In-Register Shuffles - Algorithmica

# In-Register Shuffles

Masking lets you apply operations to only a subset of vector elements. It is a very effective and frequently used data manipulation technique, but in many cases, you need to perform more advanced operations that involve permuting values inside a vector register instead of just blending them with other vectors.

The problem is that adding a separate element-shuffling instruction for each possible use case in hardware is unfeasible. What we can do though is to add just one general permutation instruction that takes the indices of a permutation and produces these indices using precomputed lookup tables.

This general idea is perhaps too abstract, so let’s jump straight to the examples.

### #Shuffles and Popcount

Population count, also known as the Hamming weight, is the count of 1 bits in a binary string.

It is a frequently used operation, so there is a separate instruction on x86 that computes the population count of a word:

const int N = (1<<12);
int a[N];

int popcnt() {
int res = 0;
for (int i = 0; i < N; i++)
res += __builtin_popcount(a[i]);
return res;
}


It also supports 64-bit integers, improving the total throughput twofold:

int popcnt_ll() {
long long *b = (long long*) a;
int res = 0;
for (int i = 0; i < N / 2; i++)
res += __builtin_popcountl(b[i]);
return res;
}


The only two instructions required are load-fused popcount and addition. They both have a high throughput, so the code processes about $8+8=16$ bytes per cycle as it is limited by the decode width of 4 on this CPU.

These instructions were added to x86 CPUs around 2008 with SSE4. Let’s temporarily go back in time before vectorization even became a thing and try to implement popcount by other means.

The naive way is to go through the binary string bit by bit:

__attribute__ (( optimize("no-tree-vectorize") ))
int popcnt() {
int res = 0;
for (int i = 0; i < N; i++)
for (int l = 0; l < 32; l++)
res += (a[i] >> l & 1);
return res;
}


As anticipated, it works just slightly faster than ⅛-th of a byte per cycle — at around 0.2.

We can try to process in bytes instead of individual bits by precomputing a small 256-element lookup table that contains the population counts of individual bytes and then query it while iterating over raw bytes of the array:

struct Precalc {
alignas(64) char counts[256];

constexpr Precalc() : counts{} {
for (int m = 0; m < 256; m++)
for (int i = 0; i < 8; i++)
counts[m] += (m >> i & 1);
}
};

constexpr Precalc P;

int popcnt() {
auto b = (unsigned char*) a; // careful: plain "char" is signed
int res = 0;
for (int i = 0; i < 4 * N; i++)
res += P.counts[b[i]];
return res;
}


It now processes around 2 bytes per cycle, rising to ~2.7 if we switch to 16-bit words (unsigned short).

This solution is still very slow compared to the popcnt instruction, but now it can be vectorized. Instead of trying to speed it up through gather instructions, we will go for another approach: make the lookup table small enough to fit inside a register and then use a special pshufb instruction to look up its values in parallel.

The original pshufb introduced in 128-bit SSE3 takes two registers: the lookup table containing 16 byte values and a vector of 16 4-bit indices (0 to 15), specifying which bytes to pick for each position. In 256-bit AVX2, instead of a 32-byte lookup table with awkward 5-bit indices, we have an instruction that independently the same shuffling operation over two 128-bit lanes.

So, for our use case, we create a 16-byte lookup table with population counts for each nibble (half-byte), repeated twice:

const reg lookup = _mm256_setr_epi8(
/* 0 */ 0, /* 1 */ 1, /* 2 */ 1, /* 3 */ 2,
/* 4 */ 1, /* 5 */ 2, /* 6 */ 2, /* 7 */ 3,
/* 8 */ 1, /* 9 */ 2, /* a */ 2, /* b */ 3,
/* c */ 2, /* d */ 3, /* e */ 3, /* f */ 4,

/* 0 */ 0, /* 1 */ 1, /* 2 */ 1, /* 3 */ 2,
/* 4 */ 1, /* 5 */ 2, /* 6 */ 2, /* 7 */ 3,
/* 8 */ 1, /* 9 */ 2, /* a */ 2, /* b */ 3,
/* c */ 2, /* d */ 3, /* e */ 3, /* f */ 4
);


Now, to compute the population count of a vector, we split each of its bytes into the lower and higher nibbles and then use this lookup table to retrieve their counts. The only thing left is to carefully sum them up:

const reg low_mask = _mm256_set1_epi8(0x0f);

int popcnt() {
int k = 0;

reg t = _mm256_setzero_si256();

for (; k + 15 < N; k += 15) {
reg s = _mm256_setzero_si256();

for (int i = 0; i < 15; i += 8) {
reg x = _mm256_load_si256( (reg*) &a[k + i] );

reg h = _mm256_and_si256(_mm256_srli_epi16(x, 4), low_mask);

reg pl = _mm256_shuffle_epi8(lookup, l);
reg ph = _mm256_shuffle_epi8(lookup, h);

}

}

int res = hsum(t);

while (k < N)
res += __builtin_popcount(a[k++]);

return res;
}


This code processes around 30 bytes per cycle. Theoretically, the inner loop could do 32, but we have to stop it every 15 iterations because the 8-bit counters can overflow.

The pshufb instruction is so instrumental in some SIMD algorithms that Wojciech Muła — the guy who came up with this algorithm — took it as his Twitter handle. You can calculate population counts even faster: check out his GitHub repository with different vectorized popcount implementations and his recent paper for a detailed explanation of the state-of-the-art.

### #Permutations and Lookup Tables

Our last major example in this chapter is the filter. It is a very important data processing primitive that takes an array as input and writes out only the elements that satisfy a given predicate (in their original order).

In a single-threaded scalar case, it is trivially implemented by maintaining a counter that is incremented on each write:

int a[N], b[N];

int filter() {
int k = 0;

for (int i = 0; i < N; i++)
if (a[i] < P)
b[k++] = a[i];

return k;
}


To vectorize it, we will use the _mm256_permutevar8x32_epi32 intrinsic. It takes a vector of values and individually selects them with a vector of indices. Despite the name, it doesn’t permute values but just copies them to form a new vector: duplicates in the result are allowed.

The general idea of our algorithm is as follows:

• calculate the predicate on a vector of data — in this case, this means performing the comparisons to get the mask;
• use the movemask instruction to get a scalar 8-bit mask;
• use this mask to index a lookup table that returns a permutation moving the elements that satisfy the predicate to the beginning of the vector (in their original order);
• use the _mm256_permutevar8x32_epi32 intrinsic to permute the values;
• write the whole permuted vector to the buffer — it may have some trailing garbage, but its prefix is correct;
• calculate the population count of the scalar mask and move the buffer pointer by that number.

First, we need to precompute the permutations:

struct Precalc {
alignas(64) int permutation[256][8];

constexpr Precalc() : permutation{} {
for (int m = 0; m < 256; m++) {
int k = 0;
for (int i = 0; i < 8; i++)
if (m >> i & 1)
permutation[m][k++] = i;
}
}
};

constexpr Precalc T;


Then we can implement the algorithm itself:

const reg p = _mm256_set1_epi32(P);

int filter() {
int k = 0;

for (int i = 0; i < N; i += 8) {
reg x = _mm256_load_si256( (reg*) &a[i] );

reg m = _mm256_cmpgt_epi32(p, x);

x = _mm256_permutevar8x32_epi32(x, permutation);
_mm256_storeu_si256((reg*) &b[k], x);


The vectorized version takes some work to implement, but it is 6-7x faster than the scalar one (the speedup is slightly less for either low or high values of P as the branch becomes predictable).
The loop performance is still relatively low — taking 4 CPU cycles per iteration — because, on this particular CPU (Zen 2), movemask, permute, and store have low throughput and all have to go through the same execution port (P2). On most other x86 CPUs, you can expect it to be ~2x faster.