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/**
* Copyright (c) 2015-present, Facebook, Inc.
* All rights reserved.
*
* This source code is licensed under the BSD+Patents license found in the
* LICENSE file in the root directory of this source tree.
*/
// -*- c++ -*-
#include "IndexScalarQuantizer.h"
#include <cstdio>
#include <algorithm>
#include <omp.h>
#include <immintrin.h>
#include "utils.h"
#include "FaissAssert.h"
namespace faiss {
/*******************************************************************
* ScalarQuantizer implementation
*
* The main source of complexity is to support combinations of 4
* variants without incurring runtime tests or virtual function calls:
*
* - 4 / 8 bits per code component
* - uniform / non-uniform
* - IP / L2 distance search
* - scalar / AVX distance computation
*
* The appropriate Quantizer object is returned via select_quantizer
* that hides the template mess.
********************************************************************/
#ifdef __AVX__
#define USE_AVX
#endif
namespace {
typedef Index::idx_t idx_t;
typedef ScalarQuantizer::QuantizerType QuantizerType;
typedef ScalarQuantizer::RangeStat RangeStat;
using DistanceComputer = ScalarQuantizer::DistanceComputer;
/*******************************************************************
* Codec: converts between values in [0, 1] and an index in a code
* array. The "i" parameter is the vector component index (not byte
* index).
*/
struct Codec8bit {
static void encode_component (float x, uint8_t *code, int i) {
code[i] = (int)(255 * x);
}
static float decode_component (const uint8_t *code, int i) {
return (code[i] + 0.5f) / 255.0f;
}
#ifdef USE_AVX
static __m256 decode_8_components (const uint8_t *code, int i) {
uint64_t c8 = *(uint64_t*)(code + i);
__m128i c4lo = _mm_cvtepu8_epi32 (_mm_set1_epi32(c8));
__m128i c4hi = _mm_cvtepu8_epi32 (_mm_set1_epi32(c8 >> 32));
// __m256i i8 = _mm256_set_m128i(c4lo, c4hi);
__m256i i8 = _mm256_castsi128_si256 (c4lo);
i8 = _mm256_insertf128_si256 (i8, c4hi, 1);
__m256 f8 = _mm256_cvtepi32_ps (i8);
__m256 half = _mm256_set1_ps (0.5f);
f8 += half;
__m256 one_255 = _mm256_set1_ps (1.f / 255.f);
return f8 * one_255;
}
#endif
};
struct Codec4bit {
static void encode_component (float x, uint8_t *code, int i) {
code [i / 2] |= (int)(x * 15.0) << ((i & 1) << 2);
}
static float decode_component (const uint8_t *code, int i) {
return (((code[i / 2] >> ((i & 1) << 2)) & 0xf) + 0.5f) / 15.0f;
}
#ifdef USE_AVX
static __m256 decode_8_components (const uint8_t *code, int i) {
uint32_t c4 = *(uint32_t*)(code + (i >> 1));
uint32_t mask = 0x0f0f0f0f;
uint32_t c4ev = c4 & mask;
uint32_t c4od = (c4 >> 4) & mask;
// the 8 lower bytes of c8 contain the values
__m128i c8 = _mm_unpacklo_epi8 (_mm_set1_epi32(c4ev),
_mm_set1_epi32(c4od));
__m128i c4lo = _mm_cvtepu8_epi32 (c8);
__m128i c4hi = _mm_cvtepu8_epi32 (_mm_srli_si128(c8, 4));
__m256i i8 = _mm256_castsi128_si256 (c4lo);
i8 = _mm256_insertf128_si256 (i8, c4hi, 1);
__m256 f8 = _mm256_cvtepi32_ps (i8);
__m256 half = _mm256_set1_ps (0.5f);
f8 += half;
__m256 one_255 = _mm256_set1_ps (1.f / 15.f);
return f8 * one_255;
}
#endif
};
#ifdef USE_AVX
uint16_t encode_fp16 (float x) {
__m128 xf = _mm_set1_ps (x);
__m128i xi = _mm_cvtps_ph (
xf, _MM_FROUND_TO_NEAREST_INT |_MM_FROUND_NO_EXC);
return _mm_cvtsi128_si32 (xi) & 0xffff;
}
float decode_fp16 (uint16_t x) {
__m128i xi = _mm_set1_epi16 (x);
__m128 xf = _mm_cvtph_ps (xi);
return _mm_cvtss_f32 (xf);
}
#else
// non-intrinsic FP16 <-> FP32 code adapted from
// https://github.com/ispc/ispc/blob/master/stdlib.ispc
float floatbits (uint32_t x) {
return *(float*)&x;
}
uint32_t intbits (float f) {
return *(uint32_t*)&f;
}
uint16_t encode_fp16 (float f) {
// via Fabian "ryg" Giesen.
// https://gist.github.com/2156668
uint32_t sign_mask = 0x80000000u;
int32_t o;
uint32_t fint = intbits(f);
uint32_t sign = fint & sign_mask;
fint ^= sign;
// NOTE all the integer compares in this function can be safely
// compiled into signed compares since all operands are below
// 0x80000000. Important if you want fast straight SSE2 code (since
// there's no unsigned PCMPGTD).
// Inf or NaN (all exponent bits set)
// NaN->qNaN and Inf->Inf
// unconditional assignment here, will override with right value for
// the regular case below.
uint32_t f32infty = 255u << 23;
o = (fint > f32infty) ? 0x7e00u : 0x7c00u;
// (De)normalized number or zero
// update fint unconditionally to save the blending; we don't need it
// anymore for the Inf/NaN case anyway.
const uint32_t round_mask = ~0xfffu;
const uint32_t magic = 15u << 23;
const uint32_t f16infty = 31u << 23;
// Shift exponent down, denormalize if necessary.
// NOTE This represents half-float denormals using single
// precision denormals. The main reason to do this is that
// there's no shift with per-lane variable shifts in SSE*, which
// we'd otherwise need. It has some funky side effects though:
// - This conversion will actually respect the FTZ (Flush To Zero)
// flag in MXCSR - if it's set, no half-float denormals will be
// generated. I'm honestly not sure whether this is good or
// bad. It's definitely interesting.
// - If the underlying HW doesn't support denormals (not an issue
// with Intel CPUs, but might be a problem on GPUs or PS3 SPUs),
// you will always get flush-to-zero behavior. This is bad,
// unless you're on a CPU where you don't care.
// - Denormals tend to be slow. FP32 denormals are rare in
// practice outside of things like recursive filters in DSP -
// not a typical half-float application. Whether FP16 denormals
// are rare in practice, I don't know. Whatever slow path your
// HW may or may not have for denormals, this may well hit it.
float fscale = floatbits(fint & round_mask) * floatbits(magic);
fscale = std::min(fscale, floatbits((31u << 23) - 0x1000u));
int32_t fint2 = intbits(fscale) - round_mask;
if (fint < f32infty)
o = fint2 >> 13; // Take the bits!
return (o | (sign >> 16));
}
float decode_fp16 (uint16_t h) {
// https://gist.github.com/2144712
// Fabian "ryg" Giesen.
const uint32_t shifted_exp = 0x7c00u << 13; // exponent mask after shift
int32_t o = ((int32_t)(h & 0x7fffu)) << 13; // exponent/mantissa bits
int32_t exp = shifted_exp & o; // just the exponent
o += (int32_t)(127 - 15) << 23; // exponent adjust
int32_t infnan_val = o + ((int32_t)(128 - 16) << 23);
int32_t zerodenorm_val = intbits(
floatbits(o + (1u<<23)) - floatbits(113u << 23));
int32_t reg_val = (exp == 0) ? zerodenorm_val : o;
int32_t sign_bit = ((int32_t)(h & 0x8000u)) << 16;
return floatbits(((exp == shifted_exp) ? infnan_val : reg_val) | sign_bit);
}
#endif
/*******************************************************************
* Quantizer: normalizes scalar vector components, then passes them
* through a codec
*/
struct Quantizer {
virtual void encode_vector(const float *x, uint8_t *code) const = 0;
virtual void decode_vector(const uint8_t *code, float *x) const = 0;
virtual ~Quantizer() {}
};
template<class Codec>
struct QuantizerUniform: Quantizer {
const size_t d;
const float vmin, vdiff;
QuantizerUniform(size_t d, const std::vector<float> &trained):
d(d), vmin(trained[0]), vdiff(trained[1])
{
}
void encode_vector(const float* x, uint8_t* code) const override {
for (size_t i = 0; i < d; i++) {
float xi = (x[i] - vmin) / vdiff;
if (xi < 0)
xi = 0;
if (xi > 1.0)
xi = 1.0;
Codec::encode_component(xi, code, i);
}
}
void decode_vector(const uint8_t* code, float* x) const override {
for (size_t i = 0; i < d; i++) {
float xi = Codec::decode_component(code, i);
x[i] = vmin + xi * vdiff;
}
}
float reconstruct_component (const uint8_t * code, int i) const
{
float xi = Codec::decode_component (code, i);
return vmin + xi * vdiff;
}
};
#ifdef USE_AVX
template<class Codec>
struct QuantizerUniform8: QuantizerUniform<Codec> {
QuantizerUniform8 (size_t d, const std::vector<float> &trained):
QuantizerUniform<Codec> (d, trained) {}
__m256 reconstruct_8_components (const uint8_t * code, int i) const
{
__m256 xi = Codec::decode_8_components (code, i);
return _mm256_set1_ps(this->vmin) + xi * _mm256_set1_ps (this->vdiff);
}
};
#endif
template<class Codec>
struct QuantizerNonUniform: Quantizer {
const size_t d;
const float *vmin, *vdiff;
QuantizerNonUniform(size_t d, const std::vector<float> &trained):
d(d), vmin(trained.data()), vdiff(trained.data() + d) {}
void encode_vector(const float* x, uint8_t* code) const override {
for (size_t i = 0; i < d; i++) {
float xi = (x[i] - vmin[i]) / vdiff[i];
if (xi < 0)
xi = 0;
if (xi > 1.0)
xi = 1.0;
Codec::encode_component(xi, code, i);
}
}
void decode_vector(const uint8_t* code, float* x) const override {
for (size_t i = 0; i < d; i++) {
float xi = Codec::decode_component(code, i);
x[i] = vmin[i] + xi * vdiff[i];
}
}
float reconstruct_component (const uint8_t * code, int i) const
{
float xi = Codec::decode_component (code, i);
return vmin[i] + xi * vdiff[i];
}
};
#ifdef USE_AVX
template<class Codec>
struct QuantizerNonUniform8: QuantizerNonUniform<Codec> {
QuantizerNonUniform8 (size_t d, const std::vector<float> &trained):
QuantizerNonUniform<Codec> (d, trained) {}
__m256 reconstruct_8_components (const uint8_t * code, int i) const
{
__m256 xi = Codec::decode_8_components (code, i);
return _mm256_loadu_ps (this->vmin + i) + xi * _mm256_loadu_ps (this->vdiff + i);
}
};
#endif
struct QuantizerFP16: Quantizer {
const size_t d;
QuantizerFP16(size_t d, const std::vector<float> & /* unused */):
d(d) {}
void encode_vector(const float* x, uint8_t* code) const override {
for (size_t i = 0; i < d; i++) {
((uint16_t*)code)[i] = encode_fp16(x[i]);
}
}
void decode_vector(const uint8_t* code, float* x) const override {
for (size_t i = 0; i < d; i++) {
x[i] = decode_fp16(((uint16_t*)code)[i]);
}
}
float reconstruct_component (const uint8_t * code, int i) const
{
return decode_fp16(((uint16_t*)code)[i]);
}
};
#ifdef USE_AVX
struct QuantizerFP16_8: QuantizerFP16 {
QuantizerFP16_8 (size_t d, const std::vector<float> &trained):
QuantizerFP16 (d, trained) {}
__m256 reconstruct_8_components (const uint8_t * code, int i) const
{
__m128i codei = _mm_loadu_si128 ((const __m128i*)(code + 2 * i));
return _mm256_cvtph_ps (codei);
}
};
#endif
Quantizer *select_quantizer (
QuantizerType qtype,
size_t d, const std::vector<float> & trained)
{
#ifdef USE_AVX
if (d % 8 == 0) {
switch(qtype) {
case ScalarQuantizer::QT_8bit:
return new QuantizerNonUniform8<Codec8bit>(d, trained);
case ScalarQuantizer::QT_4bit:
return new QuantizerNonUniform8<Codec4bit>(d, trained);
case ScalarQuantizer::QT_8bit_uniform:
return new QuantizerUniform8<Codec8bit>(d, trained);
case ScalarQuantizer::QT_4bit_uniform:
return new QuantizerUniform8<Codec4bit>(d, trained);
case ScalarQuantizer::QT_fp16:
return new QuantizerFP16_8 (d, trained);
}
} else
#endif
{
switch(qtype) {
case ScalarQuantizer::QT_8bit:
return new QuantizerNonUniform<Codec8bit>(d, trained);
case ScalarQuantizer::QT_4bit:
return new QuantizerNonUniform<Codec4bit>(d, trained);
case ScalarQuantizer::QT_8bit_uniform:
return new QuantizerUniform<Codec8bit>(d, trained);
case ScalarQuantizer::QT_4bit_uniform:
return new QuantizerUniform<Codec4bit>(d, trained);
case ScalarQuantizer::QT_fp16:
return new QuantizerFP16 (d, trained);
}
}
FAISS_THROW_MSG ("unknown qtype");
return nullptr;
}
Quantizer *select_quantizer (const ScalarQuantizer &sq)
{
return select_quantizer (sq.qtype, sq.d, sq.trained);
}
/*******************************************************************
* Quantizer range training
*/
static float sqr (float x) {
return x * x;
}
void train_Uniform(RangeStat rs, float rs_arg,
idx_t n, int k, const float *x,
std::vector<float> & trained)
{
trained.resize (2);
float & vmin = trained[0];
float & vmax = trained[1];
if (rs == ScalarQuantizer::RS_minmax) {
vmin = HUGE_VAL; vmax = -HUGE_VAL;
for (size_t i = 0; i < n; i++) {
if (x[i] < vmin) vmin = x[i];
if (x[i] > vmax) vmax = x[i];
}
float vexp = (vmax - vmin) * rs_arg;
vmin -= vexp;
vmax += vexp;
} else if (rs == ScalarQuantizer::RS_meanstd) {
double sum = 0, sum2 = 0;
for (size_t i = 0; i < n; i++) {
sum += x[i];
sum2 += x[i] * x[i];
}
float mean = sum / n;
float var = sum2 / n - mean * mean;
float std = var <= 0 ? 1.0 : sqrt(var);
vmin = mean - std * rs_arg ;
vmax = mean + std * rs_arg ;
} else if (rs == ScalarQuantizer::RS_quantiles) {
std::vector<float> x_copy(n);
memcpy(x_copy.data(), x, n * sizeof(*x));
// TODO just do a qucikselect
std::sort(x_copy.begin(), x_copy.end());
int o = int(rs_arg * n);
if (o < 0) o = 0;
if (o > n - o) o = n / 2;
vmin = x_copy[o];
vmax = x_copy[n - 1 - o];
} else if (rs == ScalarQuantizer::RS_optim) {
float a, b;
float sx = 0;
{
vmin = HUGE_VAL, vmax = -HUGE_VAL;
for (size_t i = 0; i < n; i++) {
if (x[i] < vmin) vmin = x[i];
if (x[i] > vmax) vmax = x[i];
sx += x[i];
}
b = vmin;
a = (vmax - vmin) / (k - 1);
}
int verbose = false;
int niter = 2000;
float last_err = -1;
int iter_last_err = 0;
for (int it = 0; it < niter; it++) {
float sn = 0, sn2 = 0, sxn = 0, err1 = 0;
for (idx_t i = 0; i < n; i++) {
float xi = x[i];
float ni = floor ((xi - b) / a + 0.5);
if (ni < 0) ni = 0;
if (ni >= k) ni = k - 1;
err1 += sqr (xi - (ni * a + b));
sn += ni;
sn2 += ni * ni;
sxn += ni * xi;
}
if (err1 == last_err) {
iter_last_err ++;
if (iter_last_err == 16) break;
} else {
last_err = err1;
iter_last_err = 0;
}
float det = sqr (sn) - sn2 * n;
b = (sn * sxn - sn2 * sx) / det;
a = (sn * sx - n * sxn) / det;
if (verbose) {
printf ("it %d, err1=%g \r", it, err1);
fflush(stdout);
}
}
if (verbose) printf("\n");
vmin = b;
vmax = b + a * (k - 1);
} else {
FAISS_THROW_MSG ("Invalid qtype");
}
vmax -= vmin;
}
void train_NonUniform(RangeStat rs, float rs_arg,
idx_t n, int d, int k, const float *x,
std::vector<float> & trained)
{
trained.resize (2 * d);
float * vmin = trained.data();
float * vmax = trained.data() + d;
if (rs == ScalarQuantizer::RS_minmax) {
memcpy (vmin, x, sizeof(*x) * d);
memcpy (vmax, x, sizeof(*x) * d);
for (size_t i = 1; i < n; i++) {
const float *xi = x + i * d;
for (size_t j = 0; j < d; j++) {
if (xi[j] < vmin[j]) vmin[j] = xi[j];
if (xi[j] > vmax[j]) vmax[j] = xi[j];
}
}
float *vdiff = vmax;
for (size_t j = 0; j < d; j++) {
float vexp = (vmax[j] - vmin[j]) * rs_arg;
vmin[j] -= vexp;
vmax[j] += vexp;
vdiff [j] = vmax[j] - vmin[j];
}
} else {
// transpose
std::vector<float> xt(n * d);
for (size_t i = 1; i < n; i++) {
const float *xi = x + i * d;
for (size_t j = 0; j < d; j++) {
xt[j * n + i] = xi[j];
}
}
std::vector<float> trained_d(2);
#pragma omp parallel for
for (size_t j = 0; j < d; j++) {
train_Uniform(rs, rs_arg,
n, k, xt.data() + j * n,
trained_d);
vmin[j] = trained_d[0];
vmax[j] = trained_d[1];
}
}
}
/*******************************************************************
* Similarity: gets vector components and computes a similarity wrt. a
* query vector stored in the object. The data fields just encapsulate
* an accumulator.
*/
struct SimilarityL2 {
const float *y, *yi;
explicit SimilarityL2 (const float * y): y(y) {}
/******* scalar accumulator *******/
float accu;
void begin () {
accu = 0;
yi = y;
}
void add_component (float x) {
float tmp = *yi++ - x;
accu += tmp * tmp;
}
void add_component_2 (float x1, float x2) {
float tmp = x1 - x2;
accu += tmp * tmp;
}
float result () {
return accu;
}
#ifdef USE_AVX
__m256 accu8;
void begin_8 () {
accu8 = _mm256_setzero_ps();
yi = y;
}
void add_8_components (__m256 x) {
__m256 yiv = _mm256_loadu_ps (yi);
yi += 8;
__m256 tmp = yiv - x;
accu8 += tmp * tmp;
}
void add_8_components_2 (__m256 x, __m256 y) {
__m256 tmp = y - x;
accu8 += tmp * tmp;
}
float result_8 () {
__m256 sum = _mm256_hadd_ps(accu8, accu8);
__m256 sum2 = _mm256_hadd_ps(sum, sum);
// now add the 0th and 4th component
return
_mm_cvtss_f32 (_mm256_castps256_ps128(sum2)) +
_mm_cvtss_f32 (_mm256_extractf128_ps(sum2, 1));
}
#endif
};
struct SimilarityIP {
const float *y, *yi;
/******* scalar accumulator *******/
float accu;
explicit SimilarityIP (const float * y):
y (y) {}
void begin () {
accu = 0;
yi = y;
}
void add_component (float x) {
accu += *yi++ * x;
}
void add_component_2 (float x1, float x2) {
accu += x1 * x2;
}
float result () {
return accu;
}
#ifdef USE_AVX
__m256 accu8;
void begin_8 () {
accu8 = _mm256_setzero_ps();
yi = y;
}
void add_8_components (__m256 x) {
__m256 yiv = _mm256_loadu_ps (yi);
yi += 8;
accu8 += yiv * x;
}
void add_8_components_2 (__m256 x1, __m256 x2) {
accu8 += x1 * x2;
}
float result_8 () {
__m256 sum = _mm256_hadd_ps(accu8, accu8);
__m256 sum2 = _mm256_hadd_ps(sum, sum);
// now add the 0th and 4th component
return
_mm_cvtss_f32 (_mm256_castps256_ps128(sum2)) +
_mm_cvtss_f32 (_mm256_extractf128_ps(sum2, 1));
}
#endif
};
/*******************************************************************
* DistanceComputer: combines a similarity and a quantizer to do
* code-to-vector or code-to-code comparisons
*/
template<class Quantizer, class Similarity>
struct DCTemplate : ScalarQuantizer::DistanceComputer {
Quantizer quant;
DCTemplate(size_t d, const std::vector<float> &trained):
quant(d, trained)
{}
float compute_distance (const float *x,
const uint8_t *code) override
{
Similarity sim(x);
sim.begin();
for (size_t i = 0; i < quant.d; i ++) {
float xi = quant.reconstruct_component (code, i);
sim.add_component (xi);
}
return sim.result();
}
float compute_code_distance (const uint8_t *code1,
const uint8_t *code2) override
{
Similarity sim(nullptr);
sim.begin ();
for (size_t i = 0; i < quant.d; i ++) {
float x1 = quant.reconstruct_component (code1, i);
float x2 = quant.reconstruct_component (code2, i);
sim.add_component_2 (x1, x2);
}
return sim.result ();
}
};
#ifdef USE_AVX
template<class Quantizer, class Similarity>
struct DCTemplate_8 : ScalarQuantizer::DistanceComputer {
Quantizer quant;
DCTemplate_8(size_t d, const std::vector<float> &trained):
quant(d, trained)
{}
float compute_distance (const float *x,
const uint8_t *code) override
{
Similarity sim(x);
sim.begin_8();
for (size_t i = 0; i < quant.d; i += 8) {
__m256 xi = quant.reconstruct_8_components (code, i);
sim.add_8_components (xi);
}
return sim.result_8();
}
float compute_code_distance (const uint8_t *code1,
const uint8_t *code2) override
{
Similarity sim(nullptr);
sim.begin_8 ();
for (size_t i = 0; i < quant.d; i += 8) {
__m256 x1 = quant.reconstruct_8_components (code1, i);
__m256 x2 = quant.reconstruct_8_components (code2, i);
sim.add_8_components_2 (x1, x2);
}
return sim.result_8 ();
}
};
#endif
template<class Sim>
DistanceComputer *select_distance_computer (
QuantizerType qtype,
size_t d, const std::vector<float> & trained)
{
#ifdef USE_AVX
if (d % 8 == 0) {
switch(qtype) {
case ScalarQuantizer::QT_8bit:
return new DCTemplate_8<QuantizerNonUniform8
<Codec8bit>, Sim>(d, trained);
case ScalarQuantizer::QT_4bit:
return new DCTemplate_8<QuantizerNonUniform8
<Codec4bit>, Sim>(d, trained);
case ScalarQuantizer::QT_8bit_uniform:
return new DCTemplate_8<QuantizerUniform8
<Codec8bit>, Sim>(d, trained);
case ScalarQuantizer::QT_4bit_uniform:
return new DCTemplate_8<QuantizerUniform8
<Codec4bit>, Sim>(d, trained);
case ScalarQuantizer::QT_fp16:
return new DCTemplate_8<QuantizerFP16_8, Sim>(d, trained);
}
} else
#endif
{
switch(qtype) {
case ScalarQuantizer::QT_8bit:
return new DCTemplate<QuantizerNonUniform
<Codec8bit>, Sim>(d, trained);
case ScalarQuantizer::QT_4bit:
return new DCTemplate<QuantizerNonUniform
<Codec4bit>, Sim>(d, trained);
case ScalarQuantizer::QT_8bit_uniform:
return new DCTemplate<QuantizerUniform
<Codec8bit>, Sim>(d, trained);
case ScalarQuantizer::QT_4bit_uniform:
return new DCTemplate<QuantizerUniform
<Codec4bit>, Sim>(d, trained);
case ScalarQuantizer::QT_fp16:
return new DCTemplate<QuantizerFP16, Sim>(d, trained);
}
}
FAISS_THROW_MSG ("unknown qtype");
return nullptr;
}
} // anonymous namespace
/*******************************************************************
* ScalarQuantizer implementation
********************************************************************/
ScalarQuantizer::ScalarQuantizer
(size_t d, QuantizerType qtype):
qtype (qtype), rangestat(RS_minmax), rangestat_arg(0), d (d)
{
switch (qtype) {
case QT_8bit: case QT_8bit_uniform:
code_size = d;
break;
case QT_4bit: case QT_4bit_uniform:
code_size = (d + 1) / 2;
break;
case QT_fp16:
code_size = d * 2;
break;
}
}
ScalarQuantizer::ScalarQuantizer ():
qtype(QT_8bit),
rangestat(RS_minmax), rangestat_arg(0), d (0), code_size(0)
{}
void ScalarQuantizer::train (size_t n, const float *x)
{
int bit_per_dim =
qtype == QT_4bit_uniform ? 4 :
qtype == QT_4bit ? 4 :
qtype == QT_8bit_uniform ? 8 :
qtype == QT_8bit ? 8 : -1;
switch (qtype) {
case QT_4bit_uniform: case QT_8bit_uniform:
train_Uniform (rangestat, rangestat_arg,
n * d, 1 << bit_per_dim, x, trained);
break;
case QT_4bit: case QT_8bit:
train_NonUniform (rangestat, rangestat_arg,
n, d, 1 << bit_per_dim, x, trained);
break;
case QT_fp16:
// no training necessary
break;
}
}
void ScalarQuantizer::compute_codes (const float * x,
uint8_t * codes,
size_t n) const
{
Quantizer *squant = select_quantizer (*this);
ScopeDeleter1<Quantizer> del(squant);
#pragma omp parallel for
for (size_t i = 0; i < n; i++)
squant->encode_vector (x + i * d, codes + i * code_size);
}
void ScalarQuantizer::decode (const uint8_t *codes, float *x, size_t n) const
{
Quantizer *squant = select_quantizer (*this);
ScopeDeleter1<Quantizer> del(squant);
#pragma omp parallel for
for (size_t i = 0; i < n; i++)
squant->decode_vector (codes + i * code_size, x + i * d);
}
ScalarQuantizer::DistanceComputer *ScalarQuantizer::get_distance_computer (
MetricType metric)
const
{
if (metric == METRIC_L2) {
return select_distance_computer<SimilarityL2>(qtype, d, trained);
} else {
return select_distance_computer<SimilarityIP>(qtype, d, trained);
}
}
/*******************************************************************
* IndexScalarQuantizer implementation
********************************************************************/
IndexScalarQuantizer::IndexScalarQuantizer
(int d, ScalarQuantizer::QuantizerType qtype,
MetricType metric):
Index(d, metric),
sq (d, qtype)
{
is_trained = false;
code_size = sq.code_size;
}
IndexScalarQuantizer::IndexScalarQuantizer ():
IndexScalarQuantizer(0, ScalarQuantizer::QT_8bit)
{}
void IndexScalarQuantizer::train(idx_t n, const float* x)
{
sq.train(n, x);
is_trained = true;
}
void IndexScalarQuantizer::add(idx_t n, const float* x)
{
FAISS_THROW_IF_NOT (is_trained);
codes.resize ((n + ntotal) * code_size);
sq.compute_codes (x, &codes[ntotal * code_size], n);
ntotal += n;
}
namespace {
template<class C>
void search_flat_scalar_quantizer(
const IndexScalarQuantizer & index,
idx_t n,
const float* x,
idx_t k,
float* distances,
idx_t* labels)
{
size_t code_size = index.code_size;
size_t d = index.d;
#pragma omp parallel
{
DistanceComputer *dc =
index.sq.get_distance_computer(index.metric_type);
ScopeDeleter1<DistanceComputer> del(dc);
#pragma omp for
for (size_t i = 0; i < n; i++) {
idx_t *idxi = labels + i * k;
float *simi = distances + i * k;
heap_heapify<C> (k, simi, idxi);
const float *xi = x + i * d;
const uint8_t *ci = index.codes.data ();
for (size_t j = 0; j < index.ntotal; j++) {
float accu = dc->compute_distance(xi, ci);
if (C::cmp (simi [0], accu)) {
heap_pop<C> (k, simi, idxi);
heap_push<C> (k, simi, idxi, accu, j);
}
ci += code_size;
}
heap_reorder<C> (k, simi, idxi);
}
}
};
}
void IndexScalarQuantizer::search(
idx_t n,
const float* x,
idx_t k,
float* distances,
idx_t* labels) const
{
FAISS_THROW_IF_NOT (is_trained);
if (metric_type == METRIC_L2) {
search_flat_scalar_quantizer<CMax<float, idx_t> > (*this, n, x, k, distances, labels);
} else {
search_flat_scalar_quantizer<CMin<float, idx_t> > (*this, n, x, k, distances, labels);
}
}
void IndexScalarQuantizer::reset()
{
codes.clear();
ntotal = 0;
}
void IndexScalarQuantizer::reconstruct_n(
idx_t i0, idx_t ni, float* recons) const
{
Quantizer *squant = select_quantizer (sq);
ScopeDeleter1<Quantizer> del (squant);
for (size_t i = 0; i < ni; i++) {
squant->decode_vector(&codes[(i + i0) * code_size], recons + i * d);
}
}
void IndexScalarQuantizer::reconstruct(idx_t key, float* recons) const
{
reconstruct_n(key, 1, recons);
}
/*******************************************************************
* IndexIVFScalarQuantizer implementation
********************************************************************/
IndexIVFScalarQuantizer::IndexIVFScalarQuantizer
(Index *quantizer, size_t d, size_t nlist,
QuantizerType qtype, MetricType metric):
IndexIVF (quantizer, d, nlist, 0, metric),
sq (d, qtype)
{
code_size = sq.code_size;
// was not known at construction time
invlists->code_size = code_size;
is_trained = false;
}
IndexIVFScalarQuantizer::IndexIVFScalarQuantizer ():
IndexIVF ()
{}
void IndexIVFScalarQuantizer::train_residual (idx_t n, const float *x)
{
long * idx = new long [n];
ScopeDeleter<long> del (idx);
quantizer->assign (n, x, idx);
float *residuals = new float [n * d];
ScopeDeleter<float> del2 (residuals);
#pragma omp parallel for
for (idx_t i = 0; i < n; i++) {
quantizer->compute_residual (x + i * d, residuals + i * d, idx[i]);
}
sq.train (n, residuals);
}
void IndexIVFScalarQuantizer::add_with_ids
(idx_t n, const float * x, const long *xids)
{
FAISS_THROW_IF_NOT (is_trained);
long * idx = new long [n];
ScopeDeleter<long> del (idx);
quantizer->assign (n, x, idx);
size_t nadd = 0;
Quantizer *squant = select_quantizer (sq);
ScopeDeleter1<Quantizer> del2 (squant);
#pragma omp parallel reduction(+: nadd)
{
std::vector<float> residual (d);
std::vector<uint8_t> one_code (code_size);
int nt = omp_get_num_threads();
int rank = omp_get_thread_num();
// each thread takes care of a subset of lists
for (size_t i = 0; i < n; i++) {
long list_no = idx [i];
if (list_no >= 0 && list_no % nt == rank) {
long id = xids ? xids[i] : ntotal + i;
quantizer->compute_residual (
x + i * d, residual.data(), list_no);
squant->encode_vector (residual.data(), one_code.data());
invlists->add_entry (list_no, id, one_code.data());
nadd++;
}
}
}
ntotal += nadd;
}
namespace {
void search_with_probes_ip (const IndexIVFScalarQuantizer & index,
const float *x,
const idx_t *cent_ids, const float *cent_dis,
DistanceComputer & dc,
int k, float *simi, idx_t *idxi,
bool store_pairs,
const IVFSearchParameters *params)
{
long nprobe = params ? params->nprobe : index.nprobe;
long max_codes = params ? params->max_codes : index.max_codes;
size_t code_size = index.code_size;
size_t d = index.d;
std::vector<float> decoded(d);
minheap_heapify (k, simi, idxi);
size_t nscan = 0;
for (int i = 0; i < nprobe; i++) {
idx_t list_no = cent_ids[i];
if (list_no < 0) break;
float accu0 = cent_dis[i];
const size_t list_size = index.invlists->list_size (list_no);
const uint8_t * codes = index.invlists->get_codes (list_no);
const idx_t * ids =
store_pairs ? nullptr : index.invlists->get_ids (list_no);
SimilarityIP sim(x);
for (size_t j = 0; j < list_size; j++) {
float accu = accu0 + dc.compute_distance(x, codes);
if (accu > simi [0]) {
minheap_pop (k, simi, idxi);
long id = store_pairs ? (list_no << 32 | j) : ids[j];
minheap_push (k, simi, idxi, accu, id);
}
codes += code_size;
}
nscan += list_size;
if (max_codes && nscan > max_codes)
break;
}
minheap_reorder (k, simi, idxi);
}
void search_with_probes_L2 (const IndexIVFScalarQuantizer & index,
const float *x_in,
const idx_t *cent_ids,
const Index *quantizer,
DistanceComputer & dc,
int k, float *simi, idx_t *idxi,
bool store_pairs,
const IVFSearchParameters *params)
{
long nprobe = params ? params->nprobe : index.nprobe;
long max_codes = params ? params->max_codes : index.max_codes;
size_t code_size = index.code_size;
size_t d = index.d;
std::vector<float> x(d);
maxheap_heapify (k, simi, idxi);
size_t nscan = 0;
for (int i = 0; i < nprobe; i++) {
idx_t list_no = cent_ids[i];
if (list_no < 0) break;
const size_t list_size = index.invlists->list_size (list_no);
const uint8_t * codes = index.invlists->get_codes (list_no);
const idx_t * ids =
store_pairs ? nullptr : index.invlists->get_ids (list_no);
// shift of x_in wrt centroid
quantizer->compute_residual (x_in, x.data(), list_no);
for (size_t j = 0; j < list_size; j++) {
float dis = dc.compute_distance (x.data(), codes);
if (dis < simi [0]) {
maxheap_pop (k, simi, idxi);
long id = store_pairs ? (list_no << 32 | j) : ids[j];
maxheap_push (k, simi, idxi, dis, id);
}
codes += code_size;
}
nscan += list_size;
if (max_codes && nscan > max_codes)
break;
}
maxheap_reorder (k, simi, idxi);
}
} // anonymous namespace
void IndexIVFScalarQuantizer::search_preassigned (
idx_t n, const float *x, idx_t k,
const idx_t *idx,
const float *dis,
float *distances, idx_t *labels,
bool store_pairs,
const IVFSearchParameters *params
) const
{
FAISS_THROW_IF_NOT (is_trained);
long local_nprobe = params ? params->nprobe : nprobe;
#pragma omp parallel
{
DistanceComputer *dc = sq.get_distance_computer (metric_type);
ScopeDeleter1<DistanceComputer> del(dc);
#pragma omp for
for (size_t i = 0; i < n; i++) {
if (metric_type == METRIC_INNER_PRODUCT) {
search_with_probes_ip (*this, x + i * d,
idx + i * local_nprobe,
dis + i * local_nprobe, *dc,
k, distances + i * k, labels + i * k,
store_pairs, params);
} else {
search_with_probes_L2 (*this, x + i * d,
idx + i * local_nprobe, quantizer, *dc,
k, distances + i * k, labels + i * k,
store_pairs, params);
}
}
}
}
void IndexIVFScalarQuantizer::reconstruct_from_offset (long list_no,
long offset,
float* recons) const
{
std::vector<float> centroid(d);
quantizer->reconstruct (list_no, centroid.data());
const uint8_t* code = invlists->get_single_code (list_no, offset);
sq.decode (code, recons, 1);
for (int i = 0; i < d; ++i) {
recons[i] += centroid[i];
}
}
} // namespace faiss
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