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diff --git a/third_party/jpeg-xl/lib/jxl/base/padded_bytes.h b/third_party/jpeg-xl/lib/jxl/base/padded_bytes.h
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+++ b/third_party/jpeg-xl/lib/jxl/base/padded_bytes.h
@@ -0,0 +1,197 @@
+// Copyright (c) the JPEG XL Project Authors. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#ifndef LIB_JXL_BASE_PADDED_BYTES_H_
+#define LIB_JXL_BASE_PADDED_BYTES_H_
+
+// std::vector replacement with padding to reduce bounds checks in WriteBits
+
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>  // memcpy
+
+#include <algorithm>  // max
+#include <initializer_list>
+#include <utility>  // swap
+
+#include "lib/jxl/base/cache_aligned.h"
+#include "lib/jxl/base/compiler_specific.h"
+#include "lib/jxl/base/status.h"
+
+namespace jxl {
+
+// Provides a subset of the std::vector interface with some differences:
+// - allows BitWriter to write 64 bits at a time without bounds checking;
+// - ONLY zero-initializes the first byte (required by BitWriter);
+// - ensures cache-line alignment.
+class PaddedBytes {
+ public:
+  // Required for output params.
+  PaddedBytes() : size_(0), capacity_(0) {}
+
+  explicit PaddedBytes(size_t size) : size_(size), capacity_(0) {
+    if (size != 0) IncreaseCapacityTo(size);
+  }
+
+  PaddedBytes(size_t size, uint8_t value) : size_(size), capacity_(0) {
+    if (size != 0) {
+      IncreaseCapacityTo(size);
+    }
+    if (size_ != 0) {
+      memset(data(), value, size);
+    }
+  }
+
+  PaddedBytes(const PaddedBytes& other) : size_(other.size_), capacity_(0) {
+    if (size_ != 0) IncreaseCapacityTo(size_);
+    if (data() != nullptr) memcpy(data(), other.data(), size_);
+  }
+  PaddedBytes& operator=(const PaddedBytes& other) {
+    // Self-assignment is safe.
+    resize(other.size());
+    if (data() != nullptr) memmove(data(), other.data(), size_);
+    return *this;
+  }
+
+  // default is not OK - need to set other.size_ to 0!
+  PaddedBytes(PaddedBytes&& other) noexcept
+      : size_(other.size_),
+        capacity_(other.capacity_),
+        data_(std::move(other.data_)) {
+    other.size_ = other.capacity_ = 0;
+  }
+  PaddedBytes& operator=(PaddedBytes&& other) noexcept {
+    size_ = other.size_;
+    capacity_ = other.capacity_;
+    data_ = std::move(other.data_);
+
+    if (&other != this) {
+      other.size_ = other.capacity_ = 0;
+    }
+    return *this;
+  }
+
+  void swap(PaddedBytes& other) {
+    std::swap(size_, other.size_);
+    std::swap(capacity_, other.capacity_);
+    std::swap(data_, other.data_);
+  }
+
+  void reserve(size_t capacity) {
+    if (capacity > capacity_) IncreaseCapacityTo(capacity);
+  }
+
+  // NOTE: unlike vector, this does not initialize the new data!
+  // However, we guarantee that write_bits can safely append after
+  // the resize, as we zero-initialize the first new byte of data.
+  // If size < capacity(), does not invalidate the memory.
+  void resize(size_t size) {
+    if (size > capacity_) IncreaseCapacityTo(size);
+    size_ = (data() == nullptr) ? 0 : size;
+  }
+
+  // resize(size) plus explicit initialization of the new data with `value`.
+  void resize(size_t size, uint8_t value) {
+    size_t old_size = size_;
+    resize(size);
+    if (size_ > old_size) {
+      memset(data() + old_size, value, size_ - old_size);
+    }
+  }
+
+  // Amortized constant complexity due to exponential growth.
+  void push_back(uint8_t x) {
+    if (size_ == capacity_) {
+      IncreaseCapacityTo(capacity_ + 1);
+      if (data() == nullptr) return;
+    }
+
+    data_[size_++] = x;
+  }
+
+  size_t size() const { return size_; }
+  size_t capacity() const { return capacity_; }
+
+  uint8_t* data() { return data_.get(); }
+  const uint8_t* data() const { return data_.get(); }
+
+  // std::vector operations implemented in terms of the public interface above.
+
+  void clear() { resize(0); }
+  bool empty() const { return size() == 0; }
+
+  void assign(std::initializer_list<uint8_t> il) {
+    resize(il.size());
+    memcpy(data(), il.begin(), il.size());
+  }
+
+  // Replaces data() with [new_begin, new_end); potentially reallocates.
+  void assign(const uint8_t* new_begin, const uint8_t* new_end);
+
+  uint8_t* begin() { return data(); }
+  const uint8_t* begin() const { return data(); }
+  uint8_t* end() { return begin() + size(); }
+  const uint8_t* end() const { return begin() + size(); }
+
+  uint8_t& operator[](const size_t i) {
+    BoundsCheck(i);
+    return data()[i];
+  }
+  const uint8_t& operator[](const size_t i) const {
+    BoundsCheck(i);
+    return data()[i];
+  }
+
+  uint8_t& back() {
+    JXL_ASSERT(size() != 0);
+    return data()[size() - 1];
+  }
+  const uint8_t& back() const {
+    JXL_ASSERT(size() != 0);
+    return data()[size() - 1];
+  }
+
+  template <typename T>
+  void append(const T& other) {
+    append(reinterpret_cast<const uint8_t*>(other.data()),
+           reinterpret_cast<const uint8_t*>(other.data()) + other.size());
+  }
+
+  void append(const uint8_t* begin, const uint8_t* end) {
+    if (end - begin > 0) {
+      size_t old_size = size();
+      resize(size() + (end - begin));
+      memcpy(data() + old_size, begin, end - begin);
+    }
+  }
+
+ private:
+  void BoundsCheck(size_t i) const {
+    // <= is safe due to padding and required by BitWriter.
+    JXL_ASSERT(i <= size());
+  }
+
+  // Copies existing data to newly allocated "data_". If allocation fails,
+  // data() == nullptr and size_ = capacity_ = 0.
+  // The new capacity will be at least 1.5 times the old capacity. This ensures
+  // that we avoid quadratic behaviour.
+  void IncreaseCapacityTo(size_t capacity);
+
+  size_t size_;
+  size_t capacity_;
+  CacheAlignedUniquePtr data_;
+};
+
+template <typename T>
+static inline void Append(const T& s, PaddedBytes* out,
+                          size_t* JXL_RESTRICT byte_pos) {
+  memcpy(out->data() + *byte_pos, s.data(), s.size());
+  *byte_pos += s.size();
+  JXL_CHECK(*byte_pos <= out->size());
+}
+
+}  // namespace jxl
+
+#endif  // LIB_JXL_BASE_PADDED_BYTES_H_