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Improve numerical stability of GroupNorm (#54921)
Summary: Pull Request resolved: #54921 Improve numerical stability of GroupNorm Test Plan: buck test mode/dev-nosan //caffe2/test:nn -- "GroupNorm" Reviewed By: ngimel Differential Revision: D27414438 fbshipit-source-id: 815517240ca5ea3e2beb77ced3bd862e9c83d445
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Original file line number | Diff line number | Diff line change |
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#pragma once | ||
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#include <array> | ||
#include <cstring> | ||
#include <numeric> | ||
#include <utility> | ||
#include <vector> | ||
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#include <ATen/cpu/vec/vec.h> | ||
#include <ATen/native/cpu/utils.h> | ||
#include <c10/util/SmallVector.h> | ||
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namespace at { | ||
namespace native { | ||
namespace utils { | ||
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constexpr int64_t kChunkSize = 16; | ||
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template <typename T> | ||
void AddMoments( | ||
int64_t m0_add, | ||
const T& m1_add, | ||
const T& m2_add, | ||
int64_t& m0, | ||
T& m1, | ||
T& m2) { | ||
const int64_t n = m0 + m0_add; | ||
const T c = n == 0 ? 0 : static_cast<T>(m0_add) / static_cast<T>(n); | ||
const T delta = m1_add - m1; | ||
m1 += c * delta; | ||
m2 += m2_add + delta * delta * c * static_cast<T>(m0); | ||
m0 = n; | ||
} | ||
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template <typename T> | ||
void AddMomentsVec( | ||
int64_t m0_add, | ||
const vec::Vectorized<T>& m1_add, | ||
const vec::Vectorized<T>& m2_add, | ||
int64_t& m0, | ||
vec::Vectorized<T>& m1, | ||
vec::Vectorized<T>& m2) { | ||
using Vec = vec::Vectorized<T>; | ||
const int64_t n = m0 + m0_add; | ||
const T c = n == 0 ? 0 : static_cast<T>(m0_add) / static_cast<T>(n); | ||
const Vec c_vec(c); | ||
const Vec delta = m1_add - m1; | ||
m1 += c_vec * delta; | ||
m2 += m2_add + delta * delta * c_vec * Vec(static_cast<T>(m0)); | ||
m0 = n; | ||
} | ||
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// Compute rowwise moments by Welford algorithm and cascade sum to improve | ||
// numerical stability. | ||
// https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance | ||
// https://en.wikipedia.org/wiki/Pairwise_summation | ||
template <typename T, int64_t kMaxDepth> | ||
std::pair<T, T> RowwiseMomentsImpl(const T* X, int64_t N) { | ||
using Vec = vec::Vectorized<T>; | ||
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constexpr int64_t kVecSize = Vec::size(); | ||
const int64_t n = N / kVecSize; | ||
const int64_t m = divup(n, kChunkSize); | ||
const int64_t depth = CeilLog2(m); | ||
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const Vec kZeroVec(T(0)); | ||
c10::SmallVector<int64_t, kMaxDepth> m0_stk(depth, 0); | ||
c10::SmallVector<Vec, kMaxDepth> m1_stk(depth, kZeroVec); | ||
c10::SmallVector<Vec, kMaxDepth> m2_stk(depth, kZeroVec); | ||
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for (int64_t i = 0; i < m; ++i) { | ||
const T* X_ptr = X + i * kChunkSize * kVecSize; | ||
const int64_t m0 = std::min(kChunkSize, n - i * kChunkSize); | ||
Vec m1_vec(0); | ||
Vec m2_vec(0); | ||
for (int64_t j = 0; j < m0; ++j) { | ||
const Vec x_vec = Vec::loadu(X_ptr + j * kVecSize); | ||
const Vec delta_vec = x_vec - m1_vec; | ||
const Vec c_vec = Vec(T(1) / static_cast<T>(j + 1)); | ||
m1_vec += delta_vec * c_vec; | ||
m2_vec += delta_vec * (x_vec - m1_vec); | ||
} | ||
AddMomentsVec(m0, m1_vec, m2_vec, m0_stk[0], m1_stk[0], m2_stk[0]); | ||
int64_t mask = i + 1; | ||
for (int64_t j = 1; j < depth && (mask & 1) == 0; ++j) { | ||
AddMomentsVec( | ||
m0_stk[j - 1], | ||
m1_stk[j - 1], | ||
m2_stk[j - 1], | ||
m0_stk[j], | ||
m1_stk[j], | ||
m2_stk[j]); | ||
m0_stk[j - 1] = 0; | ||
m1_stk[j - 1] = kZeroVec; | ||
m2_stk[j - 1] = kZeroVec; | ||
mask >>= 1; | ||
} | ||
} | ||
for (int64_t i = 1; i < depth; ++i) { | ||
AddMomentsVec( | ||
m0_stk[i], m1_stk[i], m2_stk[i], m0_stk[0], m1_stk[0], m2_stk[0]); | ||
} | ||
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std::array<T, kVecSize> m1_arr{}; | ||
std::array<T, kVecSize> m2_arr{}; | ||
m1_stk[0].store(m1_arr.data()); | ||
m2_stk[0].store(m2_arr.data()); | ||
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int64_t m0 = 0; | ||
T m1 = 0; | ||
T m2 = 0; | ||
for (int64_t i = n * kVecSize; i < N; ++i) { | ||
const T delta = X[i] - m1; | ||
++m0; | ||
m1 += delta / static_cast<T>(m0); | ||
m2 += delta * (X[i] - m1); | ||
} | ||
for (int64_t i = 0; i < kVecSize; ++i) { | ||
AddMoments(n, m1_arr[i], m2_arr[i], m0, m1, m2); | ||
} | ||
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return std::make_pair(m1, m2 / static_cast<T>(N)); | ||
} | ||
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template <typename T> | ||
std::pair<T, T> RowwiseMoments(const T* X, int64_t N) { | ||
using Vec = vec::Vectorized<T>; | ||
constexpr int64_t kVecSize = Vec::size(); | ||
const int64_t n = N / kVecSize; | ||
const int64_t m = divup(n, kChunkSize); | ||
const int64_t depth = CeilLog2(m); | ||
if (depth <= 4) { | ||
return RowwiseMomentsImpl<T, 4>(X, N); | ||
} else if (depth <= 8) { | ||
return RowwiseMomentsImpl<T, 8>(X, N); | ||
} else if (depth <= 16) { | ||
return RowwiseMomentsImpl<T, 16>(X, N); | ||
} else if (depth <= 32) { | ||
return RowwiseMomentsImpl<T, 32>(X, N); | ||
} else { | ||
return RowwiseMomentsImpl<T, 64>(X, N); | ||
} | ||
} | ||
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} // namespace utils | ||
} // namespace native | ||
} // namespace at |
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