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Your Performance Todo List

The most important optimisation opportunities and pitfalls to remember about

by

Jan Bielak

Your Performance Todo List

The most important optimisation opportunities and pitfalls to remember about

by

Jan Bielak

Warsaw Staszic High School, Poland

Self-taught C++ Developer

Realtime rendering

Game development

janbielak.com
github.com/janekb04
youtube.com/@janbielak

Practically Correct, Just-in-Time Shell Script Parallelization

Konstantinos Kallas, Tammam Mustafa, Jan Bielak, Dimitris Karnikis, Thurston H.Y. Dang, Michael Greenberg, Nikos Vasilakis. 16th USENIX Symposium on Operating Systems Design and Implementation (OSDI22)

Performance

1. No unnecessary work

2. Use all computing power

3. Avoid waits and stalls

4. Use hardware efficiently

No unnecessary copying

No unnecessary allocations

Use all cores

Use SIMD

Lockless data structures

Asynchronous APIs

Job Systems

Cache friendliness

Well predictable code

5. OS-level efficiency

Performance

1. No unnecessary work

2. Use all computing power

3. Avoid waits and stalls

4. Use hardware efficiently

5. OS-level efficiency

Performance

1. No unnecessary work

2. Use all computing power

3. Avoid waits and stalls

4. Use hardware efficiently

5. OS-level efficiency

Effective use of C++

Build pipeline modification

Manual hardware oriented optimisations

Your Performance Todo List

Effective use of C++

Build pipeline modification

Manual hardware oriented optimisations

Build pipeline modification

1. Enable compiler optimisations

GCC, LLVM, ICC	-O2 or -O3
MSVC	/Ox or /O2

Optimize for speed

Optimize for size

GCC, LLVM, ICC	-Os
MSVC	/O1

Optimization #1

Longer compile time

Build pipeline modification

2. Set target architecture

GCC, LLVM, ICC	-march=native -mtune=native
MSVC	/arch:IA32 or /arch:SSE or /arch:SSE2 or /arch:AVX or /arch:AVX2 or /arch:AVX512

For x86

For ARM

GCC, LLVM	-mcpu=native
MSVC	/arch:ARMv7VE or /arch:VFPv4 or /arch:armv8.0 ... or /arch:armv8.8

automatic detection of current processor's features

needs to be specified manually

Build pipeline modification

3. Use fast math

GCC, LLVM	-ffast-math (included in -Ofast)
MSVC	/fp:fast
ICC	-fp-model=fast

Faster computation

Less precise results

Non standard-compliant

Build pipeline modification

4. Disable exceptions and RTTI

GCC, LLVM, ICC	-fno-exceptions
MSVC	/EHs-c- /D_HAS_EXCEPTIONS=0

GCC, LLVM, ICC	-fno-rtti
MSVC	/GR-

No exceptions

No RTTI

Limited performance gains

Non standard-compliant

Breaks code using exceptions

Build pipeline modification

5. Enable Link Time Optimization

Compiler

Linker

}

Build pipeline modification

5. Enable Link Time Optimization

Compiler

Linker

}

GCC, LLVM	-flto
MSVC	/GL
ICC	-ipo

Build pipeline modification

6. Use Unity Builds

Compiler

Linker

}

Compiler

Build pipeline modification

6. Use Unity Builds

Compiler

Linker

}

Compiler

Unity Build

CMake	-DCMAKE_UNITY_BUILD=ON

Build pipeline modification

7. Link statically

Static Linking	Dynamic Linking

Better optimisable

More space efficient

Can be updated independently of executable

Build pipeline modification

8. Use Profile Guided Optimisation

Build pipeline

Build pipeline modification

8. Use Profile Guided Optimisation

Build pipeline

Execute

Build pipeline modification

8. Use Profile Guided Optimisation

Build pipeline

Execute

Build pipeline

GCC, LLVM	-fprofile-generate
MSVC	/GENPROFILE
ICC	-prof-gen

GCC, LLVM	-fprofile-use
MSVC	/USEPROFILE
ICC	-prof-use

Build pipeline modification

9. Try different compilers

Build pipeline modification

10. Try different standard libraries

Build pipeline modification

11. Keep your tools updated

Build pipeline modification

12. Preload with a replacement lib

env LD_PRELOAD=/usr/lib/libSUPERmalloc.so ./myprogram

env DYLD_INSERT_LIBRARIES=/usr/lib/libSUPERmalloc.dylib ./myprogram

Requires DLL injection

Windows

macOS

Linux, BSD

Build pipeline modification

13. Use binary post processing tools

LLVM BOLT

perf record

perf2bolt

Build pipeline modification

13. Use binary post processing tools

LLVM BOLT

perf record

perf2bolt

llvm-bolt

Effective use of C++

Build pipeline modification

Manual hardware oriented optimisations

Your Performance Todo List

Enable compiler optimisations
Set target architecture
Use fast math
Disable exceptions and RTTI
Enable Link Time Optimisation
Use Unity Builds
Link statically
Use Profile Guided Optimisation
Try different compilers
Try different standard library implementations
Keep your tools updated
Preload you program with a replacement library
Use binary post processing tools

Annotate your code

14. Constexpr all the things

Effective use of C++

Constant expressions:

Literals:

1, 3.0f, nullptr, "Hello"

Arithmetic:

2 + 3, 4.0 / 3.0

Sizes and alignments:

sizeof(int), alignof(std::vector<int>)

...

14. Constexpr all the things

Effective use of C++

constexpr int f(int x) { return 3 * x + 5; }

Constexpr functions:

invocation MAY be a constant expression

f(5)

int x;
std::cin >> x;
f(x);

is a constant expression

is NOT a constant expression

Is a given invocation evaluated at compile time?

if (std::is_constant_evaluated()) { ... }

if consteval { ... }

consteval int f(int x) { return 3 * x + 5; }

Immediate functions:

f(5)

int x;
std::cin >> x;
f(x);

is a constant expression

is a COMPILE ERRROR

invocation MUST be a constant expression

(inside function body)

if constexpr (compile_time_condition) {...}

If constexpr:

if constexpr (std::is_constant_evaluated()) {...}

ALWAYS TRUE

14. Constexpr all the things

Effective use of C++

constexpr std::array<int> primes{ 2, 3, 5, 7, 11 };

Constexpr variables:

variable must be initialised at its declaration

constexpr int x;
x = 3;

is a COMPILE ERRROR

implies

primes[0] = 1;

is a COMPILE ERRROR

const

constexpr int f(int x) { return x + 1; }

int main()
{
    int x1 = 3;
    constexpr int y1 = f(x1);
    
    constexpr int x2 = 3;
    constexpr int y2 = f(x2);
}

accessing it is a constant expression

is a COMPILE ERRROR

constinit std::array<int> primes{ 2, 3, 5, 7, 11 };

Constinit variables:

(by a constant a expression)

variable must be initialised at its declaration by a constant expression

14. Constexpr all the things

Effective use of C++

constexpr std::array<int> primes{ 2, 3, 5, 7, 11 };

Constexpr variables:

constinit std::array<int> primes{ 2, 3, 5, 7, 11 };

Constinit variables:

constexpr int f(int x) { return 3 * x + 5; }

Constexpr functions:

Is a given invocation evaluated at compile time?

if (std::is_constant_evaluated()) { ... }

if consteval { ... }

consteval int f(int x) { return 3 * x + 5; }

Immediate functions:

if constexpr (compile_time_condition) {...}

If constexpr:

15. Make variables const

Effective use of C++

std::vector<float>
get_mean_deltas(std::vector<float> data)
{
    float sum = 0;
    for (auto&& num : data) 
    	sum += num;
        
    for (auto& num : data)
        num -= sum / data.size();
    return data;
}

Declare variables const

15. Make variables const

Effective use of C++

std::vector<float>
get_mean_deltas(std::vector<float> data)
{
    const float sum = std::accumulate(
        data.begin(),
        data.end(),
        0.0f
    );
        
    for (auto& num : data)
        num -= sum / data.size();
    return data;
}

std::vector<float>
get_mean_deltas(std::vector<float> data)
{
    const float sum = std::accumulate(
        data.begin(),
        data.end(),
        0.0f
    );
        
    const float __mean = sum / data.size();
    for (auto& num : data)
        num -= __mean;
    return data;    
}

...so this expression is loop-invariant and can be hoisted

and no expensive division in loop!

sum is const...

~ compiler's thought process

(paraphrased)

Declare variables const

15. Make variables const

Effective use of C++

template <typename T>
class vector {
    T* begin;
    T* end;
    T* capacity;
    
    /* ... */
    
public:
    constexpr size_t size() const noexcept {
    	return end - begin;
    }
};

template <typename T>
class vector {
    T* begin;
    T* end;
    T* capacity;
    
    /* ... */
    
public:
    constexpr size_t size(this const vector& self) noexcept {
    	return self.end - self.begin;
    }
};

Declare member functions const

15. Make variables const

Effective use of C++

Copy globals to const locals

(if copying is cheap)

struct {
    /* ... */
    bool fill;
} _internal__state;

void set_draw_mode_filled();
void set_draw_mode_wireframe();

void draw_mesh(const mesh* m) {
    for (const primitive* prim = m->begin(); prim != m->end(); ++prim) {
        if(_internal__is_frontfacing(*prim)) {
            if (_internal__state.fill) {
                _internal__draw_prim_filled(*prim);
            }
            else {
                _internal__draw_prim_wireframe(*prim);
            }
        }
    }
}

15. Make variables const

Effective use of C++

Copy globals to const locals

(if copying is cheap)

void draw_mesh(const mesh* m) {
    for (const primitive* prim = m->begin(); prim != m->end(); ++prim)
        if(_internal__is_frontfacing(*prim))
            if (_internal__state.fill) 
                _internal__draw_prim_filled(*prim);
            else
                _internal__draw_prim_wireframe(*prim);
}

void draw_mesh(const mesh* m) {
    if (_internal__state.fill) 
        for (const primitive* prim = m->begin(); prim != m->end(); ++prim)
            if(_internal__is_frontfacing(*prim))
                _internal__draw_prim_filled(*prim);
    else            
        for (const primitive* prim = m->begin(); prim != m->end(); ++prim)
            if(_internal__is_frontfacing(*prim))        
                _internal__draw_prim_wireframe(*prim);
}

could modify _internal__state.fill

15. Make variables const

Effective use of C++

Copy globals to const locals

(if copying is cheap)

void draw_mesh(const mesh* m) {
    const bool fill = _internal__state.fill;
    for (const primitive* prim = m->begin(); prim != m->end(); ++prim)
        if(_internal__is_frontfacing(*prim))
            if (fill) 
                _internal__draw_prim_filled(*prim);
            else
                _internal__draw_prim_wireframe(*prim);
}

void draw_mesh(const mesh* m) {
    if (_internal__state.fill) 
        for (const primitive* prim = m->begin(); prim != m->end(); ++prim)
            if(_internal__is_frontfacing(*prim))
                _internal__draw_prim_filled(*prim);
    else            
        for (const primitive* prim = m->begin(); prim != m->end(); ++prim)
            if(_internal__is_frontfacing(*prim))        
                _internal__draw_prim_wireframe(*prim);
}

could modify _internal__state.fill

but we don't care

16. Noexcept all the things

Effective use of C++

void f();

COULD throw an exception

void f() noexcept;

WILL NEVER throw an exception

void f() noexcept(true);

void f() noexcept(false);

template <typename T>
void swap(T&& lhs, T&& rhs)
    noexcept(std::is_nothrow_move_constructible<T>
          && std::is_nothrow_move_assignable<T>)
{
    T tmp = std::move(lhs);
    lhs = std::move(rhs);
    rhs = std::move(tmp);
}

noexceptness

depends on T

16. Noexcept all the things

Effective use of C++

void f();

COULD throw an exception

void f() noexcept;

WILL NEVER throw an exception

void f() noexcept(true);

void f() noexcept(false);

template <typename T>
void swap(T&& lhs, T&& rhs)
    noexcept(std::is_nothrow_move_constructible<T>
          && std::is_nothrow_move_assignable<T>)
{
    T tmp = std::move(lhs);
    lhs = std::move(rhs);
    rhs = std::move(tmp);
}

template <typename T>
void swap(T&& lhs, T&& rhs)
    noexcept(noexcept(T(std::move(lhs)))
          && noexcept(lhs = std::move(rhs)))
{
    T tmp = std::move(lhs);
    lhs = std::move(rhs);
    rhs = std::move(tmp);
}

17. Use static for internal linkage

Effective use of C++

int counter() {
    static int counter = 0;
    return ++counter;
};

struct image {
    namespace fs = std::filesystem;
    static image from_file(fs::path path);
};

Static variables

Static member functions

17. Use static for internal linkage

Effective use of C++

static int global_value;

static void global_func();

Internal linkage variables

Internal linkage functions

a.cpp

b.cpp

// Forward declarations
extern int global_value;
void global_func();
extern int global_value2;
void global_func2();

//Use
void example() {
    global_value = 42;    
    global_func();
    global_value2 = 42;
    global_func2();
}

17. Use static for internal linkage

Effective use of C++

static int global_value;

static void global_func();

Internal linkage functions

a.cpp

b.cpp

int global_value2;

void global_func2();

// Forward declarations
extern int global_value;
void global_func();
extern int global_value2;
void global_func2();

//Use
void example() {
    global_value = 42;    
    global_func();
    global_value2 = 42;
    global_func2();
}

unresolved external symbol

17. Use static for internal linkage

Effective use of C++

18. Use [[noreturn]]

Effective use of C++

[[noreturn]] void Log::Error(const String& msg) {
    logfile << msg << '\n';
    std::cerr << msg << '\n';
    throw Engine::RuntimeError(msg);
}

19. Use [[likely]] and [[unlikely]]

Effective use of C++

void internal_work();

19. Use [[likely]] and [[unlikely]]

Effective use of C++

bool require_init = true;
void init_lib();
void internal_work();

19. Use [[likely]] and [[unlikely]]

Effective use of C++

bool require_init = true;
void init_lib();
void internal_work();


void work()
{
    if(require_init) {
        init_lib();
        require_init = false;
    }

    internal_work();
}

19. Use [[likely]] and [[unlikely]]

Effective use of C++

bool require_init = true;
void init_lib();
void internal_work();


void work()
{
    if(require_init) {
        init_lib();
        require_init = false;
    }

    internal_work();
}

Effective use of C++

C++23	[[assume(condition)]];
GCC	if (!condition) __builtin_unreachable();
MSVC, ICC	__assume(condition);
LLVM	__builtin_assume(condition);

**20. Use [[assume(condition)]];**

Effective use of C++

[[assume(condition)]];	assert(condition);
Condition must be true	Condition must be true
For the optimiser	For the programmer
If !condition then Undefined Behaviour	If !condition then std::abort() in Debug Mode noop in Release Mode

**20. Use [[assume(condition)]];**

Effective use of C++

void implementation(internal_t* obj) {
    if (obj) {
        internal_work(*obj);
    }
}

void interface(public_t* obj) {
    if (obj) {
        [[assume(obj->internal)]];
        implementation(obj->internal);
    }
}

Assume that pointer is non null*

*better use a reference

void limiter(float* samples, size_t count) {
    [[assume(samples % 32 == 0)]];
    [[assume(size > 0)]];
    
    for (int i = 0; i < count; ++i) {
        samples[i] = std::clamp(samples[i], -1.0, 1.0)
    }
}

Assume pointer alignment*

*or use std::assume_aligned

example taken from P1774 (the [[assume]] proposal)

**20. Use [[assume(condition)]];**

Effective use of C++

const char* get_name(TextureType type) {
    switch(e) {
        case TextureType::Texture2D:
            return "Texture2D";
        case TextureType::Texture3D:
            return "Texture3D";
        case TextureType::Texture2DArray:
            return "Texture2DArray";
        case TextureType::Cubemap:
            return "Cubemap";
        default:
            [[assume(false)]];
    }
}

Declare a code path unreachable

*or use std::unreachable

**20. Use [[assume(condition)]];**

21. Use __restrict

Effective use of C++

float* __restrict buffer0;

float* __restrict buffer1;

21. Use __restrict

Effective use of C++

float* __restrict buffer0;

float* __restrict buffer1;

UB if overlap

21. Use __restrict

Effective use of C++

pointer provenance

21. Use __restrict

Effective use of C++

GCC, LLVM, ICC	__attribute__((malloc))
MSVC	__declspec(restrict)

22. Make functions pure

Effective use of C++

param0

param1

output

GCC, LLVM, ICC	__attribute__((pure)) or [[gnu::pure]]
MSVC	Not Supported

22. Make functions pure

Effective use of C++

param0

param1

output

GCC, LLVM, ICC	__attribute__((pure)) or [[gnu::pure]]
MSVC	Not Supported

param0

param1

output

GCC, LLVM, ICC	__attribute__((const)) or [[gnu::const]]
MSVC	Not Supported

global state

Effective use of C++

Build pipeline modification

Manual hardware oriented optimisations

Your Performance Todo List

Enable compiler optimisations
Set target architecture
Use fast math
Disable exceptions and RTTI
Enable Link Time Optimisation
Use Unity Builds
Link statically
Use Profile Guided Optimisation
Try different compilers
Try different standard library implementations
Keep your tools updated
Preload you program with a replacement library
Use binary post processing tools

14. Use constexpr

15. Make variables const

16. Use noexcept

17. Use static for internal linkage

18. Use [[noreturn]]

19. Use [[likely]] and [[unlikely]]

20. Use [[assume]]

21. Mark pointers restrict

22. Mark functions as pure

Annotate your code

No redundant copies

23. Take parameters properly

Effective use of C++

void func(??? x);

if x can be null

if needing ownership of x

if x is copied

take by value

if x is moved from

func(x);

call site:

declaration?

take by rvalue reference

(x is only read from)

take by unique_ptr, shared_ptr

take std::optional of x

if x is modified

take by lvalue reference

if x is cheap to copy

take by value

take by const lvalue reference

if x is a range

false

false

false

false

false

false

false

true

true

true

true

true

true

true

does x need to be a contiguous array

false

true

take std::span

can x be an arbitrary range

true

take std::ranges::***

false

does x need to be a specific container

true

take the container

false

take iterator pair

does x need to be perfectly forwarded

take by "universal reference"

true

false

type&& x

type x

type& x

type x

const type& x

START HERE

23. Take parameters properly

Effective use of C++

void f(const std::string& s);

f("Hello");

f(std::string{"Hello"}.c_str());

void f(const char* s);

(safe - lifetime of temporary extended)

implicit conversion to string

(allocation)

verbose

(safe)

f(std::string{"Hello"});
f("Hello");

void f(std::string_view s);

works for both

(no copies)

(safe)

23. Take parameters properly

Effective use of C++

if x can be null

if needing ownership of x

if x is copied

take by value

if x is moved from

take by rvalue reference

(x is only read from)

take by unique_ptr, shared_ptr

take std::optional of x

if x is modified

take by lvalue reference

if x is cheap to copy

take by value

take by const lvalue reference

if x is a range

false

false

false

false

false

false

false

true

true

true

true

true

true

true

does x need to be a contiguous array

false

true

take std::span

can x be an arbitrary range

true

take std::ranges::***

false

does x need to be a specific container

true

take the container

false

take iterator pair

does x need to be perfectly forwarded

take by "universal reference"

true

false

type&& x

type x

type& x

type x

const type& x

START HERE

23. Take parameters properly

Effective use of C++

if x can be null

if needing ownership of x

if x is copied

take by value

if x is moved from

take by rvalue reference

(x is only read from)

take by unique_ptr, shared_ptr

take std::optional of x

if x is modified

take by lvalue reference

if x is cheap to copy

take by value

take by const lvalue reference

if x is a range

false

false

false

false

false

false

false

true

true

true

true

true

true

true

does x need to be a contiguous array

false

true

take std::span

can x be an arbitrary range

true

take std::ranges::***

false

does x need to be a specific container

true

take the container

false

take iterator pair

does x need to be perfectly forwarded

take by "universal reference"

true

false

type&& x

type x

type& x

type x

const type& x

if x is a readonly string

START HERE

true

take std::string_view

false

23. Take parameters properly

Effective use of C++

if x can be null

if needing ownership of x

if x is copied

take by value

if x is moved from

take by rvalue reference

(x is only read from)

take by unique_ptr, shared_ptr

take std::optional of x

if x is modified

take by lvalue reference

if x is cheap to copy

take by value

take by const lvalue reference

if x is a range

false

false

false

false

false

false

false

true

true

true

true

true

true

true

does x need to be a contiguous array

false

true

take std::span

can x be an arbitrary range

true

take std::ranges::***

false

does x need to be a specific container

true

take the container

false

take iterator pair

does x need to be perfectly forwarded

take by "universal reference"

true

false

type&& x

type x

type& x

type x

const type& x

if x is a readonly string

START HERE

true

take std::string_view

false

is x an invocable

false

true

try in this order:

std::invocable<Args...> auto&& x

return_t(*x)(Args...)

std::move_only_function&&<return_t(Args...)> x

std::function<return_t(Args...)> x

23. Take parameters properly

Effective use of C++

if x can be null

if needing ownership of x

if x is copied

take by value

if x is moved from

take by rvalue reference

(x is only read from)

take by unique_ptr, shared_ptr

take std::optional of x

if x is modified

take by lvalue reference

if x is cheap to copy

take by value

take by const lvalue reference

if x is a range

false

false

false

false

false

false

false

true

true

true

true

true

true

true

does x need to be a contiguous array

false

true

take std::span

can x be an arbitrary range

true

take std::ranges::***

false

does x need to be a specific container

true

take the container

false

take iterator pair

does x need to be perfectly forwarded

take by "universal reference"

true

false

type&& x

type x

type& x

type x

const type& x

if x is a readonly string

START HERE

true

take std::string_view

false

is x an invocable

false

true

try in this order:

std::invocable<Args...> auto&& x

return_t(*x)(Args...)

std::move_only_function&&<return_t(Args...)> x

std::function<return_t(Args...)> x

is x a raw memory address

true

false

use a raw pointer

24. Avoid allocations in loops

Effective use of C++

while (true) {
    std::string line;
    std::getline(std::cin, line);
    if (!std::cin)
        break;
    process_line(line);
}

std::string line;
while (true) {
    std::getline(std::cin, line);
    if (!std::cin)
        break;
    process_line(line);
}

std::vector<int> shiny;
for (int i = 1; i <= 100 ++i)
    if (is_shiny(i))
        shiny.push_back(i);

std::vector<int> shiny;
shiny.reserve(100);
for (int i = 1; i <= 100 ++i)
    if (is_shiny(i))
        shiny.push_back(i);

move objects out of loops

.clear() if necessary

reserve() when an upper bound on size is known ahead of time

25. Avoid copying exceptions

Effective use of C++

catch(std::exception e) {
    std::cerr << e.what() << '\n';
}

catch(const std::exception& e) {
    std::cerr << e.what() << '\n';
}

catch(mutable_err& e) {
    e.append("Caught in foo")
    throw e;
}

catch(mutable_err& e) {
    e.append("Caught in foo")
    throw;
}

catch by reference

rethrow current exception

26. Avoid copies in range-for

Effective use of C++

std::vector<std::string> names;
for (auto name : names) {
	process(name);
}

std::vector<std::string> names;
for (const auto& name : names) {
	process(name);
}

avoid copying the iterated object

27. Avoid copies in lambda captures

Effective use of C++

std::flat_set<std::string> deviceLayers;
auto supported = [deviceLayers](std::string_view layer) {
	return deviceLayers.contains(layer);
}

std::flat_set<std::string> deviceLayers;
auto supported = [&deviceLayers](std::string_view layer) {
	return deviceLayers.contains(layer);
}

capture [&object]

28. Avoid copies in str. bindings

Effective use of C++

auto [first_person, age] = *map.begin();

const auto& [first_person, age] = *map.begin();

bind reference

29. Provide ref qualified methods

Effective use of C++

template <typename T>
class simple_optional {
    T data;
    bool has_data;
public:
    /* *** */
    T& value() {
    	if (!has_data)
            throw bad_optional_access();
        return data;
    }
    const T& value() const {
        if (!has_data)
            throw bad_optional_access();
        return data;
    }
};

simple_optional<Queue> get_transfer_queue();

try {
    Queue q = get_transfer_queue().value();
    // ...

Queue gets copied

Effective use of C++

template <typename T>
class simple_optional {
    T data;
    bool has_data;
public:
    /* *** */
    T& value() & {
    	if (!has_data)
            throw bad_optional_access();
        return data;
    }
    const T& value() const& {
        if (!has_data)
            throw bad_optional_access();
        return data;
    }
    T&& value() && {
        if (!has_data)
            throw bad_optional_access();
        return std::move(data);
    }
};

simple_optional<Queue> get_transfer_queue();

try {
    Queue q = get_transfer_queue().value();
    // ...

Queue gets moved

29. Provide ref qualified methods

Effective use of C++

template <typename T>
class simple_optional {
    T data;
    bool has_data;
public:
    /* *** */
    decltype(auto) value(this auto&& self) {
        if (!self.has_data)
            throw bad_optional_access();
        return std::forward_like<decltype(self)>(self.data);
    }
};

no code duplication

29. Provide ref qualified methods

Effective use of C++

Build pipeline modification

Manual hardware oriented optimisations

Your Performance Todo List

Enable compiler optimisations
Set target architecture
Use fast math
Disable exceptions and RTTI
Enable Link Time Optimisation
Use Unity Builds
Link statically
Use Profile Guided Optimisation
Try different compilers
Try different standard library implementations
Keep your tools updated
Preload you program with a replacement library
Use binary post processing tools

14. Use constexpr

15. Make variables const

16. Use noexcept

17. Use static for internal linkage

18. Use [[noreturn]]

19. Use [[likely]] and [[unlikely]]

20. Use [[assume]]

21. Mark pointers restrict

22. Mark functions as pure

Annotate your code

No redundant copies

23. Take function parameters properly

24. Avoid allocations in loops

25. Avoid copying exceptions

26. Avoid copies in range-for

27. Avoid copies in lambda captures

28. Avoid copies in structured bindings

29. Provide && method overloads

Cache-friendly code

Memory

Is memory a contiguous sequence of bytes?

Memory

Is memory a contiguous sequence of bytes?

C++ Standard:

Process address space:

YES

(logical, virtual address space)

Virtual address space in the Physical address space:

Physical address space:

YES

Hardware caching:

Not even a sequence...

Virtual memory

Caches

Physical address space

Process address space

C++ memory model

memory page

Page table

Is memory a contiguous sequence of bytes?

C++ Standard:

Process address space:

YES

(logical, virtual address space)

Virtual address space in the Physical address space:

Physical address space:

YES

C++ Standard:

Process address space:

YES

(logical, virtual address space)

Virtual address space in the Physical address space:

Physical address space:

YES

Access virtual memory address

Translate to physical address

Get data

Virtual Memory

Physical address space

Process address space

C++ memory model

memory page

Page table

Is memory a contiguous sequence of bytes?

C++ Standard:

Process address space:

YES

(logical, virtual address space)

Virtual address space in the Physical address space:

Physical address space:

YES

C++ Standard:

Process address space:

YES

(logical, virtual address space)

Virtual address space in the Physical address space:

Physical address space:

YES

Access virtual memory address

Translate to physical address

Get data

Swap