程序员经验分享-hwhale

在 C 中对 int 进行符号扩展

2024-01-21

所以我在获取 int 字段然后对其进行符号扩展时遇到了一些麻烦。我有一个获取 int 字段的方法。

getField(int value, int hi, int lo);

Value 是我从中获取字段的 int，hi 和 lo 是字段的大小。

所以我可以在 getFieldSignExtended(int value, int hi, int lo) 中调用这个 getField 方法，但是如何对其进行符号扩展呢？

e.g. value = 7, hi = 1, lo = 0

getField(7, 1, 0);返回 3，因为二进制 7 是 111，hi 和 lo 取 0 到 1 的字段。

从 getField 返回 3 后，我得到该值等于 0x0003。

到目前为止，我所掌握的内容在积极方面有效，但在消极方面却很混乱。当我说“搞砸”时，我的意思是它根本不起作用。因此，如果我尝试在 -1 上使用它，它会显示为大整数而不是负数。

谢谢你的帮助！ :]

编辑：抱歉，我用一个矛盾的陈述让我自己和你们中的一些人感到困惑：P。固定的。


这里的字里行间有大量的阅读内容。然而，如果getField(7, 1, 0)返回 3 并且你需要getFieldSignExtended(15, 2, 0)回来-3 and getFieldSignExtended(3, 2, 0)回来+3，那么这可能就是您所追求的。

其概念是，将原始值的 hi:lo 位中的 n 位字段视为 2 的补码。如果 n 位中的第一位是 1，则您希望将 n 位字段视为负数。如果 3 位字段的第一位是 0，那么您希望将其视为正数。

#include <assert.h>
#include <limits.h>
#include <stdio.h>

extern int getFieldSignExtended(int value, int hi, int lo);

enum { INT_BITS = CHAR_BIT * sizeof(int) };

int getFieldSignExtended(int value, int hi, int lo)
{
    assert(lo >= 0);
    assert(hi > lo);
    assert(hi < INT_BITS - 1);
    int bits = (value >> lo) & ((1 << (hi - lo + 1)) - 1);
    if (bits & (1 << (hi - lo)))
        return(bits | (~0U << (hi - lo)));
    else
        return(bits);
}

这 3 个断言是直截了当的；唯一有争议的是代码拒绝处理位 31。如果您使用 hi = 31 和 lo = 0 调用它，则移位 (hi - lo + 1) 太大并且行为未定义。您还会遇到右移负数的实现定义行为。可以通过采用无符号整数参数而不执行以下操作来解决这些问题&操作如果hi - lo + 1 == INT_BITS。解决这些问题留给读者作为练习。

分配给bits将值右移，并用正确的位数屏蔽它。这(1 << (hi - lo + 1)) - 1将 1 左移多于字段中的位数，然后减 1，为字段中的每个位位置生成一串二进制 1。例如，对于 hi = 2、lo = 0，将 1 左移 3 位，得到二进制 1000；减 1 得到 0111，因此选择了正确的 3 位。所以，bits包含 n 位整数的适当位组。

The if测试检查是否设置了 n 位整数的最高有效位。如果符号位未设置，我们只需返回该值bits。如果符号位被设置，那么我们就需要执行一个棘手的计算——这个计算在这个答案的初稿中是（非常）错误的。假设我们有一个 3 位字段 = 101。作为 3 位 2 的补码，表示 -3。我们需要将其向左扩展并全为 1 以生成全尺寸-1。的价值~0所有位均为 1；当它左移时hi - lo位，它为值的非符号位留下一系列零。如果你向左移动，它也会起作用hi - lo + 1，但是需要额外的计算+ 1那是没有必要的。

我使用这个测试工具来让自己确信代码工作正常。系统测试输出是严格的（针对较小的数字）。它确保计算值与预期值匹配。 “详尽”测试并不是真正详尽的。它只测试一个值，更多的是用于观察问题（例如在我的机器上使用 hi = 31 和 lo = 0 给出错误答案 0）和模式。

static const struct
{
    int  value;
    int  hi;
    int  lo;
    int  wanted;
} tests[] =
{
    {   0x0F,  1,  0,   -1 },
    {   0x0F,  2,  0,   -1 },
    {   0x0F,  2,  1,   -1 },
    {   0x0F,  3,  1,   -1 },
    {   0x0F,  4,  2,   +3 },
    {   0x0F,  5,  0,  +15 },
    {   0x0F,  5,  1,   +7 },
    {   0x0F,  5,  2,   +3 },
    {   0x0F,  5,  3,   +1 },
    {   0x0F,  5,  4,    0 },
    {   0x03,  2,  0,   +3 },
    {   0xF3,  2,  0,   +3 },
    {   0xF3,  3,  0,   +3 },
    {   0xF3,  4,  0,  -13 },
    {   0xF3,  5,  0,  -13 },
    {   0xF3,  6,  0,  -13 },
    {   0xF3,  7,  0,  -13 },
    {   0xF3,  7,  1,   -7 },
    {   0xF3,  7,  2,   -4 },
    {   0xF3,  7,  3,   -2 },
    {   0xF3,  7,  4,   -1 },
    {   0xF3,  8,  0, 0xF3 },
};
enum { NUM_TESTS = sizeof(tests) / sizeof(tests[0]) };
static const char s_pass[] = "== PASS ==";
static const char s_fail[] = "!! FAIL !!";

static void systematic_test(void)
{
    int fail = 0;
    for (int i = 0; i < NUM_TESTS; i++)
    {
        char const *pf = s_fail;
        int actual = getFieldSignExtended(tests[i].value, tests[i].hi, tests[i].lo);
        if (actual == tests[i].wanted)
            pf = s_pass;
        else
            fail++;
        printf("%s GFSX(%+4d = 0x%.4X, %d, %d) = %+4d = 0x%.8X (wanted %+4d = 0x%.8X)\n",
               pf, tests[i].value, tests[i].value, tests[i].hi, tests[i].lo, actual, actual,
               tests[i].wanted, tests[i].wanted);
    }
    printf("%s\n", (fail == 0) ? s_pass : s_fail);
}

static void exhaustive_test(void)
{
    int value = 0x5FA03CE7;
    for (int i = 1; i < INT_BITS - 1; i++)
    {
        for (int j = 0; j < i; j++)
        {
            int actual = getFieldSignExtended(value, i, j);
            printf("%11sGFSX(%d = 0x%X, %2d, %2d) = %+10d = 0x%.8X\n", "",
                    value, value, i, j, actual, actual);
        }
    }
}

int main(void)
{
    int result1 = getFieldSignExtended(15, 2, 0);
    int result2 = getFieldSignExtended( 3, 2, 0);
    printf("GFSX(15, 2, 0) = %+d = 0x%.8X\n", result1, result1);
    printf("GFSX( 3, 2, 0) = %+d = 0x%.8X\n", result2, result2);

    printf("\nSystematic test\n");
    systematic_test();

    printf("\nExhaustive test\n");
    exhaustive_test();

    return(0);
}

这是详尽测试之前测试代码的输出，加上详尽测试输出的一小部分：

GFSX(15, 2, 0) = -1 = 0xFFFFFFFF
GFSX( 3, 2, 0) = +3 = 0x00000003

Systematic test
== PASS == GFSX( +15 = 0x000F, 1, 0) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 2, 0) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 2, 1) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 3, 1) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX( +15 = 0x000F, 4, 2) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX( +15 = 0x000F, 5, 0) =  +15 = 0x0000000F (wanted  +15 = 0x0000000F)
== PASS == GFSX( +15 = 0x000F, 5, 1) =   +7 = 0x00000007 (wanted   +7 = 0x00000007)
== PASS == GFSX( +15 = 0x000F, 5, 2) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX( +15 = 0x000F, 5, 3) =   +1 = 0x00000001 (wanted   +1 = 0x00000001)
== PASS == GFSX( +15 = 0x000F, 5, 4) =   +0 = 0x00000000 (wanted   +0 = 0x00000000)
== PASS == GFSX(  +3 = 0x0003, 2, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 2, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 3, 0) =   +3 = 0x00000003 (wanted   +3 = 0x00000003)
== PASS == GFSX(+243 = 0x00F3, 4, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 5, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 6, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 7, 0) =  -13 = 0xFFFFFFF3 (wanted  -13 = 0xFFFFFFF3)
== PASS == GFSX(+243 = 0x00F3, 7, 1) =   -7 = 0xFFFFFFF9 (wanted   -7 = 0xFFFFFFF9)
== PASS == GFSX(+243 = 0x00F3, 7, 2) =   -4 = 0xFFFFFFFC (wanted   -4 = 0xFFFFFFFC)
== PASS == GFSX(+243 = 0x00F3, 7, 3) =   -2 = 0xFFFFFFFE (wanted   -2 = 0xFFFFFFFE)
== PASS == GFSX(+243 = 0x00F3, 7, 4) =   -1 = 0xFFFFFFFF (wanted   -1 = 0xFFFFFFFF)
== PASS == GFSX(+243 = 0x00F3, 8, 0) = +243 = 0x000000F3 (wanted +243 = 0x000000F3)
== PASS ==

Exhaustive test
       GFSX(1604336871 = 0x5FA03CE7,  1,  0) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  2,  0) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  2,  1) =         -1 = 0xFFFFFFFF
       GFSX(1604336871 = 0x5FA03CE7,  3,  0) =         +7 = 0x00000007
       GFSX(1604336871 = 0x5FA03CE7,  3,  1) =         +3 = 0x00000003
       GFSX(1604336871 = 0x5FA03CE7,  3,  2) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7,  4,  0) =         +7 = 0x00000007
       GFSX(1604336871 = 0x5FA03CE7,  4,  1) =         +3 = 0x00000003
       GFSX(1604336871 = 0x5FA03CE7,  4,  2) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7,  4,  3) =         +0 = 0x00000000
       GFSX(1604336871 = 0x5FA03CE7,  5,  0) =        -25 = 0xFFFFFFE7
       GFSX(1604336871 = 0x5FA03CE7,  5,  1) =        -13 = 0xFFFFFFF3
       GFSX(1604336871 = 0x5FA03CE7,  5,  2) =         -7 = 0xFFFFFFF9
       GFSX(1604336871 = 0x5FA03CE7,  5,  3) =         -4 = 0xFFFFFFFC
       GFSX(1604336871 = 0x5FA03CE7,  5,  4) =         -2 = 0xFFFFFFFE
       GFSX(1604336871 = 0x5FA03CE7,  6,  0) =        -25 = 0xFFFFFFE7
       GFSX(1604336871 = 0x5FA03CE7,  6,  1) =        -13 = 0xFFFFFFF3
       GFSX(1604336871 = 0x5FA03CE7,  6,  2) =         -7 = 0xFFFFFFF9
       GFSX(1604336871 = 0x5FA03CE7,  6,  3) =         -4 = 0xFFFFFFFC
       GFSX(1604336871 = 0x5FA03CE7,  6,  4) =         -2 = 0xFFFFFFFE
       GFSX(1604336871 = 0x5FA03CE7,  6,  5) =         -1 = 0xFFFFFFFF
...
       GFSX(1604336871 = 0x5FA03CE7, 29, 28) =         +1 = 0x00000001
       GFSX(1604336871 = 0x5FA03CE7, 30,  0) = -543146777 = 0xDFA03CE7
       GFSX(1604336871 = 0x5FA03CE7, 30,  1) = -271573389 = 0xEFD01E73
       GFSX(1604336871 = 0x5FA03CE7, 30,  2) = -135786695 = 0xF7E80F39
       GFSX(1604336871 = 0x5FA03CE7, 30,  3) =  -67893348 = 0xFBF4079C
       GFSX(1604336871 = 0x5FA03CE7, 30,  4) =  -33946674 = 0xFDFA03CE
       GFSX(1604336871 = 0x5FA03CE7, 30,  5) =  -16973337 = 0xFEFD01E7
       GFSX(1604336871 = 0x5FA03CE7, 30,  6) =   -8486669 = 0xFF7E80F3
       GFSX(1604336871 = 0x5FA03CE7, 30,  7) =   -4243335 = 0xFFBF4079
       GFSX(1604336871 = 0x5FA03CE7, 30,  8) =   -2121668 = 0xFFDFA03C
       GFSX(1604336871 = 0x5FA03CE7, 30,  9) =   -1060834 = 0xFFEFD01E
       GFSX(1604336871 = 0x5FA03CE7, 30, 10) =    -530417 = 0xFFF7E80F
       GFSX(1604336871 = 0x5FA03CE7, 30, 11) =    -265209 = 0xFFFBF407
       GFSX(1604336871 = 0x5FA03CE7, 30, 12) =    -132605 = 0xFFFDFA03
       GFSX(1604336871 = 0x5FA03CE7, 30, 13) =     -66303 = 0xFFFEFD01
       GFSX(1604336871 = 0x5FA03CE7, 30, 14) =     -33152 = 0xFFFF7E80
       GFSX(1604336871 = 0x5FA03CE7, 30, 15) =     -16576 = 0xFFFFBF40
       GFSX(1604336871 = 0x5FA03CE7, 30, 16) =      -8288 = 0xFFFFDFA0
       GFSX(1604336871 = 0x5FA03CE7, 30, 17) =      -4144 = 0xFFFFEFD0
       GFSX(1604336871 = 0x5FA03CE7, 30, 18) =      -2072 = 0xFFFFF7E8
       GFSX(1604336871 = 0x5FA03CE7, 30, 19) =      -1036 = 0xFFFFFBF4
       GFSX(1604336871 = 0x5FA03CE7, 30, 20) =       -518 = 0xFFFFFDFA
       GFSX(1604336871 = 0x5FA03CE7, 30, 21) =       -259 = 0xFFFFFEFD
       GFSX(1604336871 = 0x5FA03CE7, 30, 22) =       -130 = 0xFFFFFF7E
       GFSX(1604336871 = 0x5FA03CE7, 30, 23) =        -65 = 0xFFFFFFBF
       GFSX(1604336871 = 0x5FA03CE7, 30, 24) =        -33 = 0xFFFFFFDF
       GFSX(1604336871 = 0x5FA03CE7, 30, 25) =        -17 = 0xFFFFFFEF
       GFSX(1604336871 = 0x5FA03CE7, 30, 26) =         -9 = 0xFFFFFFF7
       GFSX(1604336871 = 0x5FA03CE7, 30, 27) =         -5 = 0xFFFFFFFB
       GFSX(1604336871 = 0x5FA03CE7, 30, 28) =         -3 = 0xFFFFFFFD
       GFSX(1604336871 = 0x5FA03CE7, 30, 29) =         -2 = 0xFFFFFFFE
本文内容由网友自发贡献，版权归原作者所有，本站不承担相应法律责任。如您发现有涉嫌抄袭侵权的内容，请联系:hwhale#tublm.com(使用前将#替换为@)

c

在 C 中对 int 进行符号扩展 的相关文章

随机推荐

热门标签