Convert 0x1234 to 0x11223344

Question

How do I expand the hexadecimal number 0x1234 to 0x11223344 in a high-performance way?

unsigned int c = 0x1234, b;
b = (c & 0xff) << 4 | c & 0xf |          


        

Answer 1

I think that the lookup table approach suggested by Dimitri is a good choice, but I suggest to go one step further and generate the table in compile time; doing the work at compile time will obviously lessen the execution time.

First, we create a compile-time value, using any of the suggested methods:

constexpr unsigned int transform1(unsigned int x)
{
  return ((x << 8) | x);
}

constexpr unsigned int transform2(unsigned int x)
{
  return (((x & 0x00f000f0) << 4) | (x & 0x000f000f));
}

constexpr unsigned int transform3(unsigned int x)
{
  return ((x << 4) | x);
}

constexpr unsigned int transform(unsigned int x)
{
  return transform3(transform2(transform1(x)));
}

// Dimitri version, using constexprs
template  struct aarrggbb_dimitri
{
  static const unsigned int value = transform(argb);
};

// Adam Liss version
template  struct aarrggbb_adamLiss
{
  static const unsigned int value =
    (argb & 0xf000) * 0x11000 +
    (argb & 0x0f00) * 0x01100 +
    (argb & 0x00f0) * 0x00110 +
    (argb & 0x000f) * 0x00011;
};

And then, we create the compile-time lookup table with whatever method we have available, I'll wish to use the C++14 integer sequence but I don't know which compiler will the OP be using. So another possible approach would be to use a pretty ugly macro:

#define EXPAND16(x) aarrggbb::value, \
aarrggbb::value, \
aarrggbb::value, \
aarrggbb::value, \
aarrggbb::value, \
aarrggbb::value, \
aarrggbb::value, \
... and so on

#define EXPAND EXPAND16(0), \
EXPAND16(0x10), \
EXPAND16(0x20), \
EXPAND16(0x30), \
EXPAND16(0x40), \
... and so on

... and so on

See demo here.

PS: The Adam Liss approach could be used without C++11.