# 2017q1 Homework1 (clz)
contributed by <`Weinux`>
###### tags: `sysprog2017` `w1` `clz` `Weinux`

## 開發環境

* OS：Lubuntu 16.04 LTS
* Linux 系統版本: 4.4.0-63-generic
* Architecture: x86_64
* CPU 作業模式： 32-bit, 64-bit
* Byte Order: Little Endian
* CPU: Intel(R) Core(TM) i5-2410M CPU @ 2.30GHz
* Model name: Intel(R) Core(TM) i5-2410M CPU @ 2.30GHz
* Memory: 6GB (5870MB)
* Cache:
* L1d cache: 32K
* L1i cache: 32K
* L2 cache: 256K
* L3 cache: 3072K

## CLZ

比較 32-bit 數值對以下實做的效能差異：

- 原始效能比較

$ make 
$ make plot


### recursive version

#include "clz2.h"

static const int mask[]={0,8,12,14};
static const int magic[]={2,1,0,0};

unsigned clz2(uint32_t x,int c)
{
    if (!x && !c) return 32;

    uint32_t upper = (x >> (16>>c)); 
    uint32_t lower = (x & (0xFFFF>>mask[c]));
    if (c == 3) return upper ? magic[upper] : 2 + magic[lower];
    return upper ? clz2(upper, c + 1) : (16 >> (c)) + clz2(lower, c + 1);
}

### iteration version

#include "clz.h"

static inline __attribute((always_inline))
unsigned 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 technique

#include "clz.h"

static inline __attribute((always_inline))
unsigned 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;
}

### byte-shift version

#include "clz.h"

static inline __attribute((always_inline))
unsigned 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;
}

### Harley’s algorithm

#include "clz.h"

static inline __attribute((always_inline))
unsigned clz(uint32_t x)
{
    // CTZ table
#ifdef CTZ
    static uint8_t const Table[] = {
        0xFF, 0, 0xFF, 15, 0xFF, 1, 28, 0xFF,
        16, 0xFF, 0xFF, 0xFF, 2, 21, 29, 0xFF,
        0xFF, 0xFF, 19, 17, 10, 0xFF, 12, 0xFF,
        0xFF, 3, 0xFF, 6, 0xFF, 22, 30, 0xFF,
        14, 0xFF, 27, 0xFF, 0xFF, 0xFF, 20, 0xFF,
        18, 9, 11, 0xFF, 5, 0xFF, 0xFF, 13,
        26, 0xFF, 0xFF, 8, 0xFF, 4, 0xFF, 25,
        0xFF, 7, 24, 0xFF, 23, 0xFF, 31, 0xFF,
    };

    // CLZ table
#else
    static uint8_t const Table[] ={
        32,31, 0,16, 0,30, 3, 0,15, 0, 0, 0,29,10, 2, 0,
        0, 0,12,14,21, 0,19, 0, 0,28, 0,25, 0, 9, 1, 0,
        17, 0, 4, 0, 0, 0,11, 0,13,22,20, 0,26, 0, 0,18,
        5, 0, 0,23, 0,27, 0, 6,0,24, 7, 0, 8, 0, 0, 0
    };
#endif

    /* 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)];
}

## 參考資料