsSize-2;i++){
for(int j = numsSize-1;j>=i+2;j--){
int total = 0 - nums[i] - nums[j];
int index = BinarySearch(nums,i+1,j,total);
if(index >= 0){
ans = realloc(ans, sizeof(int*)*(time+1));
ans[time] = (int*) malloc(sizeof(int) * 3);
ans[time][0] = nums[i];
ans[time][1] = nums[index];
ans[time][2] = nums[j];
time++;
}
while(nums[j] == nums[j-1]){
j--;
if(j <= i+1)
break;
}
+ if(j <= i+1)
+ break;
}
i = i +1;
while(nums[i-1] == nums[i]){
i++;
if(i >= numsSize-1)
break;
}
if(i >= numsSize-1)
break;
for(int j = numsSize-1;j>=i+2;j--){
int total = 0 - nums[i] - nums[j];
int index = BinarySearch(nums,i+1,j,total);
if(index >= 0){
ans = realloc(ans, sizeof(int*)*(time+1));
ans[time] = (int*) malloc(sizeof(int) * 3);
ans[time][0] = nums[i];
ans[time][1] = nums[index];
ans[time][2] = nums[j];
time++;
}
while(nums[j] == nums[j-1]){
j--;
if(j <= i+1)
break;
}
+ if(j <= i+1)
+ break;
}
while(nums[i] == nums[i+1]){
i++;
if(i >= numsSize-2)
break;
}
if(i >= numsSize-2)
break;
}
*returnSize = time;
*returnColumnSizes = malloc(sizeof(int*) * (*returnSize));
for (int i = 0; i < (*returnSize); i++) {
(*returnColumnSizes)[i] = 3;
}
return ans;
}
```
RTL Simulation result
```
intput : [ -1 3 2 0 -2 -4 3 4 1 0 -3 ]
output : [ -4 0 4 ]
output : [ -4 1 3 ]
output : [ -3 -1 4 ]
output : [ -3 0 3 ]
output : [ -3 1 2 ]
output : [ -2 -1 3 ]
output : [ -2 0 2 ]
output : [ -1 0 1 ]
Excuting 46177 instructions, 61701 cycles, 1.336 CPI
Program terminate
- ../rtl/../testbench/testbench.v:434: Verilog $finish
Simulation statistics
=====================
Simulation time : 0.254 s
Simulation cycles: 61712
Simulation speed : 0.242961 MHz
```
ISS (Instruction Set Simulator) result
```
./rvsim --memsize 128 -l trace.log ../sw/hw3_2/hw3_2.elf
intput : [ -1 3 2 0 -2 -4 3 4 1 0 -3 ]
output : [ -4 0 4 ]
output : [ -4 1 3 ]
output : [ -3 -1 4 ]
output : [ -3 0 3 ]
output : [ -3 1 2 ]
output : [ -2 -1 3 ]
output : [ -2 0 2 ]
output : [ -1 0 1 ]
Excuting 46177 instructions, 61701 cycles, 1.336 CPI
Program terminate
Simulation statistics
=====================
Simulation time : 0.013 s
Simulation cycles: 61701
Simulation speed : 4.792 MHz
```
According to these changes, we can save 61775 - 61701 = 74 cycles
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