Iva*_*omo 1 c++ arrays pointers
我有自定义数组构造函数，如下所示：
rtc::ArrayView<const uint8_t> frame,
rtc::ArrayView<uint8_t> encrypted_frame,
uint8_t unencrypted_bytes = 10;
我如何有效地循环进入这些帧并对其进行处理？仅 for 循环是可能的选择吗？如果我们只想复制帧而不进行预处理，我知道我们可以使用std::copy. 有什么方法可以让这个迭代器处理更高效吗？
// // Copy rest of frame
// std::copy(frame.begin() + unencrypted_bytes, frame.begin() +
// (encrypted_frame.size() - 41),
// encrypted_frame.begin() + unencrypted_bytes);
// Doing XOR for Frame
for (size_t i = unencrypted_bytes; i < encrypted_frame.size() - 41; i++) {
// encrypted_frame[i] = i;
RTC_LOG(LS_INFO) << "Ivan, original frame Before XOR : " << i << " "
<< frame[i];
encrypted_frame[i] = frame[i] ^ fake_key_;
RTC_LOG(LS_INFO) << "Ivan, encrypted frame After XOR : " << i << " "
<< encrypted_frame[i];
}
下面是我的数组视图构造函数
/*
* Copyright 2015 The WebRTC 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 in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#ifndef API_ARRAY_VIEW_H_
#define API_ARRAY_VIEW_H_
#include <algorithm>
#include <array>
#include <iterator>
#include <type_traits>
#include "rtc_base/checks.h"
#include "rtc_base/type_traits.h"
namespace rtc {
// tl;dr: rtc::ArrayView is the same thing as gsl::span from the Guideline
// Support Library.
//
// Many functions read from or write to arrays. The obvious way to do this is
// to use two arguments, a pointer to the first element and an element count:
//
// bool Contains17(const int* arr, size_t size) {
// for (size_t i = 0; i < size; ++i) {
// if (arr[i] == 17)
// return true;
// }
// return false;
// }
//
// This is flexible, since it doesn't matter how the array is stored (C array,
// std::vector, rtc::Buffer, ...), but it's error-prone because the caller has
// to correctly specify the array length:
//
// Contains17(arr, arraysize(arr)); // C array
// Contains17(arr.data(), arr.size()); // std::vector
// Contains17(arr, size); // pointer + size
// ...
//
// It's also kind of messy to have two separate arguments for what is
// conceptually a single thing.
//
// Enter rtc::ArrayView<T>. It contains a T pointer (to an array it doesn't
// own) and a count, and supports the basic things you'd expect, such as
// indexing and iteration. It allows us to write our function like this:
//
// bool Contains17(rtc::ArrayView<const int> arr) {
// for (auto e : arr) {
// if (e == 17)
// return true;
// }
// return false;
// }
//
// And even better, because a bunch of things will implicitly convert to
// ArrayView, we can call it like this:
//
// Contains17(arr); // C array
// Contains17(arr); // std::vector
// Contains17(rtc::ArrayView<int>(arr, size)); // pointer + size
// Contains17(nullptr); // nullptr -> empty ArrayView
// ...
//
// ArrayView<T> stores both a pointer and a size, but you may also use
// ArrayView<T, N>, which has a size that's fixed at compile time (which means
// it only has to store the pointer).
//
// One important point is that ArrayView<T> and ArrayView<const T> are
// different types, which allow and don't allow mutation of the array elements,
// respectively. The implicit conversions work just like you'd hope, so that
// e.g. vector<int> will convert to either ArrayView<int> or ArrayView<const
// int>, but const vector<int> will convert only to ArrayView<const int>.
// (ArrayView itself can be the source type in such conversions, so
// ArrayView<int> will convert to ArrayView<const int>.)
//
// Note: ArrayView is tiny (just a pointer and a count if variable-sized, just
// a pointer if fix-sized) and trivially copyable, so it's probably cheaper to
// pass it by value than by const reference.
namespace impl {
// Magic constant for indicating that the size of an ArrayView is variable
// instead of fixed.
enum : std::ptrdiff_t { kArrayViewVarSize = -4711 };
// Base class for ArrayViews of fixed nonzero size.
template <typename T, std::ptrdiff_t Size>
class ArrayViewBase {
static_assert(Size > 0, "ArrayView size must be variable or non-negative");
public:
ArrayViewBase(T* data, size_t size) : data_(data) {}
static constexpr size_t size() { return Size; }
static constexpr bool empty() { return false; }
T* data() const { return data_; }
protected:
static constexpr bool fixed_size() { return true; }
private:
T* data_;
};
// Specialized base class for ArrayViews of fixed zero size.
template <typename T>
class ArrayViewBase<T, 0> {
public:
explicit ArrayViewBase(T* data, size_t size) {}
static constexpr size_t size() { return 0; }
static constexpr bool empty() { return true; }
T* data() const { return nullptr; }
protected:
static constexpr bool fixed_size() { return true; }
};
// Specialized base class for ArrayViews of variable size.
template <typename T>
class ArrayViewBase<T, impl::kArrayViewVarSize> {
public:
ArrayViewBase(T* data, size_t size)
: data_(size == 0 ? nullptr : data), size_(size) {}
size_t size() const { return size_; }
bool empty() const { return size_ == 0; }
T* data() const { return data_; }
protected:
static constexpr bool fixed_size() { return false; }
private:
T* data_;
size_t size_;
};
} // namespace impl
template <typename T, std::ptrdiff_t Size = impl::kArrayViewVarSize>
class ArrayView final : public impl::ArrayViewBase<T, Size> {
public:
using value_type = T;
using const_iterator = const T*;
// Construct an ArrayView from a pointer and a length.
template <typename U>
ArrayView(U* data, size_t size)
: impl::ArrayViewBase<T, Size>::ArrayViewBase(data, size) {
RTC_DCHECK_EQ(size == 0 ? nullptr : data, this->data());
RTC_DCHECK_EQ(size, this->size());
RTC_DCHECK_EQ(!this->data(),
this->size() == 0); // data is null iff size == 0.
}
// Construct an empty ArrayView. Note that fixed-size ArrayViews of size > 0
// cannot be empty.
ArrayView() : ArrayView(nullptr, 0) {}
ArrayView(std::nullptr_t) // NOLINT
: ArrayView() {}
ArrayView(std::nullptr_t, size_t size)
: ArrayView(static_cast<T*>(nullptr), size) {
static_assert(Size == 0 || Size == impl::kArrayViewVarSize, "");
RTC_DCHECK_EQ(0, size);
}
// Construct an ArrayView from a C-style array.
template <typename U, size_t N>
ArrayView(U (&array)[N]) // NOLINT
: ArrayView(array, N) {
static_assert(Size == N || Size == impl::kArrayViewVarSize,
"Array size must match ArrayView size");
}
// (Only if size is fixed.) Construct a fixed size ArrayView<T, N> from a
// non-const std::array instance. For an ArrayView with variable size, the
// used ctor is ArrayView(U& u) instead.
template <typename U,
size_t N,
typename std::enable_if<
Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
ArrayView(std::array<U, N>& u) // NOLINT
: ArrayView(u.data(), u.size()) {}
// (Only if size is fixed.) Construct a fixed size ArrayView<T, N> where T is
// const from a const(expr) std::array instance. For an ArrayView with
// variable size, the used ctor is ArrayView(U& u) instead.
template <typename U,
size_t N,
typename std::enable_if<
Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
ArrayView(const std::array<U, N>& u) // NOLINT
: ArrayView(u.data(), u.size()) {}
// (Only if size is fixed.) Construct an ArrayView from any type U that has a
// static constexpr size() method whose return value is equal to Size, and a
// data() method whose return value converts implicitly to T*. In particular,
// this means we allow conversion from ArrayView<T, N> to ArrayView<const T,
// N>, but not the other way around. We also don't allow conversion from
// ArrayView<T> to ArrayView<T, N>, or from ArrayView<T, M> to ArrayView<T,
// N> when M != N.
template <
typename U,
typename std::enable_if<Size != impl::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(U& u) // NOLINT
: ArrayView(u.data(), u.size()) {
static_assert(U::size() == Size, "Sizes must match exactly");
}
template <
typename U,
typename std::enable_if<Size != impl::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(const U& u) // NOLINT(runtime/explicit)
: ArrayView(u.data(), u.size()) {
static_assert(U::size() == Size, "Sizes must match exactly");
}
// (Only if size is variable.) Construct an ArrayView from any type U that
// has a size() method whose return value converts implicitly to size_t, and
// a data() method whose return value converts implicitly to T*. In
// particular, this means we allow conversion from ArrayView<T> to
// ArrayView<const T>, but not the other way around. Other allowed
// conversions include
// ArrayView<T, N> to ArrayView<T> or ArrayView<const T>,
// std::vector<T> to ArrayView<T> or ArrayView<const T>,
// const std::vector<T> to ArrayView<const T>,
// rtc::Buffer to ArrayView<uint8_t> or ArrayView<const uint8_t>, and
// const rtc::Buffer to ArrayView<const uint8_t>.
template <
typename U,
typename std::enable_if<Size == impl::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(U& u) // NOLINT
: ArrayView(u.data(), u.size()) {}
template <
typename U,
typename std::enable_if<Size == impl::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(const U& u) // NOLINT(runtime/explicit)
: ArrayView(u.data(), u.size()) {}
// Indexing and iteration. These allow mutation even if the ArrayView is
// const, because the ArrayView doesn't own the array. (To prevent mutation,
// use a const element type.)
T& operator[](size_t idx) const {
RTC_DCHECK_LT(idx, this->size());
RTC_DCHECK(this->data());
return this->data()[idx];
}
T* begin() const { return this->data(); }
T* end() const { return this->data() + this->size(); }
const T* cbegin() const { return this->data(); }
const T* cend() const { return this->data() + this->size(); }
std::reverse_iterator<T*> rbegin() const {
return std::make_reverse_iterator(end());
}
std::reverse_iterator<T*> rend() const {
return std::make_reverse_iterator(begin());
}
std::reverse_iterator<const T*> crbegin() const {
return std::make_reverse_iterator(cend());
}
std::reverse_iterator<const T*> crend() const {
return std::make_reverse_iterator(cbegin());
}
ArrayView<T> subview(size_t offset, size_t size) const {
return offset < this->size()
? ArrayView<T>(this->data() + offset,
std::min(size, this->size() - offset))
: ArrayView<T>();
}
ArrayView<T> subview(size_t offset) const {
return subview(offset, this->size());
}
};
// Comparing two ArrayViews compares their (pointer,size) pairs; it does *not*
// dereference the pointers.
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator==(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
return a.data() == b.data() && a.size() == b.size();
}
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator!=(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
return !(a == b);
}
// Variable-size ArrayViews are the size of two pointers; fixed-size ArrayViews
// are the size of one pointer. (And as a special case, fixed-size ArrayViews
// of size 0 require no storage.)
static_assert(sizeof(ArrayView<int>) == 2 * sizeof(int*), "");
static_assert(sizeof(ArrayView<int, 17>) == sizeof(int*), "");
static_assert(std::is_empty<ArrayView<int, 0>>::value, "");
template <typename T>
inline ArrayView<T> MakeArrayView(T* data, size_t size) {
return ArrayView<T>(data, size);
}
// Only for primitive types that have the same size and aligment.
// Allow reinterpret cast of the array view to another primitive type of the
// same size.
// Template arguments order is (U, T, Size) to allow deduction of the template
// arguments in client calls: reinterpret_array_view<target_type>(array_view).
template <typename U, typename T, std::ptrdiff_t Size>
inline ArrayView<U, Size> reinterpret_array_view(ArrayView<T, Size> view) {
static_assert(sizeof(U) == sizeof(T) && alignof(U) == alignof(T),
"ArrayView reinterpret_cast is only supported for casting "
"between views that represent the same chunk of memory.");
static_assert(
std::is_fundamental<T>::value && std::is_fundamental<U>::value,
"ArrayView reinterpret_cast is only supported for casting between "
"fundamental types.");
return ArrayView<U, Size>(reinterpret_cast<U*>(view.data()), view.size());
}
} // namespace rtc
#endif // API_ARRAY_VIEW_H_
这看起来像 WebRTC 代码。如果我不得不猜测，您正在加密 RTP 数据包的媒体字节（只是猜测）。所以你可能希望速度快一些。
我假设您认识到主循环中的 RTC_LOG 语句可能比您可以用来优化异或加密的任何其他操作更能成为循环性能杀手。如果您记录每个单独的字节，它将否定您所做的任何优化。那么让我们从这个开始吧。
for (size_t i = unencrypted_bytes; i < encrypted_frame.size() - 41; i++) {
encrypted_frame[i] = frame[i] ^ fake_key_;
}
的运算符重载[]如下所示：
T& operator[](size_t idx) const {
RTC_DCHECK_LT(idx, this->size());
RTC_DCHECK(this->data());
return this->data()[idx];
}
因此，这意味着每次迭代都会调用data()源数组和目标数组。并且operator[]重载会进行一些额外的验证检查。在发布版本中，编译器可能能够优化其中的大部分内容。但事实上我不知道这一点，因为我不知道编译器是否会像对 std:: 集合类进行操作一样优化 ArrayView。我也不知道这些 RTC_DCHECK 宏在发布版本中是否无效。
但在调试版本中，它会非常慢。因此，如果我们可以加快调试速度，我们就可以假设它会延续到您的发布版本中。
我们可以确保我们的主循环迭代字节并且不会在循环内进行任何函数调用。这将是你最大的加速。因此，这将比您所拥有的快得多：
uint8_t* frame_data = frame.data();
uint8_t* encrypted_data = encrypted_frame.data()
const size_t stop = i < encrypted_frame.size() - 41;
for (size_t i = frame_data + unencrypted_bytes; i < stop; i++) {
encrypted_data[i] = frame_data[i] ^ fake_key_;
}
您可以选择使用 std::transform 而不是 for 循环，但我认为这几乎是等效的。
同样，编译器完全有可能优化原始函数，使其与我刚刚生成的函数一样好。但由于 ArrayView 不能在本地为我编译（手头没有 webrtc 源），所以我不知道。否则，如果可以的话，我会在 Godbolt 上验证我的所有假设。
但我从经验中确实知道，在非常紧密的循环中迭代字节或单词的每个元素的函数调用，即使声明为内联，也永远不会像手动将所需的所有代码直接内联到循环中那么快。
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