Question

I don’t understand how the following C conversion functions work (and why they’re written this way); I’m fairly certain that the original author knew what he was doing:

typedef union TValue {
  uint64_t u64;
  double n;
  struct {
    uint32_t lo;    /* Lower 32 bits of number. */
    uint32_t hi;    /* Upper 32 bits of number. */
  } u32;
  [...]
} TValue;


static int32_t num2bit(double n)
{
  TValue o;
  o.n = n + 6755399441055744.0;  /* 2^52 + 2^51 */
  return (int32_t)o.u32.lo;
}

static uint64_t num2u64(double n)
{
#ifdef _MSC_VER
  if (n >= 9223372036854775808.0)  /* They think it's a feature. */
    return (uint64_t)(int64_t)(n - 18446744073709551616.0);
  else
#endif
  return (uint64_t)n;
}

• Does num2bit actually just cast a double into int32_t? Why the addition? Why write it like this?
• What is this “feature” that is alluded to in num2u64? (I believe _MSC_VER means it’s the code-path for Microsofts C compiler).

Note that those functions are not always used (depending on CPU architecture), this is for little-endian (I resolved some preprocessor macros to simplify).

Links to online browseable mirror (the code is from the LuaJIT project):
Surrounding Header file (or whole project).

Every hint is appreciated.

Answer 1

num2bit is designed to implement the Lua BitOp semantics especially wrt. modular arithmetic. The implementation-defined behavior is well under control, since LuaJIT only works for specific CPUs, platforms and compilers, anyway. Don’t use this code anywhere else.

num2u64 is a workaround for a bug/misfeature of MSVC where it always converts double to uint64_t via int64_t. This doesn’t give the desired results for numbers >= 2^63. MS considers this abomination a ‘feature’. Duh.