What's a fast way to space-out bits within a w

Question

I have a 32-bit value in the lower part of a 64-bit register; the top part is 0 (X denotes a bit with information, bits listed from LSB to MSB):

X X X  ...  X 0 0 0 0 ... 0

Now, I want to "space out" the bits with information, so that I have

X 0 X 0 X 0 ... X 0

(or if you'd rather put the 0's first, then

0 X 0 X 0 X 0 ... X

is fine too.)

What's a fast way to do that?

A multi-CPU-architecture-relevant answer would be nice, but something specific to Intel x86_64 and/or nVIDIA Pascal SM's would be the most relevant.

Answer 1

This is known as Morton number, which is a specific case of parallel expand which is in turn the reverse of compress right in the following questions

• What's the fastest way to pack 32 0/1 values into the bits of a single 32-bit variable?
• Shift masked bits to the lsb

One general solution might be

uint64_t bit_expand(uint64_t x) // 00000000ABCDEFGH
{
    x = ((x & 0xFFFF0000) << 32) | ((x & 0x0000FFFF) << 16);        // ABCD0000EFGH0000
    x = (x & 0xFF000000FF000000) | ((x & 0x00FF000000FF0000) >> 8); // AB00CD00EF00GH00
    x = (x & 0xF000F000F000F000) | ((x & 0x0F000F000F000F00) >> 4); // A0B0C0D0E0F0G0H0
    x = (x & 0xC0C0C0C0C0C0C0C0) | ((x & 0x3030303030303030) >> 2);
    x = (x & 0xA0A0A0A0A0A0A0A0) | ((x & 0x5050505050505050) >> 1);
    return x;
}

However the constant generation might be inefficient on RISC architectures because the 64-bit immediate can't be stored in a single instruction like on x86. Even on x86 the output assembly is quite long. Here is another possible implementation as described on Bit Twiddling Hacks

static const unsigned int B[] = {0x55555555, 0x33333333, 0x0F0F0F0F, 0x00FF00FF};
static const unsigned int S[] = {1, 2, 4, 8};

unsigned int x; // Interleave lower 16 bits of x and y, so the bits of x
unsigned int y; // are in the even positions and bits from y in the odd;
unsigned int z; // z gets the resulting 32-bit Morton Number.  
                // x and y must initially be less than 65536.

x = (x | (x << S[3])) & B[3];
x = (x | (x << S[2])) & B[2];
x = (x | (x << S[1])) & B[1];
x = (x | (x << S[0])) & B[0];

y = (y | (y << S[3])) & B[3];
y = (y | (y << S[2])) & B[2];
y = (y | (y << S[1])) & B[1];
y = (y | (y << S[0])) & B[0];

z = x | (y << 1);

A lookup table can also be used

#define EXPAND4(a) ((((a) & 0x8) << 4) | (((a) & 0x4) << 2) \
                  | (((a) & 0x2) << 1) | (((a) & 0x1)))

const uint8_t LUT[16] = {
    EXPAND4( 0), EXPAND4( 1), EXPAND4( 2), EXPAND4( 3),
    EXPAND4( 4), EXPAND4( 5), EXPAND4( 6), EXPAND4( 7),
    EXPAND4( 8), EXPAND4( 9), EXPAND4(10), EXPAND4(11),
    EXPAND4(12), EXPAND4(13), EXPAND4(14), EXPAND4(15)
};

output = ((uint64_t)LUT[(x >> 28) & 0xF] << 56) | ((uint64_t)LUT[(x >> 24) & 0xF] << 48)
       | ((uint64_t)LUT[(x >> 20) & 0xF] << 40) | ((uint64_t)LUT[(x >> 16) & 0xF] << 32)
       | ((uint64_t)LUT[(x >> 12) & 0xF] << 24) | ((uint64_t)LUT[(x >>  8) & 0xF] << 16)
       | ((uint64_t)LUT[(x >>  4) & 0xF] <<  8) | ((uint64_t)LUT[(x >>  0) & 0xF] <<  0);

The size of the lookup table may be increased if necessary

---

On x86 with BMI2 there's hardware support with PDEP instruction which can be accessed via the following intrinsic

output = _pdep_u64(x, 0xaaaaaaaaaaaaaaaaULL);

---

Another solution on architectures without bit deposit/expand instruction but with fast multipliers

uint64_t spaceOut8bits(uint8_t b)
{
    uint64_t MAGIC = 0x8040201008040201;
    uint64_t MASK  = 0x8080808080808080;
    uint64_t expand8bits = htobe64(((MAGIC*b) & MASK) >> 7);
    uint64_t spacedOutBits = expand8bits*0x41041 & 0xAA000000AA000000;
    return (spacedOutBits | (spacedOutBits << 24)) & 0xFFFF000000000000;
}

uint64_t spaceOut64bits(uint64_t x)
{
    return (spaceOut8bits(x >> 24) >>  0)
         | (spaceOut8bits(x >> 16) >> 16)
         | (spaceOut8bits(x >>  8) >> 32)
         | (spaceOut8bits(x >>  0) >> 48);
}

The way it works is like this

• The first step expands the input bits from abcdefgh to a0000000 b0000000 c0000000 d0000000 e0000000 f0000000 g0000000 h0000000 and store in expand8bits
• Then we move those spaced out bits close together by multiplying and masking in the next step. After that spacedOutBits will contain a0b0c0d0 00000000 00000000 00000000 e0f0g0h0 00000000 00000000 00000000. We'll merge the two bytes in the result together

The magic number for bringing the bits closer is calculated like this

  a0000000b0000000c0000000d0000000e0000000f0000000g0000000h0000000
×                                              1000001000001000001
  ────────────────────────────────────────────────────────────────
  a0000000b0000000c0000000d0000000e0000000f0000000g0000000h0000000
  00b0000000c0000000d0000000e0000000f0000000g0000000h0000000
+ 0000c0000000d0000000e0000000f0000000g0000000h0000000
  000000d0000000e0000000f0000000g0000000h0000000
  ────────────────────────────────────────────────────────────────
  a0b0c0d0b0c0d0e0c0d0e0f0d0e0f0g0e0f0g0h0f0g0h000g0h00000h0000000
& 1010101000000000000000000000000010101010000000000000000000000000
  ────────────────────────────────────────────────────────────────
  a0b0c0d0000000000000000000000000e0f0g0h0000000000000000000000000

The output assembly can be seen here. You can change the compiler to see how it's done on various architectures

There's also an alternative way on the Bit Twiddling Hacks page

z = ((x * 0x0101010101010101ULL & 0x8040201008040201ULL) * 
     0x0102040810204081ULL >> 49) & 0x5555 |
    ((y * 0x0101010101010101ULL & 0x8040201008040201ULL) * 
     0x0102040810204081ULL >> 48) & 0xAAAA;

More solutions can be found in Portable efficient alternative to PDEP without using BMI2?

Related: How to do bit striping on pixel data?

As you can see, without the availability of a bit deposit instruction it'll be quite complex in terms of operations. If you do a not of bit striping like this then it'll be better to do in parallel using SIMD