---
tags: ncku-course
---

# 2016q3 Homework1 (clz)

### Reviewed by `HaoTse`
- 尚未實作 Harley's Algorithm
- 沒有看到**驗證正確性**的結果輸出
- 沒有對效能比較圖做分析
- 缺少提出 clz 的應用
- [Initial commit](https://github.com/0140454/clz-tests/commit/34afcb41bfa2c47594853cecb10605af4781cb61) 看不出具體做了哪些事

## 各版本分析

### iterative

int clz(uint32_t x)
{
    int n = 32, c = 16;
    do {
        uint32_t y = x >> c;
        if (y) { n -= c; x = y; }
        c >>= 1;
    } while (c);
    return (n - x);
}

### binary search

int clz(uint32_t x)
{
    if (x == 0) return 32;
    int n = 0;
    if (x <= 0x0000FFFF) { n += 16; x <<= 16; }
    if (x <= 0x00FFFFFF) { n += 8; x <<= 8; }
    if (x <= 0x0FFFFFFF) { n += 4; x <<= 4; }
    if (x <= 0x3FFFFFFF) { n += 2; x <<= 2; }
    if (x <= 0x7FFFFFFF) { n += 1; x <<= 1; }
    return n;
}

利用二分法的技巧，32-bit 需要 $log_{2}32 = 5$ 次，64-bit 則是 $log_{2}64 = 6$ 次。

### byte shift

int clz(uint32_t x)
{
    if (x == 0) return 32;
    int n = 1;
    if ((x >> 16) == 0) { n += 16; x <<= 16; }
    if ((x >> 24) == 0) { n += 8; x <<= 8; }
    if ((x >> 28) == 0) { n += 4; x <<= 4; }
    if ((x >> 30) == 0) { n += 2; x <<= 2; }
    n = n - (x >> 31);
    return n;
}

### recursive

作業說明中的 recursive 並沒有指定終止條件，而且 shift offset 和 mask 不應該是固定的。

int clz(uint32_t x)
{
	/* shift upper half down, rest is filled up with 0s */
	uint16_t upper = (x >> 16);
	// mask upper half away
	uint16_t lower = (x & 0xFFFF);
	return upper ? clz(upper) : 16 + clz(lower);
}

修正後變成

static uint32_t offset[] = { 16, 8, 4, 2, 1 };
static uint32_t mask[] = { 0xFFFF, 0x00FF, 0x000F, 0x0003, 0x0001 };

static int clz_helper(uint32_t x, int i)
{
    /* shift upper half down, rest is filled up with 0s */
    uint16_t upper = (x >> offset[i]);
    // mask upper half away
    uint16_t lower = (x & mask[i]);

    if (i == 4) return !upper;

    return upper ? clz_helper(upper, i + 1) : offset[i] + clz_helper(lower, i + 1);
}

int clz(uint32_t x)
{
    if (!x) return 32;

    clz_helper(x, 0);
}

### Harley

#define u 99

int clz(uint32_t x)
{
    static uint8_t const Table[] = {
        32,31, u,16, u,30, 3, u,  15, u, u, u,29,10, 2, u,
         u, u,12,14,21, u,19, u,   u,28, u,25, u, 9, 1, u,
        17, u, 4, u, u, u,11, u,  13,22,20, u,26, u, u,18,
         5, u, u,23, u,27, u, 6,   u,24, 7, u, 8, u, 0, u
    };

    /* Propagate leftmost 1-bit to the right */
    x = x | (x >> 1);
    x = x | (x >> 2);
    x = x | (x >> 4);
    x = x | (x >> 8);
    x = x | (x >> 16);

    /* x = x * 0x6EB14F9 */
    x = (x << 3) - x;   /* Multiply by 7. */
    x = (x << 8) - x;   /* Multiply by 255. */
    x = (x << 8) - x;   /* Again. */
    x = (x << 8) - x;   /* Again. */

    return Table[x >> 26];
}

若要計算 CLZ，原本的表格應該修正成上面的表格才對。
Harley 的原理還要在研究一下...

## 測試方式

### 驗證正確性

走訪全部的 32-bit 數值，為了加速執行時間，所以使用了 OpenMP。

if (clz(0) != 32) {
    printf("Error on 0\n");
}

#pragma omp parallel for
for (uint32_t i = 1; i <= 0xFFFFFFFE; ++i) {
    if (clz(i) != __builtin_clz(i)) {
        printf("Error on %u\n", i);
    }
}

### 效能比較


## 參考資料

* [Number of leading zeros algorithms. - Hacker's Delight](http://www.hackersdelight.org/hdcodetxt/nlz.c.txt)