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'm not sure what the most efficient way would be, but this is a little shorter:

#include <stdio.h>

int main()
{
  unsigned x = 0x1234;

  x = (x << 8) | x;
  x = ((x & 0x00f000f0) << 4) | (x & 0x000f000f);
  x = (x << 4) | x;

  printf("0x1234 -> 0x%08x\n",x);

  return 0;
}

---

If you need to do this repeatedly and very quickly, as suggested in your edit, you could consider generating a lookup table and using that instead. The following function dynamically allocates and initializes such a table:

unsigned *makeLookupTable(void)
{
  unsigned *tbl = malloc(sizeof(unsigned) * 65536);
  if (!tbl) return NULL;
  int i;
  for (i = 0; i < 65536; i++) {
    unsigned x = i;
    x |= (x << 8);
    x = ((x & 0x00f000f0) << 4) | (x & 0x000f000f);
    x |= (x << 4);

    /* Uncomment next line to invert the high byte as mentioned in the edit. */
    /* x = x ^ 0xff000000; */

    tbl[i] = x;
  }
  return tbl;
}

After that each conversion is just something like:

result = lookuptable[input];

..or maybe:

result = lookuptable[input & 0xffff];

---

Or a smaller, more cache-friendly lookup table (or pair) could be used with one lookup each for the high and low bytes (as noted by @LưuVĩnhPhúc in the comments). In that case, table generation code might be:

unsigned *makeLookupTableLow(void)
{
  unsigned *tbl = malloc(sizeof(unsigned) * 256);
  if (!tbl) return NULL;
  int i;
  for (i = 0; i < 256; i++) {
    unsigned x = i;
    x = ((x & 0xf0) << 4) | (x & 0x0f);
    x |= (x << 4);
    tbl[i] = x;
  }
  return tbl;
}

...and an optional second table:

unsigned *makeLookupTableHigh(void)
{
  unsigned *tbl = malloc(sizeof(unsigned) * 256);
  if (!tbl) return NULL;
  int i;
  for (i = 0; i < 256; i++) {
    unsigned x = i;
    x = ((x & 0xf0) << 20) | ((x & 0x0f) << 16);
    x |= (x << 4);

    /* uncomment next line to invert high byte */
    /* x = x ^ 0xff000000; */

    tbl[i] = x;
  }
  return tbl;
}

...and to convert a value with two tables:

result = hightable[input >> 8] | lowtable[input & 0xff];

...or with one (just the low table above):

result = (lowtable[input >> 8] << 16) | lowtable[input & 0xff];
result ^= 0xff000000; /* to invert high byte */

If the upper part of the value (alpha?) doesn't change much, even the single large table might perform well since consecutive lookups would be closer together in the table.

---

I took the performance test code @Apriori posted, made some adjustments, and added tests for the other responses that he hadn't included originally... then compiled three versions of it with different settings. One is 64-bit code with SSE4.1 enabled, where the compiler can make use of SSE for optimizations... and then two 32-bit versions, one with SSE and one without. Although all three were run on the same fairly recent processor, the results show how the optimal solution can change depending on the processor features:

                           64b SSE4.1  32b SSE4.1  32b no SSE
-------------------------- ----------  ----------  ----------
ExpandOrig           time:  3.502 s     3.501 s     6.260 s
ExpandLookupSmall    time:  3.530 s     3.997 s     3.996 s
ExpandLookupLarge    time:  3.434 s     3.419 s     3.427 s
ExpandIsalamon       time:  3.654 s     3.673 s     8.870 s
ExpandIsalamonOpt    time:  3.784 s     3.720 s     8.719 s
ExpandChronoKitsune  time:  3.658 s     3.463 s     6.546 s
ExpandEvgenyKluev    time:  6.790 s     7.697 s    13.383 s
ExpandIammilind      time:  3.485 s     3.498 s     6.436 s
ExpandDmitri         time:  3.457 s     3.477 s     5.461 s
ExpandNitish712      time:  3.574 s     3.800 s     6.789 s
ExpandAdamLiss       time:  3.673 s     5.680 s     6.969 s
ExpandAShelly        time:  3.524 s     4.295 s     5.867 s
ExpandAShellyMulOp   time:  3.527 s     4.295 s     5.852 s
ExpandSSE4           time:  3.428 s
ExpandSSE4Unroll     time:  3.333 s
ExpandSSE2           time:  3.392 s
ExpandSSE2Unroll     time:  3.318 s
ExpandAShellySSE4    time:  3.392 s

The executables were compiled on 64-bit Linux with gcc 4.8.1, using -m64 -O3 -march=core2 -msse4.1, -m32 -O3 -march=core2 -msse4.1 and -m32 -O3 -march=core2 -mno-sse respectively. @Apriori's SSE tests were omitted for the 32-bit builds (crashed on 32-bit with SSE enabled, and obviously won't work with SSE disabled).

Among the adjustments made was to use actual image data instead of random values (photos of objects with transparent backgrounds), which greatly improved the performance of the large lookup table but made little difference for the others.

Essentially, the lookup tables win by a landslide when SSE is unnavailable (or unused)... and the manually coded SSE solutions win otherwise. However, it's also noteworthy that when the compiler could use SSE for optimizations, most of the bit manipulation solutions were almost as fast as the manually coded SSE -- still slower, but only marginally.

Answer 2

I wanted to add this link into the answer pool because I think it is extremely important when talking about optimization, to remember the hardware we are running on, as well as the technologies compiling our code for said platform.

Blog post Playing with the CPU pipeline is about looking into optimizing a set of code for the CPU pipelining. It actually shows an example of where he tries to simplify the math down to the fewest actual mathematical operations, yet it was FAR from the most optimal solution in terms of time. I have seen a couple of answers here speaking to that effect, and they may be correct, they may not. The only way to know is to actually measure the time from start to finish of your particular snippet of code, in comparison to others. Read this blog; it is EXTREMELY interesting.

I think I should mention that I am in this particular case not going to put ANY code up here unless I have truly tried multiple attempts, and actually gotten on that is particularly faster through multiple tries.

Answer 3

Here's another attempt, using eight operations:

b = (((c & 0x0F0F) * 0x0101) & 0x00F000F) + 
    (((c & 0xF0F0) * 0x1010) & 0xF000F00);
b += b * 0x10;

printf("%x\n",b); //Shows '0x11223344'

---

*Note, this post originally contained quite different code, based on Interleave bits by Binary Magic Numbers from Sean Anderson's bithacks page. But that wasn't quite what the OP was asking. so it has ben removed. The majority of the comments below refer to that missing version.

Answer 4

This can be done using SSE2 as follows:

void ExpandSSE2(unsigned __int64 in, unsigned __int64 &outLo, unsigned __int64 &outHi) {
  __m128i const mask = _mm_set1_epi16((short)0xF00F);
  __m128i const mul0 = _mm_set1_epi16(0x0011);
  __m128i const mul1 = _mm_set1_epi16(0x1000);
  __m128i       v;

  v = _mm_cvtsi64_si128(in); // Move the 64-bit value to a 128-bit register
  v = _mm_unpacklo_epi8(v, v);  // 0x12   -> 0x1212
  v = _mm_and_si128(v, mask);   // 0x1212 -> 0x1002
  v = _mm_mullo_epi16(v, mul0); // 0x1002 -> 0x1022
  v = _mm_mulhi_epu16(v, mul1); // 0x1022 -> 0x0102
  v = _mm_mullo_epi16(v, mul0); // 0x0102 -> 0x1122

  outLo = _mm_extract_epi64(v, 0);
  outHi = _mm_extract_epi64(v, 1);
}

Of course you’d want to put the guts of the function in an inner loop and pull out the constants. You will also want to skip the x64 registers and load values directly into 128-bit SSE registers. For an example of how to do this, refer to the SSE2 implementation in the performance test below.

At its core, there are five instructions, which perform the operation on four color values at a time. So, that is only about 1.25 instructions per color value. It should also be noted that SSE2 is available anywhere x64 is available.

Performance tests for an assortment of the solutions here A few people have mentioned that the only way to know what's faster is to run the code, and this is unarguably true. So I've compiled a few of the solutions into a performance test so we can compare apples to apples. I chose solutions which I felt were significantly different from the others enough to require testing. All the solutions read from memory, operate on the data, and write back to memory. In practice some of the SSE solutions will require additional care around the alignment and handling cases when there aren't another full 16 bytes to process in the input data. The code I tested is x64 compiled under release using Visual Studio 2013 running on a 4+ GHz Core i7.

Here are my results:

ExpandOrig:               56.234 seconds  // From asker's original question
ExpandSmallLUT:           30.209 seconds  // From Dmitry's answer
ExpandLookupSmallOneLUT:  33.689 seconds  // from Dmitry's answer
ExpandLookupLarge:        51.312 seconds  // A straightforward lookup table
ExpandAShelly:            43.829 seconds  // From AShelly's answer
ExpandAShellyMulOp:       43.580 seconds  // AShelly's answer with an optimization
ExpandSSE4:               17.854 seconds  // My original SSE4 answer
ExpandSSE4Unroll:         17.405 seconds  // My original SSE4 answer with loop unrolling
ExpandSSE2:               17.281 seconds  // My current SSE2 answer
ExpandSSE2Unroll:         17.152 seconds  // My current SSE2 answer with loop unrolling

In the test results above you'll see I included the asker's code, three lookup table implementations including the small lookup table implementation proposed in Dmitry's answer. AShelly's solution is included too, as well as a version with an optimization I made (an operation can be eliminated). I included my original SSE4 implementation, as well as a superior SSE2 version I made later (now reflected as the answer), as well as unrolled versions of both since they were the fastest here, and I wanted to see how much unrolling sped them up. I also included an SSE4 implementation of AShelly's answer.

So far I have to declare myself the winner. But the source is below, so anyone can test it out on their platform, and include their own solution into the testing to see if they've made a solution that's even faster.

#define DATA_SIZE_IN  ((unsigned)(1024 * 1024 * 128))
#define DATA_SIZE_OUT ((unsigned)(2 * DATA_SIZE_IN))
#define RERUN_COUNT   500

#include <cstdlib>
#include <ctime>
#include <iostream>
#include <utility>
#include <emmintrin.h> // SSE2
#include <tmmintrin.h> // SSSE3
#include <smmintrin.h> // SSE4

void ExpandOrig(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  unsigned u, v;
  do {
    // Read in data
    u  = *(unsigned const*)in;
    v  = u >> 16;
    u &= 0x0000FFFF;

    // Do computation
    u  =   (u & 0x00FF) << 4
         | (u & 0x000F)
         | (u & 0x0FF0) << 8
         | (u & 0xFF00) << 12
         | (u & 0xF000) << 16;
    v  =   (v & 0x00FF) << 4
         | (v & 0x000F)
         | (v & 0x0FF0) << 8
         | (v & 0xFF00) << 12
         | (v & 0xF000) << 16;

    // Store data
    *(unsigned*)(out)      = u;
    *(unsigned*)(out + 4)  = v;
    in                    += 4;
    out                   += 8;
  } while (in != past);
}

unsigned LutLo[256],
         LutHi[256];
void MakeLutLo(void) {
  for (unsigned i = 0, x; i < 256; ++i) {
    x        = i;
    x        = ((x & 0xF0) << 4) | (x & 0x0F);
    x       |= (x << 4);
    LutLo[i] = x;
  }
}
void MakeLutHi(void) {
  for (unsigned i = 0, x; i < 256; ++i) {
    x        = i;
    x        = ((x & 0xF0) << 20) | ((x & 0x0F) << 16);
    x       |= (x << 4);
    LutHi[i] = x;
  }
}

void ExpandLookupSmall(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  unsigned u, v;
  do {
    // Read in data
    u  = *(unsigned const*)in;
    v  = u >> 16;
    u &= 0x0000FFFF;

    // Do computation
    u = LutHi[u >> 8] | LutLo[u & 0xFF];
    v = LutHi[v >> 8] | LutLo[v & 0xFF];

    // Store data
    *(unsigned*)(out)      = u;
    *(unsigned*)(out + 4)  = v;
    in                    += 4;
    out                   += 8;
  } while (in != past);
}

void ExpandLookupSmallOneLUT(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  unsigned u, v;
  do {
    // Read in data
    u = *(unsigned const*)in;
    v = u >> 16;
    u &= 0x0000FFFF;

    // Do computation
    u = ((LutLo[u >> 8] << 16) | LutLo[u & 0xFF]);
    v = ((LutLo[v >> 8] << 16) | LutLo[v & 0xFF]);

    // Store data
    *(unsigned*)(out) = u;
    *(unsigned*)(out + 4) = v;
    in  += 4;
    out += 8;
  } while (in != past);
}

unsigned LutLarge[256 * 256];
void MakeLutLarge(void) {
  for (unsigned i = 0; i < (256 * 256); ++i)
    LutLarge[i] = LutHi[i >> 8] | LutLo[i & 0xFF];
}

void ExpandLookupLarge(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  unsigned u, v;
  do {
    // Read in data
    u  = *(unsigned const*)in;
    v  = u >> 16;
    u &= 0x0000FFFF;

    // Do computation
    u = LutLarge[u];
    v = LutLarge[v];

    // Store data
    *(unsigned*)(out)      = u;
    *(unsigned*)(out + 4)  = v;
    in                    += 4;
    out                   += 8;
  } while (in != past);
}

void ExpandAShelly(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  unsigned u, v, w, x;
  do {
    // Read in data
    u  = *(unsigned const*)in;
    v  = u >> 16;
    u &= 0x0000FFFF;

    // Do computation
    w  = (((u & 0xF0F) * 0x101) & 0xF000F) + (((u & 0xF0F0) * 0x1010) & 0xF000F00);
    x  = (((v & 0xF0F) * 0x101) & 0xF000F) + (((v & 0xF0F0) * 0x1010) & 0xF000F00);
    w += w * 0x10;
    x += x * 0x10;

    // Store data
    *(unsigned*)(out)      = w;
    *(unsigned*)(out + 4)  = x;
    in                    += 4;
    out                   += 8;
  } while (in != past);
}

void ExpandAShellyMulOp(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  unsigned u, v;
  do {
    // Read in data
    u = *(unsigned const*)in;
    v = u >> 16;
    u &= 0x0000FFFF;

    // Do computation
    u = ((((u & 0xF0F) * 0x101) & 0xF000F) + (((u & 0xF0F0) * 0x1010) & 0xF000F00)) * 0x11;
    v = ((((v & 0xF0F) * 0x101) & 0xF000F) + (((v & 0xF0F0) * 0x1010) & 0xF000F00)) * 0x11;

    // Store data
    *(unsigned*)(out) = u;
    *(unsigned*)(out + 4) = v;
    in += 4;
    out += 8;
  } while (in != past);
}

void ExpandSSE4(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  __m128i const mask0 = _mm_set1_epi16((short)0x8000),
                mask1 = _mm_set1_epi8(0x0F),
                mul = _mm_set1_epi16(0x0011);
  __m128i       u, v, w, x;
  do {
    // Read input into low 8 bytes of u and v
    u = _mm_load_si128((__m128i const*)in);

    v = _mm_unpackhi_epi8(u, u);      // Expand each single byte to two bytes
    u = _mm_unpacklo_epi8(u, u);      // Do it again for v
    w = _mm_srli_epi16(u, 4);         // Copy the value into w and shift it right half a byte
    x = _mm_srli_epi16(v, 4);         // Do it again for v
    u = _mm_blendv_epi8(u, w, mask0); // Select odd bytes from w, and even bytes from v, giving the the desired value in the upper nibble of each byte
    v = _mm_blendv_epi8(v, x, mask0); // Do it again for v
    u = _mm_and_si128(u, mask1);      // Clear the all the upper nibbles
    v = _mm_and_si128(v, mask1);      // Do it again for v
    u = _mm_mullo_epi16(u, mul);      // Multiply each 16-bit value by 0x0011 to duplicate the lower nibble in the upper nibble of each byte
    v = _mm_mullo_epi16(v, mul);      // Do it again for v

    // Write output
    _mm_store_si128((__m128i*)(out     ), u);
    _mm_store_si128((__m128i*)(out + 16), v);
    in  += 16;
    out += 32;
  } while (in != past);
}

void ExpandSSE4Unroll(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  __m128i const mask0  = _mm_set1_epi16((short)0x8000),
                mask1  = _mm_set1_epi8(0x0F),
                mul    = _mm_set1_epi16(0x0011);
  __m128i       u0, v0, w0, x0,
                u1, v1, w1, x1,
                u2, v2, w2, x2,
                u3, v3, w3, x3;
  do {
    // Read input into low 8 bytes of u and v
    u0 = _mm_load_si128((__m128i const*)(in     ));
    u1 = _mm_load_si128((__m128i const*)(in + 16));
    u2 = _mm_load_si128((__m128i const*)(in + 32));
    u3 = _mm_load_si128((__m128i const*)(in + 48));

    v0 = _mm_unpackhi_epi8(u0, u0);      // Expand each single byte to two bytes
    u0 = _mm_unpacklo_epi8(u0, u0);      // Do it again for v
    v1 = _mm_unpackhi_epi8(u1, u1);      // Do it again
    u1 = _mm_unpacklo_epi8(u1, u1);      // Again for u1
    v2 = _mm_unpackhi_epi8(u2, u2);      // Again for v1
    u2 = _mm_unpacklo_epi8(u2, u2);      // Again for u2
    v3 = _mm_unpackhi_epi8(u3, u3);      // Again for v2
    u3 = _mm_unpacklo_epi8(u3, u3);      // Again for u3
    w0 = _mm_srli_epi16(u0, 4);          // Copy the value into w and shift it right half a byte
    x0 = _mm_srli_epi16(v0, 4);          // Do it again for v
    w1 = _mm_srli_epi16(u1, 4);          // Again for u1
    x1 = _mm_srli_epi16(v1, 4);          // Again for v1
    w2 = _mm_srli_epi16(u2, 4);          // Again for u2
    x2 = _mm_srli_epi16(v2, 4);          // Again for v2
    w3 = _mm_srli_epi16(u3, 4);          // Again for u3
    x3 = _mm_srli_epi16(v3, 4);          // Again for v3
    u0 = _mm_blendv_epi8(u0, w0, mask0); // Select even bytes from w, and odd bytes from v, giving the the desired value in the upper nibble of each byte
    v0 = _mm_blendv_epi8(v0, x0, mask0); // Do it again for v
    u1 = _mm_blendv_epi8(u1, w1, mask0); // Again for u1
    v1 = _mm_blendv_epi8(v1, x1, mask0); // Again for v1
    u2 = _mm_blendv_epi8(u2, w2, mask0); // Again for u2
    v2 = _mm_blendv_epi8(v2, x2, mask0); // Again for v2
    u3 = _mm_blendv_epi8(u3, w3, mask0); // Again for u3
    v3 = _mm_blendv_epi8(v3, x3, mask0); // Again for v3
    u0 = _mm_and_si128(u0, mask1);       // Clear the all the upper nibbles
    v0 = _mm_and_si128(v0, mask1);       // Do it again for v
    u1 = _mm_and_si128(u1, mask1);       // Again for u1
    v1 = _mm_and_si128(v1, mask1);       // Again for v1
    u2 = _mm_and_si128(u2, mask1);       // Again for u2
    v2 = _mm_and_si128(v2, mask1);       // Again for v2
    u3 = _mm_and_si128(u3, mask1);       // Again for u3
    v3 = _mm_and_si128(v3, mask1);       // Again for v3
    u0 = _mm_mullo_epi16(u0, mul);       // Multiply each 16-bit value by 0x0011 to duplicate the lower nibble in the upper nibble of each byte
    v0 = _mm_mullo_epi16(v0, mul);       // Do it again for v
    u1 = _mm_mullo_epi16(u1, mul);       // Again for u1
    v1 = _mm_mullo_epi16(v1, mul);       // Again for v1
    u2 = _mm_mullo_epi16(u2, mul);       // Again for u2
    v2 = _mm_mullo_epi16(v2, mul);       // Again for v2
    u3 = _mm_mullo_epi16(u3, mul);       // Again for u3
    v3 = _mm_mullo_epi16(v3, mul);       // Again for v3

    // Write output
    _mm_store_si128((__m128i*)(out      ), u0);
    _mm_store_si128((__m128i*)(out +  16), v0);
    _mm_store_si128((__m128i*)(out +  32), u1);
    _mm_store_si128((__m128i*)(out +  48), v1);
    _mm_store_si128((__m128i*)(out +  64), u2);
    _mm_store_si128((__m128i*)(out +  80), v2);
    _mm_store_si128((__m128i*)(out +  96), u3);
    _mm_store_si128((__m128i*)(out + 112), v3);
    in  += 64;
    out += 128;
  } while (in != past);
}

void ExpandSSE2(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  __m128i const mask = _mm_set1_epi16((short)0xF00F),
                mul0 = _mm_set1_epi16(0x0011),
                mul1 = _mm_set1_epi16(0x1000);
  __m128i       u, v;
  do {
    // Read input into low 8 bytes of u and v
    u = _mm_load_si128((__m128i const*)in);

    v = _mm_unpackhi_epi8(u, u);      // Expand each single byte to two bytes
    u = _mm_unpacklo_epi8(u, u);      // Do it again for v

    u = _mm_and_si128(u, mask);
    v = _mm_and_si128(v, mask);
    u = _mm_mullo_epi16(u, mul0);
    v = _mm_mullo_epi16(v, mul0);
    u = _mm_mulhi_epu16(u, mul1);     // This can also be done with a right shift of 4 bits, but this seems to mesure faster
    v = _mm_mulhi_epu16(v, mul1);
    u = _mm_mullo_epi16(u, mul0);
    v = _mm_mullo_epi16(v, mul0);

    // write output
    _mm_store_si128((__m128i*)(out     ), u);
    _mm_store_si128((__m128i*)(out + 16), v);
    in  += 16;
    out += 32;
  } while (in != past);
}

void ExpandSSE2Unroll(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  __m128i const mask = _mm_set1_epi16((short)0xF00F),
                mul0 = _mm_set1_epi16(0x0011),
                mul1 = _mm_set1_epi16(0x1000);
  __m128i       u0, v0,
                u1, v1;
  do {
    // Read input into low 8 bytes of u and v
    u0 = _mm_load_si128((__m128i const*)(in     ));
    u1 = _mm_load_si128((__m128i const*)(in + 16));

    v0 = _mm_unpackhi_epi8(u0, u0);      // Expand each single byte to two bytes
    u0 = _mm_unpacklo_epi8(u0, u0);      // Do it again for v
    v1 = _mm_unpackhi_epi8(u1, u1);      // Do it again
    u1 = _mm_unpacklo_epi8(u1, u1);      // Again for u1

    u0 = _mm_and_si128(u0, mask);
    v0 = _mm_and_si128(v0, mask);
    u1 = _mm_and_si128(u1, mask);
    v1 = _mm_and_si128(v1, mask);

    u0 = _mm_mullo_epi16(u0, mul0);
    v0 = _mm_mullo_epi16(v0, mul0);
    u1 = _mm_mullo_epi16(u1, mul0);
    v1 = _mm_mullo_epi16(v1, mul0);

    u0 = _mm_mulhi_epu16(u0, mul1);
    v0 = _mm_mulhi_epu16(v0, mul1);
    u1 = _mm_mulhi_epu16(u1, mul1);
    v1 = _mm_mulhi_epu16(v1, mul1);

    u0 = _mm_mullo_epi16(u0, mul0);
    v0 = _mm_mullo_epi16(v0, mul0);
    u1 = _mm_mullo_epi16(u1, mul0);
    v1 = _mm_mullo_epi16(v1, mul0);

    // write output
    _mm_store_si128((__m128i*)(out     ), u0);
    _mm_store_si128((__m128i*)(out + 16), v0);
    _mm_store_si128((__m128i*)(out + 32), u1);
    _mm_store_si128((__m128i*)(out + 48), v1);

    in  += 32;
    out += 64;
  } while (in != past);
}

void ExpandAShellySSE4(unsigned char const *in, unsigned char const *past, unsigned char *out) {
  __m128i const zero      = _mm_setzero_si128(),
                v0F0F     = _mm_set1_epi32(0x0F0F),
                vF0F0     = _mm_set1_epi32(0xF0F0),
                v0101     = _mm_set1_epi32(0x0101),
                v1010     = _mm_set1_epi32(0x1010),
                v000F000F = _mm_set1_epi32(0x000F000F),
                v0F000F00 = _mm_set1_epi32(0x0F000F00),
                v0011 = _mm_set1_epi32(0x0011);
  __m128i       u, v, w, x;
  do {
    // Read in data
    u = _mm_load_si128((__m128i const*)in);
    v = _mm_unpackhi_epi16(u, zero);
    u = _mm_unpacklo_epi16(u, zero);

    // original source: ((((a & 0xF0F) * 0x101) & 0xF000F) + (((a & 0xF0F0) * 0x1010) & 0xF000F00)) * 0x11;
    w = _mm_and_si128(u, v0F0F);
    x = _mm_and_si128(v, v0F0F);
    u = _mm_and_si128(u, vF0F0);
    v = _mm_and_si128(v, vF0F0);
    w = _mm_mullo_epi32(w, v0101); // _mm_mullo_epi32 is what makes this require SSE4 instead of SSE2
    x = _mm_mullo_epi32(x, v0101);
    u = _mm_mullo_epi32(u, v1010);
    v = _mm_mullo_epi32(v, v1010);
    w = _mm_and_si128(w, v000F000F);
    x = _mm_and_si128(x, v000F000F);
    u = _mm_and_si128(u, v0F000F00);
    v = _mm_and_si128(v, v0F000F00);
    u = _mm_add_epi32(u, w);
    v = _mm_add_epi32(v, x);
    u = _mm_mullo_epi32(u, v0011);
    v = _mm_mullo_epi32(v, v0011);

    // write output
    _mm_store_si128((__m128i*)(out     ), u);
    _mm_store_si128((__m128i*)(out + 16), v);
    in  += 16;
    out += 32;
  } while (in != past);
}

int main() {
  unsigned char *const indat   = new unsigned char[DATA_SIZE_IN ],
                *const outdat0 = new unsigned char[DATA_SIZE_OUT],
                *const outdat1 = new unsigned char[DATA_SIZE_OUT],
                *      curout  = outdat0,
                *      lastout = outdat1,
                *      place;
  unsigned             start,
                       stop;

  place = indat + DATA_SIZE_IN - 1;
  do {
    *place = (unsigned char)rand();
  } while (place-- != indat);
  MakeLutLo();
  MakeLutHi();
  MakeLutLarge();

  for (unsigned testcount = 0; testcount < 1000; ++testcount) {
    // Solution posted by the asker
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandOrig(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandOrig:\t\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);

    // Dmitry's small lookup table solution
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandLookupSmall(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandSmallLUT:\t\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // Dmitry's small lookup table solution using only one lookup table
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandLookupSmallOneLUT(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandLookupSmallOneLUT:\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // Large lookup table solution
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandLookupLarge(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandLookupLarge:\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // AShelly's Interleave bits by Binary Magic Numbers solution
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandAShelly(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandAShelly:\t\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // AShelly's Interleave bits by Binary Magic Numbers solution optimizing out an addition
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandAShellyMulOp(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandAShellyMulOp:\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // My SSE4 solution
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandSSE4(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandSSE4:\t\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // My SSE4 solution unrolled
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandSSE4Unroll(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandSSE4Unroll:\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // My SSE2 solution
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandSSE2(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandSSE2:\t\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // My SSE2 solution unrolled
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandSSE2Unroll(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandSSE2Unroll:\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;

    // AShelly's Interleave bits by Binary Magic Numbers solution implemented using SSE2
    start = clock();
    for (unsigned rerun = 0; rerun < RERUN_COUNT; ++rerun)
      ExpandAShellySSE4(indat, indat + DATA_SIZE_IN, curout);
    stop = clock();
    std::cout << "ExpandAShellySSE4:\t\t" << (((stop - start) / 1000) / 60) << ':' << (((stop - start) / 1000) % 60) << ":." << ((stop - start) % 1000) << std::endl;

    std::swap(curout, lastout);
    if (memcmp(outdat0, outdat1, DATA_SIZE_OUT))
      std::cout << "INCORRECT OUTPUT" << std::endl;
  }

  delete[] indat;
  delete[] outdat0;
  delete[] outdat1;
  return 0;
}

NOTE:

I had an SSE4 implementation here initially. I found a way to implement this using SSE2, which is better because it will run on more platforms. The SSE2 implementation is also faster. So, the solution presented at the top is now the SSE2 implementation and not the SSE4 one. The SSE4 implementation can still be seen in the performance tests or in the edit history.

Answer 5

unsigned long transform(unsigned long n)
{

    /* n: 00AR
     *    00GB
     */
    n = ((n & 0xff00) << 8) | (n & 0x00ff);

    /* n: 0AR0
     *    0GB0
     */
    n <<= 4;

    /* n: AAR0
     *    GGB0
     */
    n |= (n & 0x0f000f00L) << 4;

    /* n: AARR
     *    GGBB
     */
    n |= (n & 0x00f000f0L) >> 4;

    return n;
}

The alpha and red components are shifted into the higher 2 bytes where they belong, and the result is then shifted left by 4 bits, resulting in every component being exactly where it needs to be.

With a form of 0AR0 0GB0, a bit mask and left-shift combination is OR'ed with the current value. This copies the A and G components to the position just left of them. The same thing is done for the R and B components, except in the opposite direction.

Answer 6

Perhaps this could be more simpler & efficient.

unsigned int g = 0x1234;
unsigned int ans = 0;

ans = ( ( g & 0xf000 ) << 16) + ( (g & 0xf00 ) << 12)
    + ( ( g&0xf0 ) << 8) + ( ( g&0xf ) << 4);

ans  = ( ans | ans>>4 );

printf("%p -> %p\n", g, ans);