C++11 & Performances

A few notes I took from a lecture I attended a few days ago. I found some parts very interesting, whereas basic. I share this in case some would be interested.

Memory Proximity

Modern architecture generally have several levels of cache between the CPU and RAM. On the current computer I am working on, there are 3 levels of cache as we can see with the lscpu command:

stac@debian:~>lscpu
Architecture:        x86_64
CPU op-mode(s):      32-bit, 64-bit
Byte Order:          Little Endian
CPU(s):              4
...
L1d cache:           32K
L1i cache:           32K
L2 cache:            256K
L3 cache:            3072K
NUMA node0 CPU(s):   0-3

The nearest and smallest cache L1 (64K) is divided into two parts:

• Instructions cache
• Data cache

How a CPU cache works is quite easy to understand, but behind the scene it is top notch algorithms to optimize what as to be put in so that the cores do not waste time grabbing the memory from L2, L3 or, worse, the RAM. If the CPU needs to access a part of the memory that is available in the L1 cache, fine : it is called a cache hit. If not, it is a cache miss and the CPU will try to find the memory in L2 cache. The process continues until the data is found which can end up in the RAM (the slowest location).

The number of CPU cycles to access a piece of data in memory give a clear overview of the costs:

Hence, it is very important to strive to avoid the cache misses. Let’s have a look at an exemple of an array of the following structures:

struct Person {
    char first_name[30];
    char last_name[30];
    int age;
};

The goal is to calculate the average of the ages:

constexpr int kPersonNumber = 100'000;

double average(const std::array<Person, kPersonNumber>& persons)
{
    int sum = 0;
    for (const Person& person : persons)
    {
        sum += person.age;
    }
    return sum / kPersonNumber;
}

int main(int argc, const char *argv[])
{
    std::array<Person, kPersonNumber> persons;
    for (int i=0; i < kPersonNumber; ++i)
    {
        // Fill the array of Persons
    }

    std::cout << "Average: " << average(persons) << std::endl;
    return 0;
}

The average function loops on all the structures contained in the array. This leads to a cache miss on each iteration of the loop. We can use the Cachegrind tool to visualize the number of cache misses:

> valgrind --tool=cachegrind --branch-sim=yes --cache-sim=yes --cachegrind-out-file=chg.out ./myprog
> cg_annotate chg.out `pwd`/myprog.cpp

Here is the interesting part of the output:

==XXXX== D   refs:      2,146,050  (1,397,466 rd   + 748,584 wr)
==XXXX== D1  misses:      222,386  (  119,985 rd   + 102,401 wr)
==XXXX== LLd misses:      209,999  (  108,396 rd   + 101,603 wr)
==XXXX== D1  miss rate:      10.4% (      8.6%     +    13.7%  )
==XXXX== LLd miss rate:       9.8% (      7.8%     +    13.6%  )
...
       Ir I1mr ILmr      Dr    D1mr    DLmr      Dw   D1mw   DLmw      Bc   Bcm Bi Bim  file:function
...
1,000,027    3    3 300,006 100,002 100,001 200,006 99,999 99,999 200,000    10 0   0  :main
...

D1mr stands for Data L1 miss read. And we can see the order of magnitude of the the array size.

What would be the result if we changed the structure into a struct of arrays and change the average function

struct Person {
    char first_name[kPersonNumber][30];
    char last_name[kPersonNumber][30];
    int age[kPersonNumber];
};

double average(const Person& persons)
{
    int sum = 0;
    for (int i=0; i < kPersonNumber; ++i)
    {
        sum += persons.age[i];
    }
    return sum / kPersonNumber;
}

In this new version, the ages are stored in contiguous block of memories. So, when we access persons.age array which is 400’000 bytes a big part of it can be access through the L1 cache in each iteration of the loop, reducing the number of cache misses as we can see in the Cachegrind output:

==XXXX== D   refs:      2,271,052  (1,322,467 rd   + 948,585 wr)
==XXXX== D1  misses:      129,213  (   26,807 rd   + 102,406 wr)
==XXXX== LLd misses:      112,949  (   11,368 rd   + 101,581 wr)
==XXXX== D1  miss rate:       5.7% (      2.0%     +    10.8%  )
==XXXX== LLd miss rate:       5.0% (      0.9%     +    10.7%  )
...
       Ir I1mr ILmr      Dr  D1mr  DLmr      Dw   D1mw   DLmw      Bc   Bcm Bi Bim  file:function
...
  900,026    3    3 225,006 6,252 3,098 400,006 93,749 93,749 125,000    10 0   0 :main
...

Here we can see a drastic decrease of cache misses (from 100’000 downto 6’252 for L1 cache).

Moreover, the fact the data (in our case the person’s ages is stored in 400’000 contiguous bytes can allow some compiler optimization.

If we compile the second version of the program in gcc with the following options -O2 -march=native, the average function will look like this:

average(Person const&):
  leaq 6000000(%rdi), %rax
  leaq 6400000(%rdi), %rdx
  xorl %ecx, %ecx
.L2:
  addl (%rax), %ecx
  addq $4, %rax
  cmpq %rdx, %rax
  jne .L2
  ... (make the final division here)
  ret

Here, we can see (between the label L2 and the jump to L2), that 4 bytes by 4 bytes, we add the ages in the %ecx register.

Now, if we compile the program with the flags -O3 -march=native, let us have a look at the loop:

average(Person const&):
  ...
  shrl $3, %ecx
.L4:
  addl $1, %eax
  vpaddd (%rdx), %ymm0, %ymm0
  addq $32, %rdx
  cmpl %eax, %ecx
  ja .L4
  ... (make the final division here)
  ret

We can see that we are able to pack 8 integers and to add them 8 by 8 thanks to the MMX registers. As a matter of fact the number of operations to sum all the ages is devided by 8.

Inlining and C++

Inlining is the decision taken by the compiler not to do a real call to a function, but directly write the assembler code of the function at the point it should be called.

Here is a simple example that calculate the square of the number of arguments passed to the program. We could have chosen to calculate the square of a constant but the very agressive compiler’s optimization would have calculated the result without any needs of inlining.

#include <iostream>

int square(int num)
{
    return num * num;
}

int main(int argc, const char* argv[])
{
    auto a = square(argc);
    std::cout << a << std::endl;
    return 0;
}

Compiling with gcc with no optimization (-O0 -march=native), would lead to a call to the square function (with the argument copied in %rdi register and the creation of a stack frame).

main:
  ...
  movl %eax, %edi
  call square(int)
  movl %eax, -4(%rbp)
  movl -4(%rbp), %eax
  movl %eax, %esi
  movl std::cout, %edi
  call std::basic_ostream<char, std::char_traits<char> >::operator<<(int)
  ...

The obvious optimization would be to directly multiply the value that is passed to main by itself without calling the function. This is what is done with the following gcc options -O1 -march=native:

main:
  subq $8, %rsp
  imull %edi, %edi
  movl %edi, %esi
  movl std::cout, %edi
  call std::basic_ostream<char, std::char_traits<char> >::operator<<(int)
  ...

There is no call to the square(int) function anymore: this is called inlining.

The C++ compilers strive to inline the maximum of functions to improve the program performances. But there are some C++ language features that does not fit well to this philisophy, like inheritance and polymorphism.

Let’s have a look at the following program:

#include <iostream>
#include <array>

class ValueProviderBase
{
public:
    virtual int value() =0;
};

class ConstValueProvider : public ValueProviderBase
{
public:
    virtual int value() override {return 42;}
};

constexpr int kMax = 100'000;

int main(int argc, const char* argv[])
{
    std::array<ValueProviderBase*, kMax> v;

    ConstValueProvider p;

    for (int i = 0; i < kMax; ++i) v[i] = &p;

    int sum = 0;
    const std::size_t size = v.size();
    for (int i = 0; i < size; ++i)
        sum += v[i]->value();

    std::cout << sum << std::endl;
    return 0;
}

We have an array of the child class ConstValueProvider. The goal is to compute the sum of the values. In this state, to call the method value(), the compiler has no other choice than finding the method address in the good virtual table which is done like this with no optimization.

  call std::array<ValueProviderBase*, 100000ul>::operator[](unsigned long)
  movq (%rax), %rax
  movq (%rax), %rdx
  movq (%rdx), %rdx
  movq %rax, %rdi
  call *%rdx

The std::array<>::operator[](unsigned long) returns a pointer to a pointer to the real object in memory in %rax. We dereference it and then get the address of the appropriate vtable stored in %rdx. Finally we store the address of the method in the %rdx register that is used to make the call.

The reason of this is that generally the compiler has no chance to know what really is in the array.

In our case, the array is filled with a unique subtype of ValueProviderBase. Even, static cast would not help because there could be some subclasses of ConstValueProvider in the array. This means that in C++ 98/03, the developer had no chance to help the compiler.

With C++11, we have a way to tell the compiler that the class won’t subclassed (or a particular method): it is the keyword final. This makes a big difference because if we tell the compiler to trust us on the real types contained in the array, he will be smart enought to avoid using the vtable indirection.

class ConstValueProvider : public ValueProviderBase
{
public:
    virtual int value() override final {return 42;}
};

// ...

int main(int argc, const char* argv[])
{
    // ...
    for (int i = 0; i < kMax; ++i)
        sum += static_cast<ConstValueProvider*>(v[i])->value();
    // ...
}

Without optimization requested, gcc would do a direct call to ConstValueProvider::value():

  call std::array<ValueProviderBase*, 100000ul>::operator[](unsigned long)
  movq (%rax), %rax
  movq %rax, %rdi
  call ConstValueProvider::value()

Better ! And if we set an optimization level greater than 1, the call can be inlined. And still better, the result of the whole loop can be computed at compile time:

main:
  subq $8, %rsp
  movl $4200000, %esi
  movl std::cout, %edi
  call std::basic_ostream<char, std::char_traits<char> >::operator<<(int)

The value 4200000 is directly passed in %esi to be displayed ! We got rid of any function call.

As a side note, gcc is smart enought to suggest where you should add the final keyword when the warning options -Wsuggest-final-types and -Wsuggest-final-methods are provided.

R-value References & Move Semantic

From the start, C++ used to copy objects by default. But a copies take resources and time. So a smart way to cope such copies was to use reference for instance when passing arguments to a function:

void f1(X x) {} // Pass by copy
void f2(X& x) {} // Pass by reference

But sometimes, the copy mechanism is not appropriate for your needs. For instance, imagine a vector whose size grows so much that it has to be reallocated, then all the objects were copied into the new vector before deletion of the old one. This is not a bad thing: the vector would not take the risk starting to move objects around if at some point in the mechanism, an error could occur.

Another use case is that you defined an object that handles a resource and this management must not be done by several objects: what you want here is to avoid sharing the resource ownership. What would be the meaning of sharing a std::mutex or a std::thread ?

That’s why an optimized mechanism to move the object in memory was added to the C++ standard. This mechanism relies upon ‘move constructor’ and ‘move equal operator’.

I won’t deal too long with this topic but in terms of performance I was striked by two things:

• the fact the STL is ‘move-ready’. The basic structures like std::string will be automatically moved when needed. Often, compiling a program with the last C++ standard version leads to higher performances… for free ! Morever, the containers provides some API to avoid useless copy like std::vector<>::emplace_back.
• the importance of the new C++ keyword noexcept. As a result, this keyword has to be placed after each function that does not throw exception. This give hints to the compiler to enable the move of objects like it is the case when a std::vector has to reallocate all the contained objects when growing.

template <int N>
class X {
public:
    X() : buffer_(std::make_unique<char[]>(N)) {}
    X(const X& other) : buffer_(std::make_unique<char[]>(N))
    {
        std::memcpy(buffer_.get(), other.buffer_.get(), N);
    }
    X& operator=(const X& rhs)
    {
        buffer_.reset(new char[N]);
        std::memcpy(buffer_.get(), rhs.buffer_.get(), N);
        return *this;
    }

    X(X&& other) noexcept : buffer_(std::move(other.buffer_)) {}
    X& operator=(X&& rhs) noexcept
    {
        buffer_ = std::move(rhs.buffer_);
        return *this;
    }
private:
    std::unique_ptr<char[]> buffer_;
};

constexpr int kMax = 300'000;

int main(int argc, const char *argv[])
{
    std::vector<X<1000>> v;
    v.reserve(kMax);
    for (int i = 0; i < kMax; ++i)
    {
        v.emplace_back();
    }
    // One more to for vector realloacation
    v.emplace_back();
    std::cout << v.size() << std::endl;
    return 0;
}

Using noexcept or not on the move constructor have an impact during reallocation:

• if noexcept is not used, std::vector will use the copy constructor
• if noexcept is used, std::vector will use the move constructor

In the second case, this avoids the call to std::memcpy. Hence, the better performances.

Without noexcept:

stac@debian:~/development/cpp-sandbox/vector>time ./a.out
300001
./a.out  0.15s user 0.20s system 99% cpu 0.353 total

With noexcept:

stac@debian:~/development/cpp-sandbox/vector>time ./a.out
300001
./a.out  0.05s user 0.10s system 97% cpu 0.149 total

STL Containers

Choosing a container depends upon how you will use it, so it is worth knowing about their internal structure and the complexity of the methods/functions you will call most often.

For example, the best container for appending data is std::deque because as its internal structure is an contiguous array of pointers toward arrays of contiguous memory containing the objects, there is no need for reallocation the contained objects as it is the case with std::vector. Nevertheless, if you know the number of elements that will be stored, you can use the std::vector<>::reserve method and in this case the std::vector becomes the best. Conversely, due to the tree structure of the std::set container and the fact the objects are always ordered, this is the worse container.

A new type container appeared also in C++11: std::unordered_set and its counterpart key-value std::unordered_map. Because of its structure, based on hashes, insertion is faster than in a std::set and lookup become blazing fast compared to std::vector and std::deque.

Regarding lookup of values in containers, if std::find algorithm is correct for std::vector and std::deque, it is far better to use the find method of std::set and std::unordered_set.

Note that to have similar performance with std::vector, if the elements are sorted, std::lower_bound is the best alternative to std::find algorithm.

Last but not least, it really seems that std::list due to its poor performances, must not be the first container choice.

Strings

New ABI

Strings is one of the most common objects used in programs. Its definition has changed between C++98 and C++11.

In C++98, the standard specification fostered the use of references counting to allow the copy-on-write optimization. Now that C++11, provides threading support, the standard forbids hidden states in std::string (reference counters need atomics or locks in multithreaded environment).

The std::string is no more binary compatible between gcc 4 and gcc 5. Before, a std::string only had a pointer to a structure containing the size, the capacity, the reference counter and the buffer. The later is now 32 bits structure (64-bits machine architecture).

A noteworthy fact is the Small String Optimization (SSO): if the string contains less than 16 bytes, there is no heap memory allocation (the small buffer is store in place of the pointer to the location of the char[] on the heap.

C++17 std::string_view

There is a new interesting structure coming with C++17: std::string_view. Often, you have a char[] allocated on heap and filled with some data. Generally, to avoid the overhead of the copy, you don’t dare using std::string and its facilities. So, you end up using the C primitives to read things in this char[]. Moreover, as the length of the buffer is lost (you only have a pointer to the first element of the buffer), you have to convey the size of the buffer in all the function signatures).

To cope with this limitation, std::string_view was introduced to provide a read only access to a buffer with the size and a standard API to get iterators on the buffer elements.

#include <iostream>
#include <algorithm>
#include <string_view>

int main(int argc, const char *argv[])
{
    std::string_view sv(argv[1]);
    std::cout << "Size: " << sv.size() << std::endl;
    std::cout << "Number of A: "
              << std::count(sv.begin(), sv.end(), 'A')
              << std::endl;
    return 0;
}

The result:

stac@saturne:~>./a.out jhsfhsdAAAsdfjhsdkjAAsldkfjsdlkA
Size: 32
Number of A: 6

In this example, I used the constructor based on null-terminated char[], but you can use std::string_view to work with raw contiguous memory.

The advantage of this std::string_view is it allows you to write C++ style code that looks like less old C style without the overhead of costly copies.