Assignment3: SoftCPU

contributed by <huang-me>

tags: Computer Architecture

• Assignment3: SoftCPU
  • Workflow
  • 268. Missing Number
    • Original solution
    • Optimization

---

Workflow

268. Missing Number

Since I found that compiler will calculate the result while compiling if optimization level is set to -O3, so all testing in this homework is compare under -O2 level.

Original solution

int missingNumber(int *nums, int len) {
    int res = len;
    for(int i=0; i<len; ++i) {
        res ^= i;
        res ^= nums[i];
    }
    return res;
}

missing: 2

Excuting 1610 instructions, 2086 cycles, 1.295 CPI
Program terminate
- ../rtl/../testbench/testbench.v:418: Verilog $finish

Simulation statistics
=====================
Simulation time  : 0.043 s
Simulation cycles: 2097
Simulation speed : 0.0487674 MHz

Most of the branch predict failed in for loop.

Optimization

int missingNumber(int *nums, int len) {
    int res = len;
    for(int i=0; i<len-1; i+=2) {
        res ^= i;
        res ^= nums[i];
        res ^= i+1;
        res ^= nums[i+1];
    }
    if(len%2) {
        res ^= nums[len-1];
        res ^= len-1;
    }
    return res;
}

missing: 2

Excuting 1604 instructions, 2068 cycles, 1.289 CPI
Program terminate
- ../rtl/../testbench/testbench.v:418: Verilog $finish

Simulation statistics
=====================
Simulation time  : 0.038 s
Simulation cycles: 2079
Simulation speed : 0.0547105 MHz

I use loop unrolling to decrease the amount of branch predict, and make the cycle decrease for 18 cycles.