C++ attribute: likely, unlikely (since C++20)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Declarations

Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (C++11) consteval (C++20) constinit (C++20) Storage class specifiers Translation-unit-local (C++20) class/struct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile Pack indexing specifier (C++26)     Elaborated type specifier Attributes (C++11)

Declarators Reference Pointer Array Block declarations Simple-declaration →Structured binding declaration (C++17) Alias declaration (C++11) Namespace alias definition using declaration using directive static_assert declaration (C++11) asm declaration Opaque enum declaration (C++11) Other declarations Namespace definition Function declaration Class template declaration Function template declaration Explicit template instantiation (C++11) Explicit template specialization Linkage specification Attribute declaration (C++11) Empty declaration

 

Attributes

assume (C++23)

carries_dependency (C++11)

deprecated (C++14)

fallthrough (C++17)

indeterminate (C++26)

likely (C++20)

maybe_unused (C++17)

nodiscard (C++17)

noreturn (C++11)

no_unique_address (C++20)

optimize_for_synchronized (TM TS)

unlikely (C++20)

 

Allow the compiler to optimize for the case where paths of execution including that statement are more or less likely than any alternative path of execution that does not include such a statement.

Syntax

[[likely]]	(1)
[[unlikely]]	(2)

Explanation

These attributes may be applied to labels and statements (other than declaration-statements). They may not be simultaneously applied to the same label or statement.

1) Applies to a statement to allow the compiler to optimize for the case where paths of execution including that statement are more likely than any alternative path of execution that does not include such a statement.

2) Applies to a statement to allow the compiler to optimize for the case where paths of execution including that statement are less likely than any alternative path of execution that does not include such a statement.

A path of execution is deemed to include a label if and only if it contains a jump to that label:

int f(int i)
{
    switch (i)
    {
        case 1: [[fallthrough]];
        [[likely]] case 2: return 1;
    }
    return 2;
}

i == 2 is considered more likely than any other value of i, but the [[likely]] has no effect on the i == 1 case even though it falls through the case 2: label.

Example

Run this code

#include <chrono>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <random>
 
namespace with_attributes
{
    constexpr double pow(double x, long long n) noexcept
    {
        if (n > 0) [[likely]]
            return x * pow(x, n - 1);
        else [[unlikely]]
            return 1;
    }
    constexpr long long fact(long long n) noexcept
    {
        if (n > 1) [[likely]]
            return n * fact(n - 1);
        else [[unlikely]]
            return 1;
    }
    constexpr double cos(double x) noexcept
    {
        constexpr long long precision{16LL};
        double y{};
        for (auto n{0LL}; n < precision; n += 2LL) [[likely]]
            y += pow(x, n) / (n & 2LL ? -fact(n) : fact(n));
        return y;
    }
} // namespace with_attributes
 
namespace no_attributes
{
    constexpr double pow(double x, long long n) noexcept
    {
        if (n > 0)
            return x * pow(x, n - 1);
        else
            return 1;
    }
    constexpr long long fact(long long n) noexcept
    {
        if (n > 1)
            return n * fact(n - 1);
        else
            return 1;
    }
    constexpr double cos(double x) noexcept
    {
        constexpr long long precision{16LL};
        double y{};
        for (auto n{0LL}; n < precision; n += 2LL)
            y += pow(x, n) / (n & 2LL ? -fact(n) : fact(n));
        return y;
    }
} // namespace no_attributes
 
double gen_random() noexcept
{
    static std::random_device rd;
    static std::mt19937 gen(rd());
    static std::uniform_real_distribution<double> dis(-1.0, 1.0);
    return dis(gen);
}
 
volatile double sink{}; // ensures a side effect
 
int main()
{
    for (const auto x : {0.125, 0.25, 0.5, 1. / (1 << 26)})
        std::cout
            << std::setprecision(53)
            << "x = " << x << '\n'
            << std::cos(x) << '\n'
            << with_attributes::cos(x) << '\n'
            << (std::cos(x) == with_attributes::cos(x) ? "equal" : "differ") << '\n';
 
    auto benchmark = [](auto fun, auto rem)
    {
        const auto start = std::chrono::high_resolution_clock::now();
        for (auto size{1ULL}; size != 10'000'000ULL; ++size)
            sink = fun(gen_random());
        const std::chrono::duration<double> diff =
            std::chrono::high_resolution_clock::now() - start;
        std::cout << "Time: " << std::fixed << std::setprecision(6) << diff.count()
                  << " sec " << rem << std::endl; 
    };
 
    benchmark(with_attributes::cos, "(with attributes)");
    benchmark(no_attributes::cos, "(without attributes)");
    benchmark([](double t) { return std::cos(t); }, "(std::cos)");
}

Possible output:

x = 0.125
0.99219766722932900560039115589461289346218109130859375
0.99219766722932900560039115589461289346218109130859375
equal
x = 0.25
0.96891242171064473343022882545483298599720001220703125
0.96891242171064473343022882545483298599720001220703125
equal
x = 0.5
0.8775825618903727587394314468838274478912353515625
0.8775825618903727587394314468838274478912353515625
equal
x = 1.490116119384765625e-08
0.99999999999999988897769753748434595763683319091796875
0.99999999999999988897769753748434595763683319091796875
equal
Time: 0.579122 sec (with attributes)
Time: 0.722553 sec (without attributes)
Time: 0.425963 sec (std::cos)

References

• C++23 standard (ISO/IEC 14882:2024):

• 9.12.7 Likelihood attributes [dcl.attr.likelihood]

• C++20 standard (ISO/IEC 14882:2020):

• 9.12.6 Likelihood attributes [dcl.attr.likelihood]