我正在尝试下面的代码来获取Float64
位值的32位部分。
#define LSL_HI(x) *(1+(sInt32*)&x)
#define LSL_LO(x) *(sInt32*)&x
//warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
sInt32 lx = LSL_LO(1111.2222);
[-Wstrict-aliasing]
选项是-O2
优化的一部分,我不必禁用它。
解决此问题的方法是什么?
这个宏是我从GCC数学库中获取的。
如果你想要浮点64的文字字节,你可以这样做:
#include <cstdint>
static_assert(double == 8, "non 64-bit double!");
int
main()
{
double x = 1111.2222;
uint32_t lo = static_cast<uint32_t>(reinterpret_cast<uint64_t>(x) & 0x00000000FFFFFFFF);
uint32_t hi = static_cast<uint32_t>((reinterpret_cast<uint64_t>(x) & 0xFFFFFFFF00000000) >> (8*4));
return 0;
}
我不认为这是你想要做的。如果你想要二重的分数部分和整个部分,你需要尝试其他东西。
我不确定你想实现什么,就像迪伦提到的一样。
如果你想拆开一个double(假设是IEEE 754 double(,你可以这样做(我使用的是ANSI转义序列,这可能不适合你的环境(。我有两个";将其拆分为十六进制字节";以及";分解成符号/指数/分数":
#include <algorithm>
#include <cmath>
#include <cstddef>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <limits>
#include <sstream>
#include <string>
using std::cout;
using std::fpclassify;
using std::memcpy;
using std::nan;
using std::numeric_limits;
using std::reverse;
using std::setw;
using std::size_t;
using std::string;
using std::stringstream;
using std::uint32_t;
using std::uint64_t;
namespace {
uint32_t low32_from(double d) {
char const* p = reinterpret_cast<char const*>(&d);
uint32_t result;
memcpy(&result, p, sizeof result);
return result;
}
uint32_t high32_from(double d) {
char const* p = reinterpret_cast<char const*>(&d);
p += 4;
uint32_t result;
memcpy(&result, p, sizeof result);
return result;
}
string hexstr(uint32_t value) {
char hex[] = "0123456789ABCDEF";
unsigned char buffer[4];
memcpy(buffer, &value, sizeof buffer);
auto p = &buffer[0];
stringstream ss;
char const* sep = "";
for (size_t i = 0; i < sizeof buffer; ++i) {
ss << sep << hex[(*p >> 4) & 0xF] << hex[*p & 0xF];
sep = " ";
++p;
}
return ss.str();
}
string bits(uint64_t v, size_t len) {
string s;
int group = 0;
while (len--) {
if (group == 4) { s.push_back('''); group = 0; }
s.push_back(v & 1 ? '1' : '0');
v >>= 1;
++group;
}
reverse(s.begin(), s.end());
return s;
}
string doublebits(double d) {
auto dx = fpclassify(d);
unsigned char buffer[8];
memcpy(buffer, &d, sizeof buffer);
stringstream ss;
uint64_t s = (buffer[7] >> 7) & 0x1;
uint64_t e = ((buffer[7] & 0x7FU) << 4) | ((buffer[6] >> 4) & 0xFU);
uint64_t f = buffer[6] & 0xFU;
f = (f << 8) + (buffer[5] & 0xFFU);
f = (f << 8) + (buffer[4] & 0xFFU);
f = (f << 8) + (buffer[3] & 0xFFU);
f = (f << 8) + (buffer[2] & 0xFFU);
f = (f << 8) + (buffer[1] & 0xFFU);
f = (f << 8) + (buffer[0] & 0xFFU);
ss << "sign: 33[0;32m" << bits(s, 1) << "