C attribute: unsequenced, reproducible (since C23)

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Provides the compiler with information about the access of objects by a function such that certain properties of function calls can be deduced.


[edit] Syntax

[[ unsequenced ]]
[[ __unsequenced__ ]]
[[ reproducible ]]
[[ __reproducible__ ]]
1) Indicates that a function is effectless, idempotent, stateless, and independent.
2) Indicates that a function is effectless and idempotent.

[edit] Explanation

These attributes apply to a function declarator or to a type specifier that has a function type. The corresponding attribute is a property of the function type.

[edit] Effectless

An evaluation of a function call is effectless if any store operation that is sequenced during the call is the modification of an object that synchronizes with the call; if additionally the operation is observable, all access to the object must be based on a unique pointer parameter of the function.

[edit] Idempotent

An evaluation E is idempotent if a second evaluation of E can be sequenced immediately after the original one without changing the resulting value, if any, or the observable state of the execution.

[edit] Stateless

A function F is stateless if any definition of an object of static or thread storage duration in F or in a function that is called by F is const but not volatile qualified.

[edit] Independent

A function F is independent if for any object X that is observed by a call to F through an lvalue that is not based on a parameter of the call, all accesses to X in all calls to F during the same program execution observe the same value; otherwise if the access is based on a pointer parameter, there shall be a unique such pointer parameter P such that any access to X shall be to an lvalue that is based on P.

An object X is observed by a function call if both synchronize, if X is not local to the call, if X has a lifetime that starts before the function call, and if an access of X is sequenced during the call; the last value of X, if any, that is stored before the call is said to be the value of X that is observed by the call.

[edit] Notes

These attributes exist for the purpose of compiler optimization.

If a function is reproducible, multiple subsequent calls can be treated as a single call.

If a function is unsequenced, multiple subsequent calls can be treated as a single call, and the calls can be parallelized and reordered arbitrarily.